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This chapter describes how to configure Routing Information Protocol (RIP). For a complete description of the RIP commands that appear in this chapter, refer to the "RIP Commands" chapter of the Cisco IOS IP and IP Routing Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
RIP is a relatively old, but still commonly used, interior gateway protocol created for use in small, homogeneous networks. It is a classical distance-vector routing protocol. RIP is documented in RFC 1058.
RIP uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. The Cisco IOS software sends routing information updates every 30 seconds, which is termed advertising. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by the nonupdating router as being unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the nonupdating router.
The metric that RIP uses to rate the value of different routes is hop count. The hop count is the number of routers that can be traversed in a route. A directly connected network has a metric of zero; an unreachable network has a metric of 16. This small range of metrics makes RIP an unsuitable routing protocol for large networks.
If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The network 0.0.0.0 does not exist; RIP treats 0.0.0.0 as a network to implement the default routing feature. The Cisco IOS software will advertise the default network if a default was learned by RIP, or if the router has a gateway of last resort and RIP is configured with a default metric.
RIP sends updates to the interfaces in the specified networks. If the network of an interface network is not specified, it will not be advertised in any RIP update.
The Cisco implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).
For protocol-independent features, which also apply to RIP, see the chapter "Configuring IP Routing Protocol-Independent Features" in this document.
To configure RIP, perform the tasks in the following sections. You must enable RIP. The remaining tasks are optional.
For information about the following topics, see the "Configuring IP Routing Protocol-Independent Features" chapter:
To enable RIP, use the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step 1 | Router#router rip | Enable a RIP routing process, which places you in router configuration mode. |
Step2 | Router(config-router)#network network-number | Associate a network with a RIP routing process. |
Because RIP is normally a broadcast protocol, in order for RIP routing updates to reach nonbroadcast networks, you must configure the Cisco IOS software to permit this exchange of routing information. To do so, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)#neighbor ip-address | Defines a neighboring router with which to exchange routing information. |
To control the set of interfaces with which you want to exchange routing updates, you can disable the sending of routing updates on specified interfaces by configuring the passive-interface command. See the discussion on filtering in the "Filter Routing Information" section in the "Configuring IP Routing Protocol-Independent Features" chapter.
An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via RIP. Optionally, you can limit the offset list with either an access list or an interface. To increase the value of routing metrics, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)#offset-list [access-list-number | name] {in | out} offset [type number] |
It also is possible to tune the IP routing support in the software to enable faster convergence of the various IP routing algorithms, and, hence, quicker fallback to redundant routers. The total effect is to minimize disruptions to end users of the network in situations where quick recovery is essential.
To adjust the timers, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)#timers basic update invalid holddown flush [sleeptime] | Adjusts routing protocol timers. |
The Cisco implementation of RIP Version 2 supports authentication, key management, route summarization, CIDR, and VLSMs. Key management and VLSM are described in the chapter "Configuring IP Routing Protocol-Independent Features."
By default, the software receives RIP Version 1 and Version 2 packets, but sends only Version 1 packets. You can configure the software to receive and send only Version 1 packets. Alternatively, you can configure the software to receive and send only Version 2 packets. To configure the software to send and receive packets from only one version, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)#version {1 | 2} | Configures the software to receive and send only RIP Version1 or only RIP Version 2 packets. |
The preceding task controls the default behavior of RIP. You can override that behavior by configuring a particular interface to behave differently. To control which RIP version an interface sends, use one of the following commands, beginning in interface configuration mode
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Similarly, to control how packets received from an interface are processed, use one of the following commands in interface configuration mode:
RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface.
The key chain determines the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed on that interface, not even the default authentication. Therefore, you must also perform the tasks in the section "Manage Authentication Keys" in the "Configuring IP Routing Protocol-Independent Features" chapter.
We support two modes of authentication on an interface for which RIP authentication is enabled: plain text authentication and MD5 authentication. The default authentication in every RIP Version2 packet is plain text authentication.
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NoteDo not use plain text authentication in RIP packets for security purposes, because the unencrypted authentication key is sent in every RIP Version 2 packet. Use plain text authentication when security is not an issue, for example, to ensure that misconfigured hosts do not participate in routing. |
To configure RIP authentication, use the following commands in interface configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | Router(config-if)# ip rip authentication key-chain name-of-chain | Enables RIP authentication. |
Step2 | Router(config-if)# ip rip authentication mode {text | md5} | Configures the interface to use MD5 digest authentication (or let it default to plain text authentication). |
See the "Key Management Examples" section of the "Configuring IP Routing Protocol-Independent Features" chapter for key management information and examples.
Summarizing routes in RIP Version 2 improves scalability and efficiency in large networks. Summarizing IP addresses means that there is no entry for child routes (routes that are created for any combination of the individual IP addresses contained within a summary address) in the RIP routing table, reducing the size of the table and allowing the router to handle more routes.
Summary IP address functions more efficiently than multiple individually advertised IP routes, because:
Cisco routers can summarize routes in two ways:
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NoteYou need not configure anything for autosummary to be enabled. To disable autosummary, use the Router(config-router)# no auto-summary command. |
Autosummary addressing always summarizes to the classful address boundary, while the ip summary-address command summarizes addresses on a specified interface. If autosummary addressing is enabled, autosummarization is the default behavior for interfaces on the router not associated with dial-in clients (the "backbone"), with or without the ip summary-address rip interface subcommand present.
For example, if a local IP address pool of 10.1.1.1 to 10.1.1.254 is configured on the network access server, you could configure the ip summary-address rip 10.1.1.0 255.255.255.0 command on the network access server port that provides addresses to dialup clients to cause the router to advertise 10.1.1.0/24 routes to dialup clients. Because a summary route is advertised, advertisement of the /32 host routes (installed when the dialup client connects) is suppressed so that the router does not advertise these routes to the network access server interface.
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NoteIf split horizon is enabled, neither an autosummary address nor the interface summary-address is advertised. |
In the following example configuration, the major network is 10.0.0.0. The 10 in the address defines a Class A address space, allowing space for 0.x.x.x unique hosts where x defines unique bit positions in the addresses for these hosts. The summary of the major net defines the prefix as implied by the class (A, B, or C) of the address, without any network mask. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0, 10.2.0.0 is advertised out interface E1, and 10.0.0.0 is not advertised.
int E1 ip address 10.1.1.1 255.255.255.0 ip summary-address rip 10.2.0.0 255.255.0.0 no ip split-horizon router rip network 10.0.0.0
When RIP determines that a summary address is required in the RIP database, a summary entry is created in the RIP routing database. As long as there are child routes for a summary address, the address remains in the routing database. When the last child route is removed, the summary entry also is removed from the database. This method of handling database entries reduces the number of entries in the database because each child route is not listed in an entry, and the aggregate entry itself is removed when there are no longer any valid child routes for it.
RIP version 2 route summarization requires that the lowest metric of the "best route" of an aggregated entry, or the lowest metric of all current child routes, be advertised. The best metric for aggregated summarized routes is calculated at route initialization or when there are metric modifications of specific routes at advertisement time, and not at the time the aggregated routes are advertised.
Supernet advertisement (advertising any network prefix less than its classful major network) is not allowed in RIP route summarization, other than advertising a supernet learned in the routing tables. Supernets learned on any interface that is subject to configuration are still learned. For example, the following summarization is invalid:
interface E1 .. ip summary-address rip 10.0.0.0 252.0.0.0 (invalid supernet summarization)
Each route summarization on an interface must have a unique major net, even if the subnet mask is unique. For example, the following is not permitted:
int E1 ... ip summary-address rip 10.1.0.0 255.255.0.0 ip summary-address rip 10.2.0.0 255.255.0.0 (or different mask)
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NoteThe ip summary-address eigrp command uses other options that are not applicable to RIP. Do not confuse EIGRP summary-address with the new RIP command, ip summary-address rip. |
The ip summary-address rip command causes the router to summarize a given set of routes learned via RIP version 2 or redistributed into RIP version 2. Host routes are especially applicable for summarization. To configure IP summary addressing, use the following commands in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | Router(config)#rip | Enables RIP routing. |
Step2 | Router(config)#interface e1 | Enters interface configuration mode. |
Step3 | Router(config-if)#ip summary-address rip ip_address ip_network_mask | Specifies the IP address and network mask that identify the routes to be summarized. |
See the "Route Summarization Examples" section at the end of this chapter for examples of using split horizon.
You can verify which routes are summarized for an interface using the show ip protocols command. The following example shows potential summarizations and the associated interface summary-address/network mask for Ethernet interface 2:
router#show ip protocols
Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 8 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: rip
Default version control: send version 2, receive version 2
Interface Send Recv Triggered RIP Key-chain
Ethernet2 2 2
Ethernet3 2 2
Ethernet4 2 2
Ethernet5 2 2
Automatic network summarization is not in effect
Address Summarization:
12.11.0.0/16 for Ethernet2
router#show ip rip database 11.0.0.0/8 auto-summary 11.11.11.0/24 directly connected, Ethernet2 12.0.0.0/8 auto-summary 12.11.0.0/16 int-summary ^^^^^^^^^^^^^^^^^^^^^^^^^^^ 12.11.10.0/24 directly connected, Ethernet3 12.11.11.0/24 directly connected, Ethernet4
12.11.12.0/24 directly connected, Ethernet5
RIP Version 2 supports automatic route summarization by default. The software summarizes subprefixes to the classful network boundary when crossing classful network boundaries.
If you have disconnected subnets, disable automatic route summarization to advertise the subnets. When route summarization is disabled, the software sends subnet and host routing information across classful network boundaries. To disable automatic summarization, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(confi g-router)# no auto-summary | Disables automatic summarization. |
It is possible to run IGRP and RIP concurrently. The IGRP information will override the RIP information by default because of the administrative distance of IGRP.
However, running IGRP and RIP concurrently does not work well when the network topology changes. Because IGRP and RIP have different update timers, and because they require different amounts of time to propagate routing updates, one part of the network will end up accepting and using IGRP routes and another part will end up accepting and using RIP routes. Running IGRP and RIP concurrently will result in routing loops. Even though these loops do not exist for very long, the time to live (TTL) value will quickly reach zero, and ICMP will send a "TTL exceeded" message. This message will cause most applications to stop attempting network connections.
By default, the software validates the source IP address of incoming RIP routing updates. If that source address is not valid, the software discards the routing update.
You might want to disable this feature if you have a router that is "off network" and you want to receive its updates. However, disabling this feature is not recommended under normal circumstances. To disable the default function that validates the source IP addresses of incoming routing updates, use the following command in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)# no validate-update-source | Disables the validation of the source IP address of incoming RIP routing updates. |
If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by every secondary address. One routing update is sourced per network number unless split horizon is disabled.
To enable or disable split horizon, use one of the following commands beginning in interface configuration mode:
| Command | Purposes |
|---|---|
Router(config-if)# ip split-horizon | Enables split horizon. |
Router(config-if)# no ip split-horizon | Disables split horizon. |
Split horizon for Frame Relay and SMDS encapsulation is disabled by default. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations. For all other encapsulations, split horizon is enabled by default.
See the "Route Summarization Examples" section at the end of this chapter for examples of using split horizon.
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NoteIn general, changing the state of the default is not recommended unless you are certain that your application requires making a change in order to advertise routes properly. Remember: If split horizon is disabled on a serial interface (and that interface is attached to a packet-switched network), you must disable split horizon for all routers in any relevant multicast groups on that network. |
By default, the software adds no delay between packets in a multiple-packet RIP update being sent. If you have a high-end router sending to a low-speed router, you might want to add such interpacket delay to RIP updates, in the range of 8 to 50 milliseconds. To do so, use the following command, beginning in router configuration mode:
| Command | Purpose |
|---|---|
Router(config-router)# output-delay delay | Add interpacket delay for RIP updates sent. |
Routers are used on connection-oriented networks to allow potential connectivity to many remote destinations. Circuits on the WAN are established on demand and are relinquished when the traffic subsides. Depending on the application, the connection between any two sites for user data could be short and relatively infrequent.
There are two problems using RIP to connect to a WAN:
To overcome these limitations, triggered extensions to RIP cause RIP to send information on the WAN only when there has been an update to the routing database. Periodic update packets are suppressed over the interface on which this feature is enabled. RIP routing traffic is reduced on point-to-point, serial interfaces. Therefore, you can save money on an on-demand circuit for which you are charged for usage. Triggered extensions to RIP partially support RFC 2091, Triggered Extensions to RIP to Support Demand Circuits.
To enable triggered extensions to RIP, use the following commands in global configuration mode:
| Command | Purpose |
|---|---|
interface serial number | Configures a serial interface. |
ip rip triggered | Enables triggered extensions to RIP. |
To display the contents of the RIP private database, use the following command in EXEC mode:
| Command | Purpose |
|---|---|
show ip rip database [prefix mask] | Displays the contents of the RIP private database. |
This section provides the following RIP configuration examples:
A correct (Example 1: Correct Configuration) and an incorrect (Example 2: Incorrect Configuration) configuration example of route summarization are provided.
Example 1: Correct Configuration
The following example shows how the ip summary-address rip command works with autosummary addressing in RIP. In the example, the major network is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0, so that 10.2.0.0 is advertised out interface e1 and 10.0.0.0 is not advertised.
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NoteIf split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip command) are advertised. |
router rip router int e1 ip address 10.1.1.1 255.255.255.0 (config-if)#ip summary-address rip 10.2.0.0 255.255.0.0 no ip split-horizon router rip network 10.0.0.0 neighbor 2.2.2.2 peer-group mygroup
Example 2: Incorrect Configuration
The following example shows an illegal use of the ip summary-address rip command, because both addresses to be summarized have the same major network. Each route summarization on an interface must have a unique major network, whether or not the addresses have unique address masks.
int E1 ... ip summary-address rip 10.1.0.0 255.255.0.0 ip summary-address rip 10.2.0.0 255.255.255.0
Two examples of configuring split horizon are provided.
Example 1
The following configuration shows a simple example of disabling split horizon on a serial link. In this example, the serial link is connected to an X.25 network.
interfaceserial0 encapsulationx25 noipsplit-horizon
Example 2
In the next example, Figure 18 illustrates a typical situation in which the noipsplit-horizon interface configuration command would be useful. This figure depicts two IP subnets that are both accessible via a serial interface on Router C (connected to Frame Relay network). In this example, the serial interface on Router C accommodates one of the subnets via the assignment of a secondary IP address.
The Ethernet interfaces for Router A, Router B, and Router C (connected to IP networks 12.13.50.0, 10.20.40.0, and 20.155.120.0, respectively, all have split horizon enabled by default, while the serial interfaces connected to networks 128.125.1.0 and 131.108.1.0 all have split horizon disabled by default. The partial interface configuration specifications for each router that follow Figure 18 illustrate that the ipsplit-horizon command is not explicitly configured under normal conditions for any of the interfaces.
In this example, split horizon must be disabled in order for network 128.125.0.0 to be advertised into network 131.108.0.0, and vice versa. These subnets overlap at Router C, interface S0. If split horizon were enabled on serial interface S0, it would not advertise a route back into the Frame Relay network for either of these networks.
Configuration for Router A
interface ethernet 1 ip address 12.13.50.1 ! interface serial 1 ip address 128.125.1.2 encapsulation frame-relay no ip split-horizon
Configuration for Router B
interface ethernet 2 ip address 20.155.120.1 ! interface serial 2 ip address 131.108.1.2 encapsulation frame-relay no ip split-horizon
Configuration for Router C
interface ethernet 0 ip address 10.20.40.1 ! interface serial 0 ip address 128.124.1.1 ip address 131.108.1.1 secondary encapsulation frame-relay
Posted: Wed Jul 26 18:42:44 PDT 2000
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