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This chapter describes how to configure optional IP services supported by the Cisco Optical Networking System (ONS) 15304. For a complete description of the commands in this chapter, refer to the "IP Services Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
To configure optional IP services, complete any of the following tasks:
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.
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.
To manage various aspects of IP connections, complete the appropriate tasks in the following sections:
Seethe section "ICMP Services Example" for examples of ICMP services.
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.
If this service has been disabled, you can enable it by completing the following task in interface configuration mode:
| Task | Command |
|---|---|
Enable the sending of ICMP Protocol Unreachable and Host Unreachable messages. | ip unreachables |
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 this happens, the Cisco IOS software sends an ICMP Redirect message to the packet's originator telling it that it 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 does this 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 is configured on an interface, ICMP Redirect messages are disabled by default for the interface.
If this feature was disabled, you can enable the sending of ICMP Redirect messages by completing the following task in interface configuration mode:
| Task | Command |
|---|---|
Enable the sending of ICMP Redirect messages to learn routes. | ip redirects |
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, complete the following task in interface configuration mode:
| Task | Command |
|---|---|
Enable the sending of ICMP Mask Reply messages. | ip mask-reply |
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 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 have 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 8-1.
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 8-1, suppose a router is sending IP packets over a network where the MTU in the first router is set to 1,500 bytes, but the second router is set to 512 bytes. If the datagram's "don't fragment" bit 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.
The Cisco 7000 series and Cisco 4000 series routers support fast switching of IP packets between Ethernet and FDDI interfaces. When packets are being sent from FDDI to Ethernet interfaces and you are not using IP Path MTU Discovery, FDDI packets with data lengths larger than 1500 bytes will be fragmented into multiple Ethernet packets. This will slow performance. If the majority of your traffic travels off the FDDI ring, you might want to either lower the MTU size on your host FDDI interfaces to 1,500 bytes or run IP Path MTU Discovery on your hosts.
Because the CTR card does not support the switching of frames larger than 4,472 bytes, some interoperability problems can occur if CTR cards are intermixed with other Token Ring cards on the same network. You can minimize this by setting lower (and the same) IP maximum packet sizes for all devices on the network with the ip mtu interface command.
To enable Path MTU Discovery for connections initiated by the router (when the router is acting as a host), see the section "Enable TCP Path MTU Discovery".
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.
Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu interface configuration command.
Also, all devices on a physical medium must have the same protocol MTU in order to operate.
To set the MTU packet size for a specified interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
Set the IP MTU packet size for an interface. | ip mtu bytes |
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.
IP provides a provision that allows the source IP host to specify a route through the IP network. This provision is known as source routing. Source routing is specified as an option in the IP header. If source routing is specified, the software forwards the packet according to the specified source route. This feature is employed when you want to force a packet to take a certain route through the network. The default is to perform source routing.
If IP source-route header options have been disabled, you can enable them by completing the following task in global configuration mode:
| Task | Command |
|---|---|
Enable IP source routing. | ip source-route |
You can use access lists in the following ways:
This section summarizes how to create IP access lists and how to apply them.
See the section "IP Services Configuration Examples" 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:
Step 1 Create an access list by specifying an access list number or name and access conditions.
Step 2 Apply the access list to interfaces or terminal lines.
These steps are described in the next sections.
The software supports the following styles of access lists for IP:
To create a standard access list, complete one of the following tasks in global configuration mode:
| Task | Command |
|---|---|
Define a standard IP access list using a source address and wildcard. | access-list access-list-number {deny | permit} source [source-wildcard] |
Define a standard IP access list using an abbreviation for the source and source mask of 0.0.0.0 255.255.255.255. | access-list access-list-number {deny | permit} any |
To create an extended access list, complete one of the following tasks in global configuration mode:
| Task | Command |
|---|---|
Define an extended IP access list number and the access conditions. Use the log keyword to get access list logging messages, including violations. | access-list access-list-number {deny | permit} protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [established] [log] |
Define 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. | access-list access-list-number {deny | permit} protocol any any |
Define 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. | access-list access-list-number {deny | permit} protocol host source host destination |
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] |
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.
After creating an access list, you must apply it to a line or interface, as shown in the "Apply the Access List to an Interface or Terminal Line" section.
See the "Implicit Masks in Access Lists Examples" section for more information on creating implicit masks.
 | 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 (1 to 199). This feature allows you to configure more than 99 standard IP and 100 extended IP access lists in a router. 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.
Consider the following before configuring named access lists:
To create a standard access list, complete the following tasks beginning in global configuration mode:
| Task | Command |
|---|---|
Define a standard IP access list using a name. | ip access-list standard name |
In access-list configuration mode, specify one or more conditions allowed or denied. This determines whether the packet is passed or dropped. | deny {source [source-wildcard] | any} or permit {source [source-wildcard] | any} |
Exit access-list configuration mode. | exit |
To create an extended access list, complete the following tasks beginning in global configuration mode:
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.
After creating an access list, you must apply it to a line or interface as shown in the "Apply the Access List to an Interface or Terminal Line" section.
See the "Named Access List Example" section for an example of a named access list.
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:
Complete the following task 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.
| Task | Command |
|---|---|
Restrict incoming and outgoing connections between a particular virtual terminal line (into a device) and the addresses in an access list. | access-class access-list-number {in | out} |
Complete the following task in interface configuration mode:
| Task | Command |
|---|---|
Control access to an interface. | ip access-group {access-list-number | name} {in | out} |
For outbound access lists, after receiving and routing a packet to a controlled interface, the software checks the source address of the packet against the access list. If the access list permits the address, the software transmits 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.
Cisco's 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.
Cisco's 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.
To enable IP accounting, complete one of the following tasks for each interface in interface configuration mode:
| Task | Command |
|---|---|
Enable basic IP accounting. | ip accounting |
Enable IP accounting with the ability to identify IP traffic that fails IP access lists. | ip accounting access-violations |
To configure other IP accounting functions, complete one or more of the following tasks in global configuration mode:
| Task | Command |
|---|---|
Set the maximum number of accounting entries to be created. | ip accounting-threshold threshold |
Filter accounting information for hosts. | ip accounting-list ip-address wildcard |
Control the number of transit records that will be stored in the IP accounting database. | ip accounting-transits count |
To display IP access violations for a specific IP accounting database, complete the following task in EXEC mode:
| Task | Command |
|---|---|
Display IP access-violation information. | show ip accounting [checkpoint] access-violations |
To display IP access violations, you must give the access-violations keyword on the command. If you do not specify the keyword, the command defaults to displaying the number of packets that have passed access lists and were routed. The access violations output displays the number of the access list failed by the last packet for the source and destination pair. The number of packets reveals how aggressive the attack is upon a specific destination.
Use the EXEC command show ip accounting to display the active accounting database. To display the checkpointed database, use the show ip accounting checkpoint EXEC command. The clear ip accounting EXEC command clears the active database and creates the checkpointed database.
To tune IP performance, complete any of the following tasks. To configure various switching options, refer to the "Switching Paths" chapter in the Switching Services Configuration Guide.
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). This feature only compresses the TCP header, so it has no effect on UDP packets or other protocol headers. The TCP header compression technique, described fully in RFC 1144, is supported on serial lines using HDLC or PPP encapsulation. You must enable compression on both ends of a serial connection.
You can optionally specify outgoing packets to be compressed only if TCP incoming packets on the same interface are compressed. If you do not specify this option, the Cisco IOS software will compress all traffic. The default is no compression.
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.
To enable compression, complete either of the following optional tasks in interface configuration mode:
| Task | Command |
|---|---|
Enable TCP header compression. | ip tcp header-compression [passive] |
Specify the total number of header compression connections that can exist on an interface. | ip tcp compression-connections number |
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 dial-up 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, complete the following task in global configuration mode:
| Task | Command |
|---|---|
Set the amount of time the Cisco IOS software will wait to attempt to establish a TCP connection. | ip tcp synwait-time seconds |
Path MTU Discovery is a method for maximizing the use of available bandwidth in the network between the end points 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, complete the following task in interface configuration mode:
| Task | Command |
|---|---|
Enable Path MTU Discovery. | ip tcp path-mtu-discovery [age-timer {minutes | infinite}] |
Customers using TCP connections to move bulk data between systems on distinct subnets would benefit most by enabling this feature. This might include customers using RSRB with TCP encapsulation, STUN, X.25 Remote Switching (also known as XOT or X.25 over TCP), and some protocol translation configurations.
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 "Understand Path MTU Discovery" section.
The age-timer is a time interval for how often TCP should re-estimate 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.
The TCP selective acknowledgment feature improves performance in the event that multiple packets are lost from one TCP window of data.
Prior to this feature, with the limited information available from cumulative acknowledgments, a TCP sender could learn about only one lost packet per round trip time. An aggressive sender could choose to retransmit packets early, but such retransmitted segments might have already been successfully received.
Prior to selective acknowledgment, if TCP lost packets 4 and 7 out of an 8-packet window, TCP would receive acknowledgment of only packets 1, 2, and 3. Packets 4 through 8 would have to be resent. With selective acknowledgment, TCP receives acknowledgment of packets 1, 2, 3, 5, 6, and 8. Only packets 4 and 7 have to be resent.
Refer to RFC 2018 for more detailed information on TCP selective acknowledgment.
The feature is used only when multiple packets are dropped within one TCP window. There is no performance impact when the feature is enabled but not used. To enable TCP selective acknowledgment, complete the following task in global configuration mode:
| Task | Command |
|---|---|
Enable TCP selective acknowledgment. |
Refer to RFC 1323 for more detailed information on TCP timestamp.
To enable TCP timestamp, perform the following task in global configuration mode:
| Task | Command |
|---|---|
Enable TCP timestamp. |
If you want to use TCP header compression over a serial line, TCP timestamp and TCP selective acknowledgment must be disabled. Both features are disabled by default. To disable TCP selective acknowledgment after it is enabled, refer to the "Enable TCP Selective Acknowledgment" section.
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). Cisco does not recommend that you change this value. However, you could change that value by performing the following task in global configuration mode:
| Task | Command |
|---|---|
Set the TCP maximum read size for Telnet or rlogin. | ip tcp chunk-size characters |
| Task | Command |
|---|---|
Set the TCP window size. | ip tcp window-size bytes |
| Task | Command |
|---|---|
Set the TCP outgoing queue size. | ip tcp queuemax packets |
To monitor and maintain your network, perform the tasks in the following sections:
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.
The following table lists the tasks associated with clearing caches, tables, and databases. Complete the following tasks as needed in EXEC mode:
| Task | Command |
|---|---|
Clear the active IP accounting or checkpointed database when IP accounting is enabled. | clear ip accounting [checkpoint] |
Clear TCP statistics. |
The system counts how many packets pass each line of an access list; the counters are displayed by the show access-lists command. You can clear the counters of an access list by completing the following task in EXEC mode:
| Task | Command |
|---|---|
Clear the access list counters. | clear access-list counters access-list-number | name |
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 Network Protocols Command Reference, Part 1 for details about the commands listed in these tasks. Complete any of the following tasks in privileged EXEC mode:
| Task | Command |
|---|---|
Display the contents of one or all current access lists. | show access-lists [access-list-number | name] |
Display the contents of current IP access lists. | show ip access-list [access-list-number | name] |
Display the active IP accounting or checkpointed database. | show ip accounting [checkpoint] |
Show statistics on TCP header compression. | |
Display IP protocol statistics. | show ip traffic |
Display TCP statistics. |
The following sections provide IP configuration examples:
The example that follows changes some of the ICMP defaults for the first Ethernet interface 0. Disabling the sending of redirects could mean that you do not think your devices on this segment will ever have 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 1 no ip unreachables no ip redirects
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 1 ip access-group 2 in
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. This 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
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 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 1 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 mail host's address is 128.88.1.2. The keyword established 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 1 ip access-group 102 in
The following configuration creates a standard access list named Internet_filter and an extended access list named marketing_group:
interface Ethernet 1 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
Posted: Thu Feb 24 12:24:07 PST 2000
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