Configuring Routing Information Protocol

Routing Information Protocol uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. 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.

A router that is running RIP can receive a default network via an update from another router that is running RIP, or the router can source (generate) the default network itself with RIP. In both cases, the default network is advertised through RIP to other RIP neighbors.

The Cisco implementation of RIP Version 2 supports plain text and Message Digest 5 (MD5) authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).

RIP sends routing-update messages at regular intervals and when the network topology changes. When a router receives a RIP routing update that includes changes to an entry, the router updates its routing table to reflect the new route. The metric value for the path is increased by 1, and the sender is indicated as the next hop. RIP routers maintain only the best route (the route with the lowest metric value) to a destination. After updating its routing table, the router immediately begins transmitting RIP routing updates to inform other network routers of the change. These updates are sent independently of the regularly scheduled updates that RIP routers send.

RIP uses a single routing metric (hop count) to measure the distance between the source and a destination network. Each hop in a path from source to destination is assigned a hop count value, which is typically 1. When a router receives a routing update that contains a new or changed destination network entry, the router adds 1 to the metric value indicated in the update and enters the network in the routing table. The IP address of the sender is used as the next hop. 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 authentication, key management, route summarization, CIDR, and VLSMs. For more information about managing authentication keys see the "Managing Authentication Keys" section of the "Configuring IP Routing Protocol-Independent Feature" module.

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 override the default behavior, you can configure which RIP version an interface sends. Similarly, you can also control how packets received from an interface are processed.

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 "Managing Authentication Keys" in the "Configuring IP Routing Protocol-Independent Features" module.

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 Version 2 packet is plain text authentication.

Note	Do 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.

RIP is normally a broadcast protocol, and 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 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 router configuration command. See the discussion on filtering in the "Filter Routing Information" section in the "Configuring IP Routing Protocol-Independent Features" module.

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:

Routing protocols use several timers that determine such variables as the frequency of routing updates, the length of time before a route becomes invalid, and other parameters. You can adjust these timers to tune routing protocol performance to better suit your internetwork needs. You can make the following timer adjustments:

• The rate (time in seconds between updates) at which routing updates are sent
• The interval of time (in seconds) after which a route is declared invalid
• The interval (in seconds) during which routing information regarding better paths is suppressed
• The amount of time (in seconds) that must pass before a route is removed from the routing table
• The amount of time for which routing updates will be postponed

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

In addition, an address family can have explicitly specified timers that apply to that address-family (or VRF) only. The timers basic command must be specified for an address family or the system defaults for the timers basic command are used regardless of what is configured for RIP routing. The VRF does not inherit the timer values from the base RIP configuration. The VRF will always use the system default timers unless explicitly changed using the timers basic command.

See the "Address Family Timers Example" section at the end of this chapter for examples of adjusting timers for an address family (VRF).

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 for the following reasons:

• The summarized routes in the RIP database are processed first.
• Any associated child routes that are included in a summarized route are skipped as RIP looks through the routing database, reducing the processing time required. Cisco routers can summarize routes in two ways:
• Automatically, by summarizing subprefixes to the classful network boundary when crossing classful network boundaries (automatic summary).

Note	Automatic summary is enabled by default.

• As specifically configured, advertising a summarized local IP address pool on the specified interface (on a network access server) so that the address pool can be provided to dialup clients.

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.

The ip summary-address rip routerconfiguration 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.

See the "Route Summarization Example" 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 EXEC command. You can check summary address entries in the RIP database. These entries will appear in the database only if relevant child routes are being summarized. To display summary address entries in the RIP routing database entries if there are relevant routes being summarized based upon a summary address, use the show ip rip database command in EXEC mode. When the last child route for a summary address becomes invalid, the summary address is also removed from the routing table.

Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and Switched Multimegabit Digital System [SMDS]), situations can arise for which this behavior is less than ideal. For these situations, you may want to disable split horizon with RIP.

If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by the secondary address. One routing update is sourced per network number unless split horizon is disabled.

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.

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.

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.

You can prevent your router from receiving fraudulent route updates by configuring neighbor router authentication. When configured, neighbor authentication occurs whenever routing updates are exchanged between neighbor routers. This authentication ensures that a router receives reliable routing information from a trusted source.

Without neighbor authentication, unauthorized or deliberately malicious routing updates could compromise the security of your network traffic. A security compromise could occur if an unfriendly party diverts or analyzes your network traffic. For example, an unauthorized router could send a fictitious routing update to convince your router to send traffic to an incorrect destination. This diverted traffic could be analyzed to learn confidential information about your organization or merely used to disrupt your organization's ability to effectively communicate using the network. Neighbor authentication prevents any such fraudulent route updates from being received by your router.

When neighbor authentication has been configured on a router, the router authenticates the source of each routing update packet that it receives. This is accomplished by the exchange of an authenticating key (sometimes referred to as a password) that is known to both the sending and the receiving router.

There are two types of neighbor authentication used: plain text authentication and Message Digest Algorithm Version 5 (MD5) authentication. Both forms work in the same way, with the exception that MD5 sends a "message digest" instead of the authenticating key itself. The message digest is created using the key and a message, but the key itself is not sent, preventing it from being read while it is being transmitted. Plain text authentication sends the authenticating key itself over the wire.

In plain text authentication, each participating neighbor router must share an authenticating key. This key is specified at each router during configuration. Multiple keys can be specified with some protocols; each key must then be identified by a key number.

In general, when a routing update is sent, the following authentication sequence occurs:

1. A router sends a routing update with a key and the corresponding key number to the neighbor router. In protocols that can have only one key, the key number is always zero. The receiving (neighbor) router checks the received key against the same key stored in its own memory.
2. If the two keys match, the receiving router accepts the routing update packet. If the two keys do not match, the routing update packet is rejected.

MD5 authentication works similarly to plain text authentication, except that the key is never sent over the wire. Instead, the router uses the MD5 algorithm to produce a "message digest" of the key (also called a "hash"). The message digest is then sent instead of the key itself. This ensures that nobody can eavesdrop on the line and learn keys during transmission.

Another form of neighbor router authentication is to configure key management using key chains. When you configure a key chain, you specify a series of keys with lifetimes, and the Cisco IOS software rotates through each of these keys. This decreases the likelihood that keys will be compromised. To find complete configuration information for key chains, refer to the "Managing Authentication Keys" section in the Configuring IP Routing Protocol-Independent Features module of the Cisco IOS IP Routing: Protocol-Independent Configuration Guide.

The IP-RIP Delay Start feature is used on Cisco routers to delay the initiation of RIPv2 neighbor sessions until the network connectivity between the neighbor routers is fully operational, thereby ensuring that the sequence number of the first MD5 packet that the router sends to the non-Cisco neighbor router is 0. The default behavior for a router configured to establish RIPv2 neighbor sessions with a neighbor router using MD5 authentication is to start sending MD5 packets when the physical interface is up.

The IP-RIP Delay Start feature is often used when a Cisco router is configured to establish a RIPv2 neighbor relationship using MD5 authentication with a non-Cisco device over a Frame Relay network. When RIPv2 neighbors are connected over Frame Relay, it is possible for the serial interface connected to the Frame Relay network to be up while the underlying Frame Relay circuits are not yet ready to transmit and receive data. When a serial interface is up and the Frame Relay circuits are not yet operational, any MD5 packets that the router attempts to transmit over the serial interface are dropped. When MD5 packets are dropped because the Frame Relay circuits over which the packets need to be transmitted are not yet operational, the sequence number of the first MD5 packet received by the neighbor router after the Frame Relay circuits become active will be greater than 0. Some non-Cisco routers will not allow an MD5-authenticated RIPv2 neighbor session to start when the sequence number of the first MD5 packet received from the other router is greater than 0.

The differences in vendor implementations of MD5 authentication for RIPv2 are probably a result of the ambiguity of the relevant RFC (RFC 2082) with regards to packet loss. RFC 2082 suggests that routers should be ready to accept either a sequence number of 0 or a sequence number higher than the last sequence number received. For more information about MD5 message reception for RIPv2, see section 3.2.2 of RFC 2082 at the following url: http://www.ietf.org/rfc/rfc2082.txt .

An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via RIP. This is done to provide a local mechanism for increasing the value of routing metrics. Optionally, you can limit the offset list with either an access list or an interface.

Routing protocols use several timers that determine such variables as the frequency of routing updates, the length of time before a route becomes invalid, and other parameters. You can adjust these timers to tune routing protocol performance to better suit your internetwork needs. You can make the following timer adjustments:

• The rate (time in seconds between updates) at which routing updates are sent
• The interval of time (in seconds) after which a route is declared invalid
• The interval (in seconds) during which routing information regarding better paths is suppressed
• The amount of time (in seconds) that must pass before a route is removed from the routing table
• The amount of time for which routing updates will be postponed

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.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    router rip

4.    network ip-address

5.    neighbor ip-address

6.    offset-list [access-list-number | access-list-name] {in | out} offset[interface-type interface-number]

7.    timers basic update invalid holddown flush [sleeptime]

8.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router rip Example: Router(config)# router rip	Enables a RIP routing process and enters router configuration mode.
Step 4	network ip-address Example: Router(config-router)# network 10.1.1.0	Associates a network with a RIP routing process.
Step 5	neighbor ip-address Example: Router(config-router)# neighbor 1.1.1.2	Defines a neighboring router with which to exchange routing information.
Step 6	offset-list [access-list-number | access-list-name] {in | out} offset[interface-type interface-number] Example: Router(config-router)# offset-list 98 in 1 Ethernet 1/0	(Optional) Applies an offset to routing metrics.
Step 7	timers basic update invalid holddown flush [sleeptime] Example: Router(config-router)# timers basic 1 2 3 4	(Optional) Adjusts routing protocol timers.
Step 8	end Example: Router(config-router)# end	Exits router configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    router rip

4.    version {1 | 2}

5.    exit

6.    interface type number

7.    ip rip send version [1] [2]

8.    ip rip receive version [1] [2]

9.    ip rip authentication key-chain name-of-chain

10.    ip rip authentication mode {text | md5}

11.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router rip Example: Router(config)# router rip	Enters router configuration mode.
Step 4	version {1 | 2} Example: Router(config-router)# version 1	Configures an interface to send only RIP Version 1 packets.
Step 5	exit Example: Router(config-router)# exit	Exits the router configuration mode and enters the global configuration mode.
Step 6	interface type number Example: Router(config)# interface Ethernet 3/0	Enters interface configuration mode.
Step 7	ip rip send version [1] [2] Example: Router(config-if)# ip rip send version 1	Configures an interface to send only RIP Version 1 packets.
Step 8	ip rip receive version [1] [2] Example: Router(config-if)# ip rip receive version 1	Configures an interface to accept only RIP Version 1 packets.
Step 9	ip rip authentication key-chain name-of-chain Example: Router(config-if)# ip rip authentication key-chain chainname	Enables RIP authentication.
Step 10	ip rip authentication mode {text | md5} Example: Router(config-if)# ip rip authentication mode md5	Configures the interface to use MD5 digest authentication (or let it default to plain text authentication).
Step 11	end Example: Router(config-if)# end	Exits interface configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    ip summary-address rip ip-address network-mask

5.    exit

6.    router rip

7.    no auto-summary

8.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface Ethernet 3/0	Enters the interface configuration mode.
Step 4	ip summary-address rip ip-address network-mask Example: Router(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0	Specifies the IP address and network mask that identify the routes to be summarized.
Step 5	exit Example: Router(config-if)# exit	Exits the interface configuration mode.
Step 6	router rip Example: Router(config)# router rip	Enters the router configuration mode.
Step 7	no auto-summary Example: Router(config-router)# no auto-summary	Used in router configuration mode, disables automatic summarization.
Step 8	end Example: Router(config-router)# end	Exits router configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    ip split-horizon

5.    no ip split-horizon

6.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface Ethernet 3/0	Enters interface configuration mode.
Step 4	ip split-horizon Example: Router(config-if)# ip split-horizon	Enables split horizon.
Step 5	no ip split-horizon Example: Router(config-if)# no ip split-horizon	Disables split horizon.
Step 6	end Example: Router(config-if)# end	Exits interface configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    ip split-horizon

5.    exit

6.    router rip

7.    no validate-update-source

8.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface Ethernet 3/0	Enters interface configuration mode.
Step 4	ip split-horizon Example: Router(config-if)# ip split-horizon	Enables split horizon.
Step 5	exit Example: Router(config-if)# exit	Exits interface configuration mode.
Step 6	router rip Example: Router(config)# router rip	Enters router configuration mode.
Step 7	no validate-update-source Example: Router(config-router)# no validate-update-source	Disables the validation of the source IP address of incoming RIP routing updates.
Step 8	end Example: Router(config-router)# end	Exits router configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    exit

5.    router rip

6.    output-delay milliseconds

7.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface Ethernet 3/0	Enters interface configuration mode.
Step 4	exit Example: Router(config-if)# exit	Exits interface configuration mode.
Step 5	router rip Example: Router(config)# router rip	Enters router configuration mode.
Step 6	output-delay milliseconds Example: Router(config-router)# output-delay 8	Configures interpacket delay for outbound RIP updates.
Step 7	end Example: Router(config-router)# end	Exits router configuration mode and returns to privileged EXEC mode.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface serial controller-number

4.    ip rip triggered

5.    end

6.    show ip rip database [prefix mask]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface serial controller-number Example: Router(config)# interface serial3/0	Configures a serial interface.
Step 4	ip rip triggered Example: Router(config-if)# ip rip triggered	Enables triggered extensions to RIP.
Step 5	end Example: Router(config-if)# end	Returns to privileged EXEC mode.
Step 6	show ip rip database [prefix mask] Example: Router# show ip rip database	Displays the contents of the RIP private database.

The tasks in this section explain how to configure a router to use the IP-RIP Delay Start feature on a Frame Relay interface.

Timesaver

Cisco routers allow an MD5-authenticated RIPv2 neighbor session to start when the sequence number of the first MD5 packet received from the other router is greater than 0. If you are using only Cisco routers in your network, you do not need to use the IP-RIP Delay Start feature.

• Prerequisites
  
• Restrictions
  
• Configuring RIPv2
  
• Configuring Frame Relay on a Serial Subinterface
  
• Configuring IP with MD5 Authentication for RIPv2 and IP-RIP Delay on a Frame Relay Subinterface