Routing Information Protocol (RIP)

Routing Information Protocol, or RIP , is a distance-vector protocol that uses hop-count metrics to get from one router to the next . RIP is limited to 15 hop counts. RIP is mostly used in the routing of global Internet and is an Interior Gateway Protocol ( IGP ) . What this means is that RIP performs routing in a single autonomous system. On the other side is the Exterior Gateway Protocol ( EGP ) , which is used to perform routing updates on different autonomous systems.

RIPv2 is another version of RIP. The main difference between the two is that RIP is used only for classful routing and RIPv2 is used for classless routing. RIP sends routing updates at regular intervals and when a routing table has changed. Routers running the RIP protocol maintain the best route because it is the shortest or the one with fewest hops. After the router updates its routing table, the router sends information about the routes it knows out to all its interfaces, informing all of its neighboring routers. This process is done independently of the other updates.

RIP uses a single routing metric. RIP measures the distance between the source and destination network. When a router receives an update that has new or changed information in it, the router will add a metric to that source address, which is the IP address of the sender. In the path from source to destination, each interface to the destination would be defined as one additional hop in the hop count, with a maximum hop count of 15. The next hop would be 16, and then it would be sent back as an unreachable network or destination address.

By implementing a hop count limit, the RIP protocol prevents routing loops from an infinite cycle. In order for RIP to adjust for rapid changes in the network, it implements the split horizon and hold-down timers to prevent incorrect routing information from being propagated. The RIP hop count also helps prevent routing loops in an infinite circle. RIP also uses timers to help regulate its performance. These timers include

• The routing-update timer , usually set to 30 seconds, clocks the periodic update intervals. To prevent collisions, the router puts in a random number of seconds each time the timer is reset.

• The route-timeout ; when this timeout expires , the route is determined to be unreachable.

• The route-flush timer ; an unreachable route is maintained in the routing table until this timeout expires. The update timer is 30 seconds, the invalid timer is 90 seconds, the hold-down timer is 100 seconds, and the flush timer is 270 seconds.

Troubleshooting RIP

Two versions of RIP exist, the distinction between them is that RIP1 had limited use and scalability. RIP2 is almost the same, but the new version is able to support Classless Interdomain Routing (CIDR), route summarization, and variable-length subnet masks (VLSMs). The following example shows a few commands used in troubleshooting RIP, as well as the output. The show ip route command displays the routing table of the networks and how they are reaching other networks.

 DCSRTR#  DCSRTR#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,  B - BGPD - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o - ODR Gateway of last resort is not set      172.16.0.0/24 is subnetted, 6 subnets R       currentip  [120/2] via 172.16.20.2, 00:00:12, Serial0  R       10.10.20.0  [120/2] via 172.16.20.2, 00:00:12, Serial0   R       10.10.20.0  [120/2] via 172.16.20.2, 00:00:12, Serial0   R       172.16.30.0  [120/2] via 172.16.20.2, 00:00:12, Serial0   C       172.16.30.0 is directly connected, Serial0   C       10.10.20.0 is directly connected, Ethernet0  DCSRTR# 

This is one of the best commands for learning the routes the router knows about and discovering how the router learned the routes. In the highlighted commands above, notice the letter preceding the route information. This letter indicates how the route was learned (the letters and their meaning are listed in the first lines of the example). The item Serial0 is the interface through which the data exits to get to the network specified.

When using the show ip protocol command, you will be able to identify the protocol or protocols running on the router, as the following example demonstrates :

 DCSRTR#show ip protocol Routing Protocol is "rip"   Sending updates every 30 seconds, next due in 13 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 1, receive version 1     Interface             Send  Recv  Triggered RIP  Key-chain     FastEthernet0/0       1     1     Serial0/0.1           1     1     FastEthernet0/1       1     1   Automatic network summarization is in effect   Maximum path: 4   Routing for Networks:     10.0.0.0     64.0.0.0     66.0.0.0   Routing Information Sources:     Gateway         Distance      Last Update     10.1.1.1             120      00:00:14     10.1.2.55            120      00:00:19   Distance: (default is 120) DCSRTR# 

The above shows the status and protocols running on each interface. Notice that the beginning of the output shows how often updates are sent to RIP enabled routers in the network.