Table of Contents

IP Services Task List
Managing IP Connections

Enabling ICMP Protocol Unreachable Messages
Enabling ICMP Redirect Messages
Enabling ICMP Mask Reply Messages
Understanding Path MTU Discovery
Setting the MTU Packet Size
Enabling IP Source Routing
Configuring Simplex Ethernet Interfaces
Configuring a DRP Server Agent

Enabling the DRP Server Agent
Limiting the Source of DRP Queries
Configuring Authentication of DRP Queries and Responses

Filtering IP Packets

Creating Standard and Extended Access Lists Using Numbers
Creating Standard and Extended Access Lists Using Names
Applying Time Ranges to Access Lists
Including Comments About Entries in Access Lists
Applying Access Lists

Controlling Access to a Line or Interface
Controlling Policy Routing and the Filtering of Routing Information
Controlling Dialer Functions

Configuring HSRP

Enabling HSRP
Configuring HSRP Group Attributes
Changing the HSRP MAC Refresh Interval
Enabling HSRP MIB Traps

Configuring IP Accounting
Configuring TCP Performance Parameters

Compressing TCP Packet Headers

Expressing TCP Header Compression
Changing the Number of TCP Header Compression Connections

Setting the TCP Connection Attempt Time
Enabling TCP Path MTU Discovery
Enabling TCP Selective Acknowledgment
Enabling TCP Time Stamp
Setting the TCP Maximum Read Size
Setting the TCP Window Size
Setting the TCP Outgoing Queue Size

Configuring IP over WANs
Configuring the MultiNode Load Balancing Forwarding Agent

MultiNode Load Balancing Forwarding Agent Configuration Task List

Enabling Cisco Express Forwarding
Enabling NetFlow Switching
Enabling IP Multicast Routing
Configuring the Router as a Forwarding Agent

Monitoring and Maintaining the IP Network

Clearing Caches, Tables, and Databases
Monitoring and Maintaining the DRP Server Agent
Clearing the Access List Counters
Displaying System and Network Statistics
Monitoring the MNLB Forwarding Agent

IP Services Configuration Examples

ICMP Services Example
Simplex Ethernet Interfaces Example
DRP Server Agent Example
Numbered Access List Examples

Implicit Masks in Access Lists Examples
Extended Access List Examples

Named Access List Example
Time Range Applied to an IP Access List Example
Commented IP Access List Entry Examples
HSRP MAC Refresh Interval Examples

No Switch or Learning Bridge Present Example
Switch or Learning Bridge Present Example

HSRP MIB Trap Example
MNLB Forwarding Agent Examples

Forwarding Agent Configuration for FA2
Services Manager Configuration for SM

Configuring IP Services

This chapter describes how to configure optional IP services. For a complete description of the IP services commands in this chapter, refer to the "IP Services Commands" chapter of the Cisco IOS IP and IP Routing Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

IP Services Task List

To configure optional IP services, perform any of the tasks in the following sections:

• Managing IP Connections

• Filtering IP Packets

• Configuring HSRP

• Configuring IP Accounting

• Configuring TCP Performance Parameters

• Configuring IP over WANs

• Configuring the MultiNode Load Balancing Forwarding Agent

• Monitoring and Maintaining the IP Network

Remember that not all the tasks in these sections are required. The tasks you must perform will depend on your network and your needs.

At the end of this chapter, the examples in the "IP Services Configuration Examples" section illustrate how you might configure your network using IP.

Managing IP Connections

The IP suite offers a number of services that control and manage IP connections. Internet Control Message Protocol (ICMP) provides many of these services. ICMP messages are sent by routers or access servers to hosts or other routers when a problem is discovered with the Internet header. For detailed information on ICMP, see RFC 792.

• Enabling ICMP Protocol Unreachable Messages

• Enabling ICMP Redirect Messages

• Enabling ICMP Mask Reply Messages

• Understanding Path MTU Discovery

• Setting the MTU Packet Size

• Enabling IP Source Routing

• Configuring Simplex Ethernet Interfaces

• Configuring a DRP Server Agent

See the "ICMP Services Example" section at the end of this chapter for examples of ICMP services.

Enabling ICMP Protocol Unreachable Messages

If the Cisco IOS software receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP Protocol Unreachable message back to the source. Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP Host Unreachable message to the source. This feature is enabled by default.

To enable this service if it has been disabled, use the following command in interface configuration mode:

Command	Purpose
ip unreach ables	Enable the sending of ICMP Protocol Unreachable and Host Unreachable messages.

Enabling ICMP Redirect Messages

Routes are sometimes less than optimal. For example, it is possible for the router to be forced to resend a packet through the same interface on which it was received. If the router resends a packet through the same interface on which it was received, the Cisco IOS software sends an ICMP Redirect message to the originator of the packet telling the originator that the router is on a subnet directly connected to the receiving device, and that it must forward the packet to another system on the same subnet. The software sends an ICMP Redirect message to the packet's originator because the originating host presumably could have sent that packet to the next hop without involving this device at all. The Redirect message instructs the sender to remove the receiving device from the route and substitute a specified device representing a more direct path. This feature is enabled by default. However, when Hot Standby Router Protocol (HSRP) is configured on an interface, ICMP Redirect messages are disabled by default for the interface.

To enable the sending of ICMP Redirect messages if this feature was disabled, use the following command in interface configuration mode:

Command	Purpose
ip redirects	Enable the sending of ICMP Redirect messages to learn routes.

Enabling ICMP Mask Reply Messages

Occasionally, network devices must know the subnet mask for a particular subnetwork in the internetwork. To achieve this information, such devices can send ICMP Mask Request messages. These messages are responded to by ICMP Mask Reply messages from devices that have the requested information. The Cisco IOS software can respond to ICMP Mask Request messages if this function is enabled.

To enable the sending of ICMP Mask Reply messages, use the following command in interface configuration mode:

Command	Purpose
ip mask-reply	Enable the sending of ICMP Mask Reply messages.

Understanding Path MTU Discovery

The Cisco IOS software supports the IP Path MTU Discovery mechanism, as defined in RFC 1191. IP Path MTU Discovery allows a host to dynamically discover and cope with differences in the maximum allowable maximum transmission unit (MTU) size of the various links along the path. Sometimes a router is unable to forward a datagram because it requires fragmentation (the packet is larger than the MTU you set for the interface with the ip mtu command), but the "don't fragment" (DF) bit is set. The Cisco IOS software sends a message to the sending host, alerting it to the problem. The host will need to fragment packets for the destination so that they fit the smallest packet size of all the links along the path. This technique is shown in Figure 15.

IP Path MTU Discovery is useful when a link in a network goes down, forcing the use of another, different MTU-sized link (and different routers). As shown in Figure 15, suppose a router is sending IP packets over a network where the MTU in the first router is set to 1500 bytes, but the second router is set to 512 bytes. If the "Don't fragment" bit of the datagram is set, the datagram would be dropped because the 512-byte router is unable to forward it. All packets larger than 512 bytes are dropped in this case. The second router returns an ICMP Destination Unreachable message to the source of the datagram with its Code field indicating, "Fragmentation needed and DF set." To support IP Path MTU Discovery, it would also include the MTU of the next hop network link in the low-order bits of an unused header field.

IP Path MTU Discovery is also useful when a connection is first being established and the sender has no information at all about the intervening links. It is always advisable to use the largest MTU that the links will bear; the larger the MTU, the fewer packets the host must send.

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Note   IP Path MTU Discovery is a process initiated by end hosts. If an end host does not support IP Path MTU Discovery, the receiving device will have no mechanism available to avoid fragmenting datagrams generated by the end host.

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If a router that is configured with a small MTU on an outbound interface receives packets from from a host that is configured with a large MTU (for example, receiving packets from a Token Ring interface and forwarding them to an outbound Ethernet interface), the router fragments received packets that are larger than the MTU of the outbound interface. Fragmenting packets slows the performance of the router. To keep routers in your network from fragmenting received packets, run IP Path MTU Discovery on all hosts and routers in your network, and always configure the largest possible MTU for each router interface type.

To enable Path MTU Discovery for connections initiated by the router (when the router is acting as a host), see the section "Enabling TCP Path MTU Discovery" later in this chapter.

Setting the MTU Packet Size

All interfaces have a default MTU packet size. You can adjust the IP MTU size so that if an IP packet exceeds the MTU set for an interface, the Cisco IOS software will fragment it.

Enabling IP Source Routing

The Cisco IOS software examines IP header options on every packet. It supports the IP header options Strict Source Route, Loose Source Route, Record Route, and Time Stamp, which are defined in RFC 791. If the software finds a packet with one of these options enabled, it performs the appropriate action. If it finds a packet with an invalid option, it sends an ICMP Parameter Problem message to the source of the packet and discards the packet.

Configuring Simplex Ethernet Interfaces

You can configure simplex Ethernet interfaces. This feature is useful for setting up dynamic IP routing over a simplex circuit (a circuit that receives only or sends only). When a route is learned on a receive-only interface, the interface designated as the source of the route is converted to the interface you specify. When packets are routed out this specified interface, they are sent to the IP address of the source of the routing update. To reach this IP address on a transmit-only Ethernet link, a static Address Resolution Protocol (ARP) entry mapping this IP address to the hardware address of the other end of the link is required.

To assign a transmit interface to a receive-only interface, use the following command in interface configuration mode:

Command	Purpose
transmit-interfac_(_IREFOBJ:1001075_) _e type number	Assign a transmit interface to a receive-only interface.

See the "Simplex Ethernet Interfaces Example" section at the end of this chapter for an example of configuring a simplex Ethernet interface.

Configuring a DRP Server Agent

The Director Response Protocol (DRP) is a simple User Datagram Protocol (UDP)-based application developed by Cisco Systems. It enables Cisco's DistributedDirector product to query routers (DRP Server Agents) in the field for Border Gateway Protocol (BGP) and Interior Gateway Protocol (IGP) routing table metrics between distributed servers and clients. DistributedDirector, a separate standalone product, uses DRP to transparently redirect end-user service requests to the topologically closest responsive server. DRP enables DistributedDirector to provide dynamic, scalable, and "network intelligent" Internet traffic load distribution between multiple geographically dispersed servers.

• Enabling the DRP Server Agent

• Limiting the Source of DRP Queries

• Configuring Authentication of DRP Queries and Responses

To monitor and maintain the DRP Server Agent, see the section "Monitoring and Maintaining the DRP Server Agent" later in this chapter.

For an example of configuring a DRP Server Agent, see the section "DRP Server Agent Example" at the end of this chapter.

Enabling the DRP Server Agent

The DRP Server Agent is disabled by default. To enable it, use the following command in global configuration mode:

Command	Purpose
ip drp server	Enable the DRP Server Agent.

Limiting the Source of DRP Queries

As a security measure, you can limit the source of valid DRP queries. If a standard IP access list is applied to the interface, the Server Agent will respond only to DRP queries originating from an IP address in the list. If no access list is configured, the server agent will answer all queries.

If both an access group and a key chain (described in the next section) have been configured, both security mechanisms must allow access before a request is processed.

To limit the source of valid DRP queries, use the following command in global configuration mode:

Command	Purpose
ip drp access-group access-list-number	Control the sources of valid DRP queries by applying a standard IP access list.

Configuring Authentication of DRP Queries and Responses

Another available security measure is to configure the DRP Server Agent to authenticate DRP queries and responses. You define a key chain, identify the keys that belong to the key chain, and specify how long each key is valid. To do so, use the following commands beginning in global configuration mode:

Filtering IP Packets

Packet filtering helps control packet movement through the network. Such control can help limit network traffic and restrict network use by certain users or devices. To permit or deny packets from crossing specified interfaces, we provide access lists.

See the "IP Services Configuration Examples" section at the end of this chapter for examples of configuring IP access lists.

An access list is a sequential collection of permit and deny conditions that apply to IP addresses. The Cisco IOS software tests addresses against the conditions in an access list one by one. The first match determines whether the software accepts or rejects the address. Because the software stops testing conditions after the first match, the order of the conditions is critical. If no conditions match, the software rejects the address.

The two steps involved in using access lists are as follows:

1. Create an access list by specifying an access list number or name and access conditions.

2. Apply the access list to interfaces or terminal lines.

These steps are described in the next sections.

Creating Standard and Extended Access Lists Using Numbers

Caution Release 11.1 and later releases introduced substantial changes to IP access lists. These extensions are backward compatible; migrating from a release earlier than Release 11.1 to the current image will convert your access lists automatically. However, previous releases are not upwardly compatible with these changes. Thus, if you save an access list with the current image and then use older software, the resulting access list will not be interpreted correctly. This could cause you severe security problems. Save your old configuration file before booting Release 11.1 or later images.

The software supports the following styles of access lists for IP:

• Standard IP access lists use source addresses for matching operations.

• Extended IP access lists use source and destination addresses for matching operations, and optional protocol type information for finer granularity of control.

• Dynamic extended IP access lists grant access per user to a specific source or destination host basis through a user authentication process. In essence, you can allow user access through a firewall dynamically, without compromising security restrictions. Dynamic access lists and lock-and-key access are described in the "Configuring Traffic Filters" chapter of the Cisco IOS Security Configuration Guide.

• Reflexive access lists allow IP packets to be filtered based on session information. Reflexive access lists contain temporary entries, and are nested within an extended, named IP access list. For information on reflexive access lists, refer to the "Configuring IP Session Filtering (Reflexive Access Lists)" chapter in the Cisco IOS Security Configuration Guide and the "Reflexive Access List Commands" chapter in the Cisco IOS Security Command Reference publication.

For an example of a standard IP access list using logs, see the section "Numbered Access List Examples" at the end of this chapter.

To create an extended access list, use the following commands in global configuration mode:

	Command	Purpose
Step 1	access-list access-list-number remark remark	Indicates the purpose of the deny or permit statement.1
Step 2	access-list access-list-number {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tostos] [established] [log] [time-range time-range-name] or access-list access-list-number {deny | permit} protocol any any [log] [time-range time-range-name] or access-list access-list-number {deny | permit} protocolhost source host destination [log] [time-rangetime-range-name] or access-list access-list-number [dynamic dynamic-name [timeout minutes]] {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [established] [log] [time-range time-range-name]	Defines an extended IP access list number and the access conditions. Use the log keyword to get access list logging messages, including violations. Specifies a time range to restrict when the permit or deny statement is in effect. or Defines an extended IP access list using an abbreviation for a source and source wildcard of 0.0.0.0 255.255.255.255, and an abbreviation for a destination and destination wildcard of 0.0.0.0 255.255.255.255. or Defines an extended IP access list using an abbreviation for a source and source wildcard of source 0.0.0.0, and an abbreviation for a destination and destination wildcard of destination 0.0.0.0. or Defines a dynamic access list. For information about lock-and-key access, refer to the "Configuring Traffic Filters" chapter in the Cisco IOS Security Configuration Guide.

1This example configures the remark before the deny or permit statement. The remark can be configured after the deny or permit statement.

After an access list is created initially, any subsequent additions (possibly entered from the terminal) are placed at the end of the list. In other words, you cannot selectively add or remove access list command lines from a specific access list.

Note   When creating an access list, remember that, by default, the end of the access list contains an implicit deny statement for everything if it did not find a match before reaching the end. Further, with standard access lists, if you omit the mask from an associated IP host address access list specification, 0.0.0.0 is assumed to be the mask.

Note   Autonomous switching is not used when you have extended access lists.

After creating an access list, you must apply it to a line or interface, as shown in the section "Applying Access Lists" later in this chapter.

See the "Implicit Masks in Access Lists Examples" section at the end of this chapter for examples of implicit masks.

Creating Standard and Extended Access Lists Using Names

Caution Named access lists will not be recognized by any software release prior to Cisco IOS Release 11.2.

You can identify IP access lists with an alphanumeric string (a name) rather than a number. Named access lists allow you to configure more IP access lists in a router than if you were to use numbered access lists. If you identify your access list with a name rather than a number, the mode and command syntax are slightly different. Currently, only packet and route filters can use a named list.

• Access lists specified by name are not compatible with older releases.

• Not all access lists that accept a number will accept a name. Access lists for packet filters and route filters on interfaces can use a name.

• A standard access list and an extended access list cannot have the same name.

• Numbered access lists are also available, as described in the previous section, "Creating Standard and Extended Access Lists Using Numbers."

To create a standard access list, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1	ip access-list standard name	Defines a standard IP access list using a name.
Step 2	access-list access-list-number remark remark	Indicates the purpose of the deny or permit statement.1
Step 3	deny {source [source-wildcard] | any}[log] or permit {source [source-wildcard] | any}[log]	In access-list configuration mode, specifies one or more conditions allowed or denied, which determines whether the packet is passed or dropped.
Step 4	exit	Exits access-list configuration mode.

1This example configures the remark before the deny or permit statement. The remark can be configured after the deny or permit statement.

To create an extended access list, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1	ip access-list extended name	Defines an extended IP access list using a name.
Step 2	access-list access-list-number remark remark	Indicates the purpose of the deny or permit statement.1
Step 3	deny | permit protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [established] [log] [time-rangetime-range-name] or deny | permit protocol any any [log] [time-rangetime-range-name] or deny | permit protocol host source host destination[log] [time-range time-range-name] or dynamic dynamic-name [timeout minutes] {deny|permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [established] [log] [time-rangetime-range-name]	In access-list configuration mode, specifies the conditions allowed or denied. Use the log keyword to get access list logging messages, including violations. Specifies a time range to restrict when the permit or deny statement is in effect. or Defines an extended IP access list using an abbreviation for a source and source wildcard of 0.0.0.0 255.255.255.255, and an abbreviation for a destination and destination wildcard of 0.0.0.0 255.255.255.255. or Defines an extended IP access list using an abbreviation for a source and source wildcard of source 0.0.0.0, and an abbreviation for a destination and destination wildcard of destination 0.0.0.0. or Defines a dynamic access list. For information about lock-and-key access, refer to the "Configuring Traffic Filters" chapter in the Cisco IOS Security Configuration Guide.

1This example configures the remark before the deny or permit statement. The remark can be configured after the deny or permit statement.

Note   Autonomous switching is not used when you have extended access lists.

After you initially create an access list, you place any subsequent additions (possibly entered from the terminal) at the end of the list. In other words, you cannot selectively add access list command lines to a specific access list. However, you can use no permit and no deny commands to remove entries from a named access list.

Note   When making the standard and extended access list, remember that, by default, the end of the access list contains an implicit deny statement for everything if it did not find a match before reaching the end. Further, with standard access lists, if you omit the mask from an associated IP host address access list specification, 0.0.0.0 is assumed to be the mask.

After creating an access list, you must apply it to a line or interface, as shown in section "Applying Access Lists," later in this chapter.

See the "Named Access List Example" section at the end of this chapter for an example of a named access list.

Applying Time Ranges to Access Lists

It is now possible to implement access lists based on the time of day and week using the time-range command. To do so, first define the name and times of the day and week of the time range, then reference the time range by name in an access list to apply restrictions to the access list.

Currently, IP and Internetwork Packet Exchange (IPX) named or numbered extended access lists are the only functions that can use time ranges. The time range allows the network administrator to define when the permit or deny statements in the access list are in effect. Prior to this feature, access list statements were always in effect once they were applied. The time-range keyword and argument are referenced in the named and numbered extended access list task tables in the previous sections, "Creating Standard and Extended Access Lists Using Numbers" and "Creating Standard and Extended Access Lists Using Names." The time-range command is configured in the "Performing Basic System Management" chapter of the Cisco IOS Configuration Fundamentals Configuration Guide. See the "Time Range Applied to an IP Access List Example" section at the end of this chapter for a configuration examples of IP time ranges.

There are many possible benefits of using time ranges, such as the following:

• The network administrator has more control over permitting or denying a user access to resources. These resources could be an application (identified by an IP address/mask pair and a port number), policy routing, or an on-demand link (identified as interesting traffic to the dialer).

• Network administrators can set time-based security policy, including the following:

  • Perimeter security using the Cisco IOS Firewall feature set or access lists

  • Data confidentiality with Cisco Encryption Technology or IP Security Protocol (IPSec)

• Policy-based routing and queueing functions are enhanced.

• When provider access rates vary by time of day, it is possible to automatically reroute traffic cost effectively.

• Service providers can dynamically change a Committed Access Rate (CAR) configuration to support the quality of service (QoS) Service Level Agreements (SLAs) that are negotiated for certain times of day.

• Network administrators can control logging messages. Access list entries can log traffic at certain times of the day, but not constantly. Therefore, administrators can simply deny access without needing to analyze many logs generated during peak hours.

Including Comments About Entries in Access Lists

It is now possible to include comments (remarks) about entries in any IP access list using the remark command. The remarks make the access list easier for the network administrator to understand and scan. Each remark line is limited to 100 characters.

The remark can go before or after a permit or deny statement. You should be consistent about where you put the remark so it is clear which remark describes which permit or deny statement. For example, it would be confusing to have some remarks before the associated permit or deny statements and some remarks after the associated statements. The standard and extended access list task tables in the previous sections, "Creating Standard and Extended Access Lists Using Numbers" and "Creating Standard and Extended Access Lists Using Names," include the remark command. See the "Commented IP Access List Entry Examples" section at the end of this chapter for examples of commented IP access list entries.

Remember to apply the access list to an interface or terminal line after the access list is created. See the following section, "Applying Access Lists," for more information.

Applying Access Lists

After creating an access list, you must reference the access list to make it work. The several ways to use an access list are described in the following sections:

• Controlling Access to a Line or Interface

• Controlling Policy Routing and the Filtering of Routing Information

• Controlling Dialer Functions

Controlling Access to a Line or Interface

After you create an access list, you can apply it to one or more interfaces. Access lists can be applied on either outbound or inbound interfaces. The following two tables show how to accomplish this task for both terminal lines and network interfaces. Remember the following:

To restrict access to a virtual terminal line and the addresses in an access list, use the following command in line configuration mode. Only numbered access lists can be applied to lines. Set identical restrictions on all the virtual terminal lines, because a user can attempt to connect to any of them.

For inbound access lists, after receiving a packet, the Cisco IOS software checks the source address of the packet against the access list. If the access list permits the address, the software continues to process the packet. If the access list rejects the address, the software discards the packet and returns an ICMP Host Unreachable message.

When you apply an access list that has not yet been defined to an interface, the software will act as if the access list has not been applied to the interface and will accept all packets. Remember this behavior if you use undefined access lists as a means of security in your network.

Controlling Policy Routing and the Filtering of Routing Information

To use access lists to control policy routing and the filtering of routing information, see the "Configuring IP Routing Protocol-Independent Features" chapter of this guide.

Controlling Dialer Functions

To use access lists to control dialer functions, refer to the Cisco IOS Dial Services Configuration Guide: Terminal Services and Cisco IOS Dial Services Configuration Guide: Network Services publications.

Configuring HSRP

HSRP provides high network availability because it routes IP traffic from hosts on Ethernet, FDDI, or Token Ring networks without relying on the availability of any single router. HSRP is used in a group of routers for selecting an active router and a standby router. (An active router is the router of choice for routing packets; a standby router is a router that takes over the routing duties when an active router fails, or when preset conditions are met.)

HSRP is useful for hosts that do not support a router discovery protocol (such as ICMP Router Discovery Protocol [IRDP]) and cannot switch to a new router when their selected router reloads or loses power. Because existing TCP sessions can survive the failover, this protocol also provides a more transparent recovery for hosts that dynamically choose a next hop for routing IP traffic.

When the HSRP is configured on a network segment, it provides a virtual Media Access Control (MAC) address and an IP address that is shared among routers in a group of routers that is running HSRP. One of these devices is selected by the protocol to be the active router. The active router receives and routes packets destined for the group's MAC address. For n routers running HSRP, there are n + 1 IP and MAC addresses assigned.

HSRP detects when the designated active router fails, at which point a selected standby router assumes control of the Hot Standby group's MAC and IP addresses. A new standby router is also selected at that time.

Devices that are running HSRP send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers.

When HSRP is configured on an interface, ICMP Redirect messages are disabled by default for the interface.

You can configure multiple Hot Standby groups on an interface, thereby making fuller use of the redundant routers. To do so, specify a group number for each Hot Standby command you configure for the interface.

Note   Token Ring interfaces allow up to three Hot Standby groups each, the group numbers being 0, 1, and 2.

Note   The Cisco 1000 series, Cisco 2500 series, Cisco 3000 series, and Cisco 4000 series routers that use Lance Ethernet hardware do not support multiple Hot Standby groups on a single Ethernet interface.

HSRP is supported over Inter-Switch Link (ISL) encapsulation. Refer to the "Configuring Routing Between VLANs with ISL Encapsulation" chapter in the Cisco IOS Switching Services Configuration Guide.

Use the commands in the following sections to configure HSRP:

• Enabling HSRP (Required)

• Configuring HSRP Group Attributes

• Changing the HSRP MAC Refresh Interval

• Enabling HSRP MIB Traps

For more information about HSRP and how to configure it on a Cisco router, see the chapter "Using HSRP for Fault-Tolerant IP Routing" in the Cisco CCIE Fundamentals: Case Studies publication.

Enabling HSRP

To enable the HSRP on an interface, use the following command in interface configuration mode:

Command	Purpose
standby [group-number] ip [ip-address [secondary]]	Enable the HSRP.

Configuring HSRP Group Attributes

To configure other Hot Standby group attributes that affect how the local router participates in HSRP, use one or more of the following commands in interface configuration mode:

Command	Purpose
standby [group-number] timers hellotime holdtime	Configure the time between hello packets and the hold time before other routers declare the active router to be down.
standby [group-number] priority priority [preempt [delay [minimum | sync] delay]] or standby [group-number] [priority priority] preempt [delay [minimum | sync] delay]	Set the Hot Standby priority used in choosing the active router. Specify that, if the local router has priority over the current active router, the local router should attempt to take its place as the active router. Configure a preemption delay, after which the Hot Standby router preempts and becomes the active router.
standby [group-number] track type number [interface-priority]	Configure the interface to track other interfaces, so that if one of the other interfaces goes down, the device's Hot Standby priority is lowered.
standby [group-number] authentication string	Select an authentication string to be carried in all HSRP messages.
standby use-bia [scope interface]	Configure HSRP to use the burned-in address of an interface as its virtual MAC address instead of the preassigned MAC address (on Ethernet and FDDI) or the functional address (on Token Ring).

Changing the HSRP MAC Refresh Interval

When HSRP runs over FDDI, you can change the interval at which a packet is sent to refresh the MAC cache on learning bridges or switches. HSRP hello packets use the burned-in address (BIA) instead of the MAC virtual address. Refresh packets keep the MAC cache on switches and learning bridges current.

You can change the refresh interval on FDDI rings to a longer or shorter interval, thereby using bandwidth more efficiently. You can prevent the sending of any MAC refresh packets if you don't need them (if you have FDDI but do not have a learning bridge or switch). When changing the HSRP MAC refresh interval, be aware of the following items:

• This feature applies to HSRP running over FDDI only.

• You need not configure the MAC refresh interval if you have the standby use-bia command configured.

By default, a packet is sent every 10 seconds to refresh the MAC cache on learning bridges or switches. To change the interval, use the following command in interface configuration mode:

For examples of this feature, see the section "HSRP MAC Refresh Interval Examples" at the end of this chapter.

Enabling HSRP MIB Traps

With Cisco IOS Release 12.0(3)T, the software supports the HSRP Management MIB feature. HSRB MIB supports Simple Network Management Protocol (SNMP) get operations, to allow network devices to get reports about HSRP groups in a network from the network management station.

Enabling HSRP MIB trap support is done from the command-line interface (CLI), and the MIB is used for getting the reports. A trap notifies the network management station when a router leaves or enters the active or standby state. When an entry is configured from the CLI, the RowStatus for that group in the MIB immediately goes to the active state.

The Cisco IOS software supports a read-only version of the MIB, and set operations are not supported.

This feature supports four MIB tables:

• cHsrpGrpEntry table defined in CISCO-HSRP-MIB.my

• cHsrpExtIfTrackedEntry, cHsrpExtSecAddrEntry, and cHsrpExtIfEntry defined in CISCO-HSRP-EXT-MIB.my

For descriptions of supported MIBs and how to use MIBs, see Cisco's MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

The cHsrpGrpEntry table consists of all the group information defined in RFC 2281, Cisco Hot Standby Router Protocol; the other tables consist of the Cisco extensions to RFC 2281, which are defined in CISCO-HSRP-EXT-MIB.my.

To enable HSRP MIB trap support, use the following commands in global configuration mode:

	Command	Purpose
Step 1	Router(config)# snmp-server enable traps hsrp	Enables the router to send SNMP traps and informs, and HSRP notifications.
Step 2	Router(config)# snmp-server host host community-string hsrp	Specifies the recipient of an SNMP notification operation, and that HSRP notifications be sent to the host.

See the section "HSRP MIB Trap Example" later in this document for an example of how to configure HSRP MIB trap support in your network.

Configuring IP Accounting

Our IP accounting support provides basic IP accounting functions. By enabling IP accounting, users can see the number of bytes and packets switched through the Cisco IOS software on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the software or terminating in the software is not included in the accounting statistics. To maintain accurate accounting totals, the software maintains two accounting databases: an active and a checkpointed database.

Our IP accounting support also provides information identifying IP traffic that fails IP access lists. Identifying IP source addresses that violate IP access lists alerts you to possible attempts to breach security. The data also indicates that you should verify IP access list configurations. To make this feature available to users, you must enable IP accounting of access list violations using the ip accounting access-violations command. Users can then display the number of bytes and packets from a single source that attempted to breach security against the access list for the source destination pair. By default, IP accounting displays the number of packets that have passed access lists and were routed.

Configuring TCP Performance Parameters

To tune IP performance, complete any of the tasks in the following sections. To configure various switching options, refer to the "Cisco IOS Switching Paths" chapter in the Cisco IOS Switching Services Configuration Guide.

• Compressing TCP Packet Headers

• Setting the TCP Connection Attempt Time

• Enabling TCP Path MTU Discovery

• Enabling TCP Selective Acknowledgment

• Enabling TCP Time Stamp

• Setting the TCP Maximum Read Size

• Setting the TCP Window Size

• Setting the TCP Outgoing Queue Size

Compressing TCP Packet Headers

You can compress the headers of your TCP/IP packets in order to reduce their size, thereby increasing performance. Header compression is particularly useful on networks with a large percentage of small packets (such as those supporting many Telnet connections).

To enable TCP header compression, use the following command in interface configuration mode:

Command	Purpose
ip tcp header-compression [passive]	Enable TCP header compression.

The ip tcp header-compression only compresses the TCP header; it has no effect on UDP packets or other protocol headers. The TCP header compression technique is supported on serial lines using High-Level Data Link Control (HDLC) or PPP encapsulation. You must enable compression on both ends of a serial connection.

By using the passive keyword, you can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same interface are compressed. If you specify the command without the passive keyword, the software will compress all traffic. Without the command, the default is no compression.

---

Note   Fast processors can handle several fast interfaces, such as T1s, that are running header compression. However, you should think carefully about your network's traffic characteristics before compressing TCP headers. You might want to use the monitoring commands to compare network utilization before and after enabling TCP header compression.

---

Expressing TCP Header Compression

Before Cisco IOS Release 12.0(7)T, if compression of TCP headers was enabled, compression was performed in the process switching path. Compression performed in the process switching path meant that packets traversing interfaces that had TCP header compression enabled were queued and passed up to the process to be switched. This procedure slowed down transmission of the packet, and therefore some users preferred to fast switch uncompressed TCP packets.

In Cisco IOS Release 12.1, if TCP header compression is enabled, it occurs by default in the fast-switched path or the Cisco Express Forwarding (CEF)-switched path, depending on which switching method is enabled on the interface.

Changing the Number of TCP Header Compression Connections

You also can specify the total number of header compression connections that can exist on an interface. You should configure one connection for each TCP connection through the specified interface.

When specifying the total number of header compression connections that can exist on an interface, be aware of the following:

• By default, for Frame Relay encapsulation, there can be 256 TCP header compression connections (128 calls). The maximum value is fixed, not configurable.

• By default, for PPP or HDLC encapsulation, the software allows 32 TCP header compression connections (16 calls). This default can be increased to a maximum of 256 TCP header compression connections.

To specify the number of connections, use the following command in interface configuration mode:

Command	Purpose
ip tcp compression-connections number	Specify the total number of TCP header compression connections that can exist on an interface.

Setting the TCP Connection Attempt Time

You can set the amount of time the Cisco IOS software will wait to attempt to establish a TCP connection. In previous versions of software, the system would wait a fixed 30 seconds when attempting to establish a connection. This amount of time is not sufficient in networks that have dialup asynchronous connections (such as a network consisting of dial-on-demand links that are implemented over modems) because it will affect your ability to Telnet over the link (from the router) if the link must be brought up.

Because the connection attempt time is a host parameter, it does not pertain to traffic going through the device, just to traffic originated at the device.

To set the TCP connection attempt time, use the following command in global configuration mode:

Command	Purpose
ip tcp synwait-time seconds	Set the amount of time the Cisco IOS software will wait to attempt to establish a TCP connection.

Enabling TCP Path MTU Discovery

Path MTU Discovery is a method for maximizing the use of available bandwidth in the network between the endpoints of a TCP connection, and is described in RFC 1191. By default, this feature is disabled. Existing connections are not affected when this feature is turned on or off. To enable Path MTU Discovery, use the following command in interface configuration mode:

The ip tcp path-mtu-discovery command is to enable Path MTU Discovery for connections initiated by the router when it is acting as a host. For a discussion of how the Cisco IOS software supports Path MTU Discovery when the device is acting as a router, see the section "Understanding Path MTU Discovery" earlier in this chapter.

The age-timer is a time interval for how often TCP should reestimate the Path MTU with a larger maximum segment size (MSS). The default path MTU Discovery age-timer is 10 minutes; its maximum is 30 minutes. You can turn off the age-timer by setting it to infinite.

Enabling TCP Selective Acknowledgment

The TCP selective acknowledgment feature improves performance in the event that multiple packets are lost from one TCP window of data.

The TCP selective acknowledgment mechanism helps improve performance. The receiving TCP host returns selective acknowledgment packets to the sender, informing the sender of data that has been received. In other words, the receiver can acknowledge packets received out of order. The sender can then resend only the missing data segments (instead of everything since the first missing packet).

Refer to RFC 2018 for more detailed information on TCP selective acknowledgment.

Enabling TCP Time Stamp

The TCP time-stamp option provides better TCP round-trip time measurements. Because the time stamps are always sent and echoed in both directions and the time-stamp value in the header is always changing, TCP header compression will not compress the outgoing packet. To allow TCP header compression over a serial link, the TCP time-stamp option is disabled.

Refer to RFC 1323 for more detailed information on TCP time stamp.

If you want to use TCP header compression over a serial line, TCP time stamp and TCP selective acknowledgment must be disabled. Both features are disabled by default. To disable TCP selective acknowledgment once it is enabled, see the previous "Enabling TCP Selective Acknowledgment" section.

Setting the TCP Maximum Read Size

By default, for Telnet and rlogin, the maximum number of characters that TCP reads from the input queue at once is a very large number (the largest possible 32-bit positive number). We do not recommend that you change this value. However, you could change that value by using the following command in global configuration mode:

Setting the TCP Window Size

The default TCP window size is 2144 bytes. We recommend you keep the default value, unless you know your router is sending large packets (greater than 536 bytes). To change the default window size, use the following command in global configuration mode:

Setting the TCP Outgoing Queue Size

The default TCP outgoing queue size per connection is 5 segments if the connection has a TTY associated with it (like a Telnet connection). If there is no TTY connection associated with it, the default queue size is 20 segments. To change the 5-segment default value, use the following command in global configuration mode:

Configuring IP over WANs

You can configure IP over X.25, Switched Multimegabit Data Service (SMDS), Frame Relay, and dial-on-demand routing (DDR) networks. When configuring IP over X.25, SMDS, or Frame Relay, configure the address mappings as described in the appropriate chapters of the Cisco IOS Wide-Area Networking Configuration Guide. For DDR, refer to the "Terminal Services" chapter of the Cisco IOS Dial Configuration Guide: Terminal Services publication.

Configuring the MultiNode Load Balancing Forwarding Agent

The MultiNode Load Balancing (MNLB) forwarding agent is the Cisco IOS-based packet redirector component of the MNLD Feature Set for LocalDirector, a product in the Cisco family of load balancing solutions.

The forwarding agent discovers the destination of specific connection requests and forwards packets between the client and the chosen destination. When a forwarding agent receives a connection request, the request is forwarded to the MNLB services manager, the LocalDirector-based component of the MNLD Feature Set for LocalDirector. The services manager makes the load-balancing decision and sends the forwarding agent the optimal destination. After the destination is specified, session data is forwarded directly to the destination by the forwarding agent, without further services manager participation. There is no limit to the number of forwarding agents that can be configured in the MNLD Feature Set for LocalDirector.

The MNLD Feature Set for LocalDirector comprises hardware and software that runs on multiple network components. The services manager runs on the Cisco LocalDirector chassis and makes the load-balancing decisions. The forwarding agents run on Cisco IOS router and switch platforms and forward packets to and from the selected destination. Separating the decision-making and packet-forwarding tasks enables much faster packet throughput. The underlying Cisco architecture, ContentFlow architecture, enables the following:

Configure the forwarding agent only if you are installing the MNLD Feature Set for LocalDirector. If you are installing the MNLD Feature Set for LocalDirector, refer to the MultiNode Load Balancing Feature Set for LocalDirector User Guide for information about which other hardware and software components are required.

The MNLB forwarding agent is an implementation of the Cisco ContentFlow architecture flow delivery agent (FDA).

Refer to the MultiNode Load Balancing Feature Set for LocalDirector User Guide for more information about how the forwarding agent is configured and for more information about the product.

MultiNode Load Balancing Forwarding Agent Configuration Task List

The following sections describe MNLB forwarding agent configuration tasks:

• Enabling Cisco Express Forwarding (Required)

• Enabling NetFlow Switching (Optional but strongly recommended)

• Enabling IP Multicast Routing (Required)

• Configuring the Router as a Forwarding Agent (Required)

Enabling Cisco Express Forwarding

Cisco Express Forwarding (CEF) is advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions.

To enable CEF, use the following command in global configuration mode:

Command	Purpose
Router(config) ip cef distributed	Enables CEF.

---

Note   When you enable CEF globally, all interfaces that support CEF are enabled by default. If you want to turn off CEF on a particular interface, you can do so.

---

Refer to the "Cisco Express Forwarding" part of the Cisco IOS Switching Services Configuration Guide for more information on how to configure CEF.

Enabling NetFlow Switching

You must enable NetFlow switching on all interfaces that will carry ContentFlow traffic. To enable NetFlow switching, use the following commands beginning in interface configuration mode:

Normally the size of the NetFlow cache will meet your needs. However, you can increase or decrease the number of entries maintained in the cache by using the following command in global configuration mode:

Enabling IP Multicast Routing

You must enable IP multicast routing on all interfaces to the services manager.

Configuring the Router as a Forwarding Agent

To configure the router as a forwarding agent, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1	Router(config)# ip casa control-address igmp-address	Specifies the IP address and Internet Group Management Protocol (IGMP) address of the forwarding agent. The recommended IGMP address is 224.0.1.2.
Step 2	Router(config-casa)# forwarding-agent pools initial_affinity_pool max_affinity_pool	Adjusts the memory allocated for the affinity pools of the forwarding agent. The default pool size is 5000, and there is no maximum pool size.
Step 3	Router(config-casa)# forwarding-agent number [password [timeout]]	Specifies the port number. The default is port 1637.

Note   The forwarding agent IGMP address and port must match the IGMP address and port configured on the services manager using the ip igmp join-group command.

Monitoring and Maintaining the IP Network

To monitor and maintain your network, perform the tasks in the following sections:

• Clearing Caches, Tables, and Databases

• Monitoring and Maintaining the DRP Server Agent

• Clearing the Access List Counters

• Displaying System and Network Statistics

• Monitoring the MNLB Forwarding Agent

Clearing Caches, Tables, and Databases

You can remove all contents of a particular cache, table, or database. Clearing a cache, table, or database can become necessary when the contents of the particular structure have become or are suspected to be invalid.

To clear caches, tables, and databases, use the following commands as needed in EXEC mode:

Command	Purpose
clear ip accounting [checkpoint]	Clear the active IP accounting or checkpointed database when IP accounting is enabled.
clear tcp statistics	Clear TCP statistics.

Monitoring and Maintaining the DRP Server Agent

To monitor and maintain the DRP Server Agent, use the following commands in EXEC mode:

Command	Purpose
clear ip drp	Clear statistics being collected on DRP requests and responses.
show ip drp	Display information about the DRP Server Agent.

Clearing the Access List Counters

The system counts how many packets pass each line of an access list; the counters are displayed by the show access-lists command. To clear the counters of an access list, use the following command in EXEC mode:

Command	Purpose
clear access-list counters {access-list-number | name}	Clear the access list counters.

Displaying System and Network Statistics

You can display specific statistics such as the contents of IP routing tables, caches, and databases. The resulting information can be used to determine resource utilization and to solve network problems.

These tasks are summarized in the table that follows. See the "IP Services Commands" chapter in the Cisco IOS IP and IP Routing Command Reference publication for details about the commands listed in these tasks. To display specific statistics such as the contents of IP routing tables, caches, and databases, use any of the following commands in privileged EXEC mode:

Command	Purpose
show access-lists [access-list-number | name]	Display the contents of one or all current access lists.
show ip access-list [access-list-number | name]	Display the contents of current IP access lists.
show ip accounting [checkpoint]	Display the active IP accounting or checkpointed database.
show ip redirects	Display the address of the default router and the address of hosts for which an ICMP Redirect message has been received.
show ip tcp header-compression	Display statistics on TCP header compression.
show ip traffic	Display IP protocol statistics.
show standby [interface [group]] [brief]	Display the status of the standby router.
show tcp statistics	Display TCP statistics.

Monitoring the MNLB Forwarding Agent

To monitor the status of the forwarding agent, use the following commands in EXEC mode:

Command	Purpose
Router#show ip casa affinities	Display the status of affinities.
Router#show ip casa oper	Display the operational status of the forwarding agent.
Router#show ip casa stats	Display statistical information about the forwarding agent.
Router#show ip casa wildcard	Display information about wildcard blocks.

IP Services Configuration Examples

The following sections provide IP configuration examples:

• ICMP Services Example

• Simplex Ethernet Interfaces Example

• DRP Server Agent Example

• Numbered Access List Examples

• Named Access List Example

• Time Range Applied to an IP Access List Example

• Commented IP Access List Entry Examples

• HSRP MAC Refresh Interval Examples

• HSRP MIB Trap Example

• MNLB Forwarding Agent Examples

ICMP Services Example

The following example changes some of the ICMP defaults for the first Ethernet interface 0. Disabling the sending of redirects could mean that you do not expect your devices on this segment to ever need to send a redirect. Disabling the Unreachables messages will have a secondary effect—it also will disable IP Path MTU Discovery, because path discovery works by having the Cisco IOS software send Unreachables messages. If you have a network segment with a small number of devices and an absolutely reliable traffic pattern—which could easily happen on a segment with a small number of little-used user devices—you would be disabling options that your device would be unlikely to use anyway.

interface ethernet 0
 no ip unreachables
 no ip redirects

Simplex Ethernet Interfaces Example

The following is an example of configuring a simplex Ethernet interface. Figure 16 illustrates how to configure IP on two routers sharing transmit-only and receive-only Ethernet connections.

interface ethernet 0
 ip address 128.9.1.1
!
interface ethernet 1
 ip address 128.9.1.1
 transmit-interface ethernet 0
!
!use show interfaces command to find router2-MAC-address-E0
arp 128.9.1.4 router2-MAC-address-E0 arpa

interface ethernet 0
 ip address 128.9.1.2
 transmit-interface ethernet 1
!
interface ethernet 1
 ip address 128.9.1.2
!
!use show interfaces command to find router1-MAC-address-E1
arp 128.9.1.1 router1-MAC-address-E1 arpa

DRP Server Agent Example

The following example enables the DRP Server Agent. Sources of DRP queries are limited by access list 1, which permits only queries from the host at 33.45.12.4. Authentication is also configured for the DRP queries and responses.

ip drp server
access-list 1 permit 33.45.12.4
ip drp access-group 1
ip drp authentication key-chain mktg
key chain mktg
 key 1
  key-string internal

Numbered Access List Examples

In the following example, network 36.0.0.0 is a Class A network whose second octet specifies a subnet; that is, its subnet mask is 255.255.0.0. The third and fourth octets of a network 36.0.0.0 address specify a particular host. Using access list 2, the Cisco IOS software would accept one address on subnet 48 and reject all others on that subnet. The last line of the list shows that the software would accept addresses on all other network 36.0.0.0 subnets.

access-list 2 permit 36.48.0.3
access-list 2 deny 36.48.0.0  0.0.255.255 
access-list 2 permit 36.0.0.0  0.255.255.255 
interface ethernet 0
 ip access-group 2 in
 

The following example defines access lists 1 and 2, both of which include logging:

interface ethernet 0
 ip address 1.1.1.1 255.0.0.0
 ip access-group 1 in
 ip access-group 2 out
!
access-list 1 permit 5.6.0.0 0.0.255.255 log
access-list 1 deny 7.9.0.0 0.0.255.255 log
!
access-list 2 permit 1.2.3.4 log
access-list 2 deny 1.2.0.0 0.0.255.255 log
 

Suppose the interface receives 10 packets from 5.6.7.7 and 14 packets from 1.2.23.21. The first log will look like this:

list 1 permit 5.6.7.7 1 packet
list 2 deny 1.2.23.21 1 packet
 

Five minutes later, the console will receive this log:

list 1 permit 5.6.7.7 9 packets
list 2 deny 1.2.23.21 13 packets

Implicit Masks in Access Lists Examples

IP access lists contain implicit masks. For instance, if you omit the mask from an associated IP host address access list specification, 0.0.0.0 is assumed to be the mask. Consider the following example configuration:

access-list 1 permit 0.0.0.0
access-list 1 permit 131.108.0.0
access-list 1 deny 0.0.0.0 255.255.255.255
 

For this example, the following masks are implied in the first two lines:

access-list 1 permit 0.0.0.0 0.0.0.0
access-list 1 permit 131.108.0.0 0.0.0.0
 

The last line in the configuration (using the deny keyword) can be left off, because IP access lists implicitly deny all other access. Leaving off the last line in the configuration is equivalent to finishing the access list with the following command statement:

access-list 1 deny 0.0.0.0 255.255.255.255
 

The following access list only allows access for those hosts on the three specified networks. It assumes that subnetting is not used; the masks apply to the host portions of the network addresses. Any hosts with a source address that does not match the access list statements will be rejected.

access-list 1 permit 192.5.34.0  0.0.0.255
access-list 1 permit 128.88.0.0  0.0.255.255
access-list 1 permit 36.0.0.0  0.255.255.255
! (Note: all other access implicitly denied)
 

To specify a large number of individual addresses more easily, you can omit the address mask that is all zeros from the access-list global configuration command. Thus, the following two configuration commands are identical in effect:

access-list 2 permit 36.48.0.3
access-list 2 permit 36.48.0.3  0.0.0.0

Extended Access List Examples

In the following example, the first line permits any incoming TCP connections with destination ports greater than 1023. The second line permits incoming TCP connections to the Simple Mail Transfer Protocol (SMTP) port of host 128.88.1.2. The last line permits incoming ICMP messages for error feedback.

access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.0.0 0.0.255.255 gt 1023
access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.1.2 0.0.0.0 eq 25
access-list 102 permit icmp 0.0.0.0 255.255.255.255 128.88.0.0 255.255.255.255
interface ethernet 0
 ip access-group 102 in
 

For another example of using an extended access list, suppose you have a network connected to the Internet, and you want any host on an Ethernet to be able to form TCP connections to any host on the Internet. However, you do not want IP hosts to be able to form TCP connections to hosts on the Ethernet except to the mail (SMTP) port of a dedicated mail host.

SMTP uses TCP port 25 on one end of the connection and a random port number on the other end. The same two port numbers are used throughout the life of the connection. Mail packets coming in from the Internet will have a destination port of 25. Outbound packets will have the port numbers reversed. The fact that the secure system behind the router always will be accepting mail connections on port 25 is what makes it possible to separately control incoming and outgoing services. The access list can be configured on either the outbound or inbound interface.

In the following example, the Ethernet network is a Class B network with the address 128.88.0.0, and the address of the mail host is 128.88.1.2. The established keyword is used only for the TCP protocol to indicate an established connection. A match occurs if the TCP datagram has the ACK or RST bits set, which indicate that the packet belongs to an existing connection.

access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.0.0 0.0.255.255 established
access-list 102 permit tcp 0.0.0.0 255.255.255.255 128.88.1.2 0.0.0.0 eq 25
interface ethernet 0
 ip access-group 102 in

Named Access List Example

The following configuration creates a standard access list named Internet_filter and an extended access list named marketing_group:

interface Ethernet0/5
 ip address 2.0.5.1 255.255.255.0
 ip access-group Internet_filter out
 ip access-group marketing_group in
...
ip access-list standard Internet_filter
 permit 1.2.3.4
 deny any
ip access-list extended marketing_group
 permit tcp any 171.69.0.0 0.0.255.255 eq telnet
 deny tcp any any
 permit icmp any any
 deny udp any 171.69.0.0 0.0.255.255 lt 1024
 deny ip any any log

Time Range Applied to an IP Access List Example

The following example denies Hypertext Transfer Protocol (HTTP) traffic on Monday through Friday between the hours of 8:00 a.m. and 6:00 p.m. on IP. The example allows UDP traffic on Saturday and Sunday from noon to 8:00 p.m. only.

time-range no-http
 periodic weekdays 8:00 to 18:00
!
time-range udp-yes
 periodic weekend 12:00 to 20:00
!
ip access-list extended strict
 deny tcp any any eq http time-range no-http
 permit udp any any time-range udp-yes
!
interface ethernet 0
 ip access-group strict in

Commented IP Access List Entry Examples

In the following example of a numbered access list, the workstation belonging to Jones is allowed access and the workstation belonging to Smith is not allowed access:

access-list 1 remark Permit only Jones workstation through
access-list 1 permit 171.69.2.88
access-list 1 remark Do not allow Smith workstation through
access-list 1 deny 171.69.3.13
 

In the following example of a numbered access list, the Winter and Smith workstations are not allowed to browse the web:

access-list 100 remark Do not allow Winter to browse the web
access-list 100 deny host 171.69.3.85 any eq http
access-list 100 remark Do not allow Smith to browse the web
access-list 100 deny host 171.69.3.13 any eq http
 

In the following example of a named access list, the Jones subnet is not allowed access:

ip access-list standard prevention
remark Do not allow Jones subnet through
deny 171.69.0.0 0.0.255.255
 

In the following example of a named access list, the Jones subnet is not allowed to use outbound Telnet:

ip access-list extended telnetting
remark Do not allow Jones subnet to telnet out
deny tcp 171.69.0.0 0.0.255.255 any eq telnet

HSRP MAC Refresh Interval Examples

The following sections provide HSRP MAC refresh interval examples:

• No Switch or Learning Bridge Present Example

• Switch or Learning Bridge Present Example

No Switch or Learning Bridge Present Example

The following HSRP example of changing the MAC refresh interval is applicable if there is no switch or learning bridge in your network. It prevents the sending of refresh packets.

interface fddi 1/0/0
 ip address 10.1.1.1 255.255.255.0
 standby ip 10.1.1.250
 standby mac-refresh 0

Switch or Learning Bridge Present Example

The following HSRP example of changing the MAC refresh interval is applicable if there is a switch or learning bridge in your network. It will reduce the number of extra packets you send to refresh the MAC cache on the switch or learning bridge to two per minute.

interface fddi 1/0/0
 ip address 10.1.1.1 255.255.255.0
 standby ip 10.1.1.250
 standby mac-refresh 30

HSRP MIB Trap Example

The following example shows how to configure HSRP on two routers and enable the HSRP MIB trap feature. As in many environments, one router is preferred as the active one by configuring it at a higher priority level and enabling preemption. In this example, the active router is referred to as the primary router. The second router is referred to as the backup router.

Primary Router

interface Ethernet1
 ip address 15.1.1.1 255.255.0.0
 no ip redirects
 standby priority 200 preempt
 standby preempt
 standby ip 15.1.1.3
snmp-server enable traps hsrp
snmp-server host yourhost.cisco.com public hsrp
 

Backup Router

 interface Ethernet1
 ip address 15.1.1.2 255.255.0.0
 no ip redirects
 standby priority 101
 standby ip 15.1.1.3
snmp-server enable traps hsrp
snmp-server host myhost.cisco.com public hsrp

MNLB Forwarding Agent Examples

This section provides the following configuration examples:

• Forwarding Agent Configuration for FA2

• Services Manager Configuration for SM

The network configured is shown in Figure 17.

Forwarding Agent Configuration for FA2

The following is a sample of a router configured as a forwarding agent. In this example all disabled interfaces have been omitted to simplify the display.

FA2#wr t                 
Building configuration...
 
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
service udp-small-servers
service tcp-small-servers
!
hostname FA2
!
!
microcode CIP flash slot0:cip26-5
microcode reload
ip subnet-zero
ip flow-cache feature-accelerate
no ip domain-lookup
!         
ip cef distributed
ip casa 206.10.20.34 224.0.1.2
 forwarding-agent 1637 
!
interface Ethernet0/0
 ip address 172.26.56.18 255.255.255.0
 no ip directed-broadcast
 ip route-cache flow
 ip igmp join-group 224.0.1.2
 no ip mroute-cache
!
interface Ethernet0/1
 ip address 172.26.56.37 255.255.255.0
 no ip directed-broadcast
!
!
!
router eigrp 777
 network 172.26.0.0
!
 
no ip classless
!
line con 0
 exec-timeout 0 0
 transport input none
line aux 0
line vty 0 4
 exec-timeout 0 0
login
!         
end

Services Manager Configuration for SM

SM# wr t
Building configuration...
: Saved
: LocalDirector 420 Version 3.0.0.127
syslog output 20.3
no syslog console
enable password 000000000000000000000000000000 encrypted
hostname SM
no shutdown ethernet 0
no shutdown ethernet 1
no shutdown ethernet 2
no shutdown ethernet 3
interface ethernet 0 auto
interface ethernet 1 auto
interface ethernet 2 auto
interface ethernet 3 auto
mtu 0 1500
mtu 1 1500
mtu 2 1500
mtu 3 1500
multiring all
no secure 0
no secure 1
no secure 2 
no secure 3
ping-allow 0
ping-allow 1
ping-allow 2
ping-allow 3
ip address 172.26.56.19 255.255.255.248
route 172.26.10.249 255.255.255.255 172.26.56.20 1
route 206.10.20.33 255.255.255.255 172.26.56.17 1
route 206.10.20.34 255.255.255.255 172.26.56.18 1
no rip passive
failover ip address 0.0.0.0
failover
password cisco
telnet 161.0.0.0 255.0.0.0
no snmp-server contact
no snmp-server location
casa service-manager port 1638
casa service-manager multicast-ttl 60
tftp-server 172.26.10.249 /tftpboot/LD 
virtual 172.26.56.13:0:0:tcp is
virtual 172.26.56.2:0:0:tcp is
redirection 172.26.56.13:0:0:tcp dispatched casa wildcard-ttl 60 fixed-ttl 60 igmp 224.0.1.2 port 1637
redirection 172.26.56.2:0:0:tcp dispatched casa wildcard-ttl 60 fixed-ttl 60 igmp 224.0.1.2 port 1637
real 172.26.56.34:0:0:tcp is
real 172.26.56.33:0:0:tcp is
real 172.26.56.6:0:0:tcp is
real 172.26.56.10:0:0:tcp is
bind 172.26.56.13:0:0:tcp 172.26.56.33:0:0:tcp
bind 172.26.56.13:0:0:tcp 172.26.56.34:0:0:tcp
bind 172.26.56.2:0:0:tcp 172.26.56.10:0:0:tcp
bind 172.26.56.2:0:0:tcp 172.26.56.6:0:0:tcp
: end

Posted: Mon Sep 11 12:46:58 PDT 2000
Copyright 1989-2000©Cisco Systems Inc.