Networking

Cisco administration 101: Know the basics about RIPv2

When it comes to internal routing protocols, Routing Information Protocol version 2 (RIPv2) is one of the most common routing protocols in use today. David Davis reviews some RIPv2 fundamentals every network administrator needs to know and tells you how to configure this protocol.

When it comes to internal routing protocols, Routing Information Protocol version 2 (RIPv2) is one of the most common routing protocols in use today. In addition, RIPv2 is one of the easiest protocols to configure.

Meant for smaller networks, RIPv2 does have a number of limitations. However, version 2 boasts some critical improvements over the pervious version. RIPv2 is a valuable tool that provides quick and simple IP routing, and it should be in every network administrator's toolbox.

This week, I want to review some common things every network administrator needs to know about RIPv2. We'll start with RIP fundamentals and then discuss how to configure this protocol.

What type of routing protocol is RIP?

Based on RFC 1388, 1723, and 2453, RIP is a distance-vector routing protocol. Its primarily limitation is that it can't support a network that has more than 15 hops. RIP assumes that anything more than 15 hops is infinity, so it considers the route invalid. Despite this limitation, RIP works great for basic route communications between devices.

Its other benefit is that it's so widespread. Many routers come with RIP by default, even many small office routers. In addition, many firewalls support RIP, but not OSPF or EIGRP.

Here are some important RIP facts:

  • RIP's administrative distance is 120 for both RIPv1 and RIPv2.
  • RIPv2 sends routing updates via multicast address 224.0.0.9.
  • Cisco routers don't enable RIPv2 by default. To use RIPv2, you must use the ver 2 command in RIP Router Configuration Mode.
  • RIP automatically summarizes routing updates. You can disable this by using the no auto-summary command.
  • RIP uses hop count as its metric.

How does RIP work?

With RIP, a router sends its full routing table to all other connected routers every 30 seconds. Triggered updates can also occur if a router goes down before the 30-second timer has expired.

RIP performs "routing by rumor" and is more prone to loops than other routing protocols. That's because a RIP router sends its entire routing table to every other router. All other routers do the same, and because there's no real neighbor relationship or calculation of routes, the routers have little firsthand knowledge of available networks.

Because of this, it can turn out like the old telephone game you used to play as a kid. At the end of the game, the resulting message is usually quite different from the original. This type of problem can affect RIP because, unlike OSPF (a link-state protocol), RIP routers don't calculate their own routes; they must trust their neighbor's routes.

What's new in RIP version 2?

RIPv2 boasts the following enhancements:

  • Support for variable length subnet masks (VLSM) (Because of this, RIP doesn't assume that all networks are classful.)
  • Multicast routing updates
  • Authentication with an encrypted password for routing updates

How do I configure RIP?

Unlike some other routing protocols, RIP doesn't use any kind of autonomous system numbers to identify areas of the network that are under a single administrative domain. Because of this, entering RIP Routing Configuration Mode is very simple. Here's an example:

Router(config)# router rip
Router(config-router)#

Once in RIP Configuration Mode, you need to specify that you're using RIPv2. To do so, enter the ver 2command at the config-router prompt. Here's an example:

Router(config-router)# ver 2

The most common network administrator's task is to specify which networks RIP will advertise and listen on. You can define this using the network command. Here's an example:

Router(config-router)# network 10.0.0.0

The 10.0.0.0 parameter is the network IP address covering any interfaces that you want RIP to perform the following three functions on:

  • Send out routing updates.
  • Listen for incoming routing updates.
  • Include the network that the interface is on in any routing updates sent from the router.

If you would prefer not to send out advertisements on an interface, use the passive interface command.

How do I see what's going on with RIP?

After you've configured RIP, you need to know how to check its status. Here's a list of some common RIP commands.

show ip route

While this command shows the entire routing table, you can use it to see which RIP routes are in the routing table with the show ip route r command.

show ip protocols

Here's an example:

Router-2# show ip protocols 
Routing Protocol is "rip"
  Sending updates every 30 seconds, next due in 25 seconds
  Invalid after 180 seconds, hold down 180, flushed after 240
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Redistributing: rip
  Default version control: send version 2, receive version 2
    Interface             Send  Recv  Triggered RIP  Key-chain
    Serial0/1             2     2                                    
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    1.0.0.0
  Routing Information Sources:
    Gateway         Distance

show running-config

Here's an example:

Router-2# show running-config | begin router rip
router rip
 version 2
 network 1.0.0.0

debug ip rip

You can use this command to see what's going on deep within RIP.

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David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.

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