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4 years ago Enabling RIP and Configuring RIP Parameters
SUMMARY STEPS
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1. enable
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2. configure terminal
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3. router rip
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4. network ip-address
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5. neighbor ip-address
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6. offset-list [access-list-number | access-list-name] {in | *out*} offset [interface-type interface-number]
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7. timers basic update invalid holddown flush [sleeptime]
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8. end
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
| Step 1 | enable Example:Device> enable |
Enables privileged EXEC mode.Enter your password if prompted. |
| Step 2 | configure terminal Example:Device# configure terminal |
Enters global configuration mode. |
| Step 3 | router rip Example:Device(config)# router rip |
Enables a RIP routing process and enters router configuration mode. |
| Step 4 | network ip-address Example:Device(config-router)# network 10.1.1.0 |
Associates a network with a RIP routing process. |
| Step 5 | neighbor ip-address Example:Device(config-router)# neighbor 10.1.1.2 |
Defines a neighboring device with which to exchange routing information. |
| Step 6 | offset-list [access-list-number | access-list-name] {in | *out*} offset [interface-type interface-number] Example:Device(config-router)# offset-list 98 in 1 Ethernet 1/0 |
(Optional) Applies an offset list to routing metrics. |
| Step 7 | timers basic update invalid holddown flush [sleeptime] Example:Device(config-router)# timers basic 1 2 3 4 |
(Optional) Adjusts routing protocol timers. |
| Step 8 | end Example:Device(config-router)# end |
Exits router configuration mode and returns to privileged EXEC mode. |
RIP Overview
The Routing Information Protocol (RIP) uses broadcast UDP data packets to exchange routing information. Cisco software sends routing information updates every 30 seconds, which is termed advertising. If a device does not receive an update from another device for 180 seconds or more, the receiving device marks the routes served by the nonupdating device as unusable. If there is still no update after 240 seconds, the device removes all routing table entries for the nonupdating device.
A device that is running RIP can receive a default network via an update from another device that is running RIP, or the device can source the default network using RIP. In both cases, the default network is advertised through RIP to other RIP neighbors.
The Cisco implementation of RIP Version 2 (RIPv2) supports plain text and message digest algorithm 5 (MD5) authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).
RIP Routing Updates
The Routing Information Protocol (RIP) sends routing-update messages at regular intervals and when the network topology changes. When a device receives a RIP routing update that includes changes to an entry, the device 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 devices maintain only the best route (the route with the lowest metric value) to a destination. After updating its routing table, the device immediately begins transmitting RIP routing updates to inform other network devices of the change. These updates are sent independently of the regularly scheduled updates that RIP devices send.
RIP Routing Metric
The Routing Information Protocol (RIP) uses a single routing metric to measure the distance between the source and the destination network. Each hop in a path from the source to the destination is assigned a hop-count value, which is typically 1. When a device receives a routing update that contains a new or changed destination network entry, the device 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 an interface network is not specified in the routing table, it will not be advertised in any RIP update.
Authentication in RIP
The Cisco implementation of the Routing Information Protocol (RIP) Version 2 (RIPv2) supports authentication, key management, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).
By default, the software receives RIP Version 1 (RIPv1) and RIPv2 packets, but sends only RIPv1 packets. You can configure the software to receive and send only RIPv1 packets. Alternatively, you can configure the software to receive and send only RIPv2 packets. To override the default behavior, you can configure the RIP version that an interface sends. Similarly, you can also control how packets received from an interface are processed.
RIPv1 does not support authentication. If you are sending and receiving RIP v2 packets, you can enable RIP authentication on an interface.
The key chain determines the set of keys that can be used on the interface. Authentication, including default authentication, is performed on that interface only if a key chain is configured. For more information on key chains and their configuration, see the “Managing Authentication Keys” section in the “Configuring IP Routing Protocol-Independent Features” chapter in the Cisco IOS IP Routing: Protocol-Independent Configuration Guide.
Cisco supports two modes of authentication on an interface on which RIP is enabled: plain-text authentication and message digest algorithm 5 (MD5) authentication. Plain-text authentication is the default authentication in every RIPv2 packet.
Exchange of Routing Information
Routing Information Protocol (RIP) is normally a broadcast protocol, and for RIP routing updates to reach nonbroadcast networks, you must configure the Cisco 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.
You can use an offset list to increase 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.
Routing protocols use several timers that determine variables such 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 about 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
You can adjust the IP routing support in the Cisco software to enable faster convergence of various IP routing algorithms, and hence, cause quicker fallback to redundant devices. 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 timers that explicitly apply to that address family (or Virtual Routing and Forwarding [VRF]) instance). The timers-basic command must be specified for an address family or the system defaults for the timers-basic command are used regardless of the timer that 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 the timers are explicitly changed using the timers-basic command.
RIP Route Summarization
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:
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.
Split Horizon Mechanism
Normally, devices 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. The split horizon mechanism blocks information about routes from being advertised by a device out of any interface from which that information originated. This behavior usually optimizes communications among multiple devices, particularly when links are broken. However, with nonbroadcast networks, such as Frame Relay and the Switched Multimegabit Digital System (SMDS), situations can arise for which this behavior is less than ideal. In such situations, you may want to disable split horizon with the Routing Information Protocol (RIP).
If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by the secondary address. If split horizon is enabled, one routing update is sourced per network number.
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.
Interpacket Delay for RIP Updates
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.
RIP Optimization over WAN Circuits
Devices 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.
Source IP Addresses of RIP Routing Updates
By default, the Cisco software validates the source IP address of incoming Routing Information Protocol (RIP) routing updates. If the source address is not valid, the software discards the routing update. You must disable this functionality if you want to receive updates from a device that is not part of this network. However, disabling this functionality is not recommended under normal circumstances.
Neighbor Router Authentication
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:
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.
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.
IP-RIP Delay Start Overview
The IP-RIP Delay Start feature is used on Cisco devices to delay the initiation of Routing Information Protocol Version 2 (RIPv2) neighbor sessions until the network connectivity between the neighbor devices is fully operational, thereby ensuring that the sequence number of the first message digest algorithm 5 (MD5) packet that the device sends to the non-Cisco neighbor device is 0. The default behavior for a device configured to establish RIPv2 neighbor sessions with a neighbor device 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 device 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 device 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 device after the Frame Relay circuits become active will be greater than 0. Some non-Cisco devices will not allow an MD5-authenticated RIPv2 neighbor session to start when the sequence number of the first MD5 packet received from the other device 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 respect to packet loss. RFC 2082 suggests that devices 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.
The IP-RIP Delay Start feature is supported over other interface types such as Fast Ethernet and Gigabit Ethernet.
Cisco devices allow an MD5-authenticated RIPv2 neighbor session to start when the sequence number of the first MD5 packet received from the other device is greater than 0. If you are using only Cisco devices in your network, you do not need to use the IP-RIP Delay Start feature.
Offset-list
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.
Timers
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:
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.