RIP is a distance vector routing protocol. As all routing protocols are using a metric, the one used by RIP is the hop count. The shortest the hop count, the more likely RIP will chose that path to reach the destination network. However, RIP’s maximum hop count is 15(CCNA exam question). Any route with a hop count greater than 15 is considered unreachable. Routing updates are broadcasted by default every 30 seconds. They are sent in UDP packets with both source and destination port numbers set to 520. Because the maximum datagram size is only 504 bytes, there’s a maximum of 25 routes that can be announced in a single packet. Having more routes will cause the router to send more packets.
There are only two message types used by RIP. Request message and Response message. The names are as descriptive as they can be. When a RIP enabled router interface comes up, it sends out a Request message. The other RIP enabled routers in the network are responding with Response messages. When the first router receives the Response messages, it installs the new received routes in it’s routing table. If the router already has a route in it’s table but it gets one with a better hop count, the old route is replaced. After that, the router sends its own routing table to its neighbors.
The default Administrative Distance for RIP is 120(another CCNA exam question). In routing, the AD is used as a reference of trustworthiness. The lower the value, the higher the priority of that route. For example, if you have a route received through RIP, which has an AD of 120, and you have the same route received from a higher priority protocol like OSPF, which has a default AD of 110, OSPF will be in charge to route the packets through its route, even if the routing protocol metric is greater. Due to its lack of scaling capabilities, RIP is the least-preferred protocol from all Interior Gateway Protocols (IGPs).
There are three versions of RIP. RIPv1, RIPv2 and RIPng, but only the first two are required for the CCNA certification.
RIPv1 is the first version of this protocol. The main disadvantage of this protocol is that it’s a Classful Routing Protocol, meaning that you cannot use Variable Length Subnet Masking (VLSM).
The configuration of the RIPv1 protocol is pretty straight forward. You must enable RIP routing with the router rip global configuration command, then specify the networks you want to announce with the network network-address command. RIPv1 assumes the default subnet mask for the IP address you specify. The default masks are: class A – 255.0.0.0, class B – 255.255.0.0, class C – 255.255.255.0. Please note that if you will enter a classless IP address like 192.168.1.32 the router IOS will convert it automatically to the classless one, 192.168.1.0.
To check that RIP is receiving updates from other routers, you use the show ip route command. RIP routes can be easily identified as they are prefixed with R. Also, show ip protocols can give you plenty of information if you want to check if you correctly advertise your routes to others.
Router#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
Gateway of last resort is not set