IPv4 Routing Protocol Selection

This section describes the process of choosing routing protocols for your network and discusses the concepts of redistribution, filtering, and administrative distance.

Choosing Your Routing Protocol

To decide which routing protocol is best for your network, you need to first look at your requirements. You can then compare your requirements to the specifications for the available routing protocols, as detailed in the previous sections and summarized earlier in Table 3-2, and choose the routing protocol that best meets your needs.

Recall that Chapter 1, "Network Design," described the hierarchical model in which a network is divided into three layers: core, distribution, and access. Because each layer provides different services, they typically have different routing requirements and therefore use different routing protocols. The specific network function performed at each of these layers is as follows:

• Access layer Provides end-user and workgroup access to the resources of the network.

• Distribution layer Implements the organization's policies (including filtering of traffic) and provides connections between workgroups and between the workgroups and the core.

• Core layer Provides high-speed links between distribution-layer devices and to core resources. The core layer typically uses redundancy to ensure high network availability.

Thus, the different routing protocols suitable at each layer are as follows:

• In the core layer, a fast-converging routing protocol is required: EIGRP, OSPF, and IS-IS are the possible choices. OSPF and IS-IS require a hierarchical topology with areas defined; EIGRP supports a hierarchical topology but doesn't require it. EIGRP is Cisco-proprietary, so it can only be supported if all routers are Cisco routers. IS-IS requires OSI addresses to be configured, which is not a common skill.

• In the distribution layer, any of the interior routing protocols are suitable, depending on the specific network requirements. For example, if it is an all-Cisco network and has a mixture of link types so that VLSMs would be appropriate, EIGRP would be the logical choice. Because the distribution layer routes between the core and access layers, it might also have to redistribute (share with) and/or filter between the routing protocols running in those layers, as described in the next section.

• The access layer typically includes RIPv2, IGRP, EIGRP, OSPF, or static routes. The devices in this layer are typically less powerful (in terms of processing and memory capabilities) and therefore support smaller routing tablesthus, the distribution layer should filter routes sent to this layer. Remember that EIGRP is not suitable for use in a dial-up network and that distance vector routing protocols have issues in NBMA networks.

Redistribution, Filtering, and Administrative Distance

Key Point

If two (or more) routing protocols are run in the same network, information from one routing protocol can be redistributed with, or shared with, another routing protocol. Routers that participate in more than one routing protocol perform the redistribution.

Redistribution can be bidirectionalthe information from each routing protocol is shared with the other. It can also be performed in only one direction, with default routes used in the other direction. You must be careful not to introduce routing loops when you use redistribution.

Key Point

Routes can be filtered to prevent specific routes from being advertised. In other words, the router can exclude specific routes from the routing updates it sends to other specific routers.

Route filtering is useful when redistribution is being used, to help prevent routing loops.

For example, consider the network in Figure 3-17, with IGRP running in the upper part and RIPv2 running in the lower part. Both Routers A and B are configured to pass IGRP information into the RIPv2 network, and RIPv2 into the IGRP network, with the intention that all devices can reach all networks.

Figure 3-17. Routers A and B Are Redistributing Between IGRP and RIPv2

A problem can occur if both Routers A and B redistribute the full content of their routing tables, because more than one path exists between the IGRP and RIPv2 networks. For example, Router B can pass information about network 10.0.0.0 to Router E, which can pass it to Router D, which can pass it to Router C, which can pass it to Router A. Router A is connected to network 10.0.0.0, but depending on how the redistribution is configured, Router A might think that the path to some of the subnets of network 10.0.0.0 through Router C is betterthrough the IGRP network. If Router A passed this information to Router F, and so on, traffic from the RIPv2 part of the network might loop around the entire network before ending up where it startedin other words, the potential exists for a routing loop. Specific route filtering can be configured to avoid thisyou must know your network and ensure that you are not introducing problems.

Because each routing protocol uses different metrics, you can't compare one metric with anotherfor example, how do you compare whether 3 RIP hops are better than an OSPF cost of 10? Thus, when multiple routing protocols are run on Cisco routers, another parameter, called the administrative distance, compares the routing protocols.

Key Point

Cisco routers use the administrative distance in the path selection process when they learn two or more routes to the same destination network or subnet from different sources of routing information, such as different routing protocols. The administrative distance rates the believability of the sources of routing information.

The administrative distance is a value between 0 and 255; the lower the value, the higher the believability of the source of the routing information.

Table 3-4 lists the default administrative distance of the sources of routing information, including routing protocols, supported by Cisco routers.

Table 3-3. Administrative Distance of Routing Protocols

Route Source	Default Distance
Connected interface, static route out an interface	0
Static route to an address	1
EIGRP summary route	5
EBGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIPv1, RIPv2	120
External EIGRP	170
Internal BGP	200
Unknown	255

For example, consider a router that receives a route to network 10.0.0.0 from RIPv2 (with an administrative distance of 120) and also receives a route to the same network from IGRP (with an administrative distance of 100). The router uses the administrative distance to determine that IGRP is more believable; the router therefore puts the IGRP route into its routing table.

The administrative distance can be changed from its default value, either for all routes from a routing protocol or for specific routes. This can help to eliminate routing loops.

Note

Many details about routing protocol operation and configuration, including redistribution, filtering, and administrative distances, are provided in the book CCNP Self-Study: Building Scalable Cisco Internetworks (BSCI), Second Edition, by Paquet & Teare, Cisco Press, 2003.