IP Routing Protocols

Routing protocols are used to pass information about the structure of the network between routers. Cisco routers support the following IP routing protocols RIP (versions 1 and 2), IGRP, EIGRP, IS-IS, OSPF, and BGP. This section compares routing protocols and calls out key
differences between them.

Administrative Distance
Cisco routers are capable of supporting several IP routing protocols concurrently. When identical prefixes are discovered from two or more separate sources, Administrative Distance (AD) is used to discriminate between the paths. AD is a poor choice of words; trustworthiness is a better name. Routers use paths with the lower AD. The following figure lists the default values for various routing protocols. Of course, there are several ways to change AD for a routing protocol or for a specific route.


Building the Routing Table
The router builds a routing table by ruling out invalid routes and considering the remaining advertisements. The procedure is:

1. For each route received, verify the next hop. If invalid, discard the route.
2. If multiple, valid routes are advertised by a routing protocol, choose the lowest metric.
3. Routes are identical if they advertise the same prefix and mask, so 192.168.0.0/16 and 192.168.0.0/24 are separate paths and are each placed into the routing table.
4. If more than one specific valid route is advertised by different routing protocols, choose the path with the lowest AD.

Comparing Routing Protocols
Two things should always be considered in choosing a routing protocol: fast convergence speed and support for VLSM. EIGRP, OSPF, and IS-IS meet these criteria. Although all three meet the minimum, there are still important distinctions, as described below:

EIGRP is proprietary, but it is simple to configure and support.
OSPF is an open standard, but it is difficult to implement and support.
There are few books on IS-IS and even fewer engineers with experience who use it. IS-IS is therefore uncommon.