RIP v2 builds upon the original RIP specification and adds a number of features, the most significant of which is the sharing of subnet mask information. Thus, RIP v2 supports VLSM. Figure 4.1 illustrates the packet formats for both RIP and RIP v2. Note that there are many similarities between the two in order to facilitate interoperability.

FIGURE 4.1  The RIP and RIP v2 packet formats

Consider the network illustrated in Figure 4.2. This network is a radical departure from the hierarchical model, but it is an excellent model from which to describe and understand RIP and hop count. Note that this topology would be considered a partial mesh or complex mesh, as opposed to a full mesh.

The numbers on each line reflect the hop count for each router hop. Therefore, the hop count from Router A to Router B is 3, while the hop count from Router A to Router C is 1. In this scenario, the designer has manipulated the hop counters to reflect policy, which was likely related to the bandwidth of the circuit. While this drawing does not so indicate, assume that a hop count of 1 is a full T1 circuit and higher numbers reflect proportionally lower bandwidth paths.

FIGURE 4.2  Complex Mesh network with RIP

RIP uses hop count only to determine the path. Using Figure 4.2, determine the path that Router A would use to send packets to Router G. You will find that the path A-C-F-G, with a hop count of 7, would be used over the other routes. Note that the hop count values do not surpass 15—a hop count of 16 marks the route as unavailable in RIP.

It is important to note that RIP networks designed with the hierarchical model would have a maximum default hop count of 6—easily within the 15-hop limitation. Other designs, especially those that manipulate the hop metric, may exceed this limitation more easily.

Convergence time is an important consideration in selecting a routing protocol. RIP is one of the slower routing protocols in terms of convergence, although the hierarchical design model also works to facilitate the fastest possible convergence.

Network Design with IGRP

The Interior Gateway Routing Protocol (IGRP) is quite common in large, enterprise-scale, corporate networks. However, like EIGRP, the protocol is proprietary to Cisco and requires a commitment to the Cisco platform. Many companies are reluctant to make such a business decision, and designers will likely need to deploy an open-standard protocol, such as OSPF. In addition, IGRP, and its successor, EIGRP, tolerate arbitrary topologies better than OSPF—however, designers should strive to follow the hierarchical model in order to improve convergence and troubleshooting.

It is unlikely that a designer would select IGRP for a completely new network design, but it might still be warranted for reasons that will be presented in this section. It is much more likely that the use of IGRP will be based on previous deployments of the protocol and the required integration that the network will demand. A recent Cisco survey found IGRP and EIGRP in over 50 percent of networks.

IGRP is a more advanced protocol than RIP, which it was designed to replace. It is a distance-vector protocol that uses a 24-bit metric value to determine the best route, with a maximum of 254 hops (default value of 100 hops). This is greatly enhanced over RIP’s 15-hop-based metric. Other benefits include load balancing and path determination, where the protocol can select from multiple default networks. IGRP is also more tolerant of nonhierarchical topologies; unlike EIGRP, IGRP can support arbitrary topology configurations. However, both protocols operate better when deployed with a strong design. It is important to note that complex mesh configurations will impact convergence in both IGRP and EIGRP, but the redundancy benefits of these designs may offset the negatives.

As with RIP, IGRP transmits the entire routing table with each update, which by default occurs every 90 seconds (compared to RIP at every 30 seconds). These updates may contain 104 route entries (within a 1,500-byte packet), which is a clear improvement over IP RIP, which includes only 25 routes. Unfortunately, the entire routing table is sent each time. Of more importance in advanced networks, IGRP does not support VLSM and is classful. Finally, IGRP uses the concept of split-horizon to prevent routing loops. By default this feature is on. However, the architect may disable it to support point-to-multipoint installations.

	Some texts state that split-horizon is disabled automatically with some topologies, such as SMDS. This is incorrect. You should use the show ip interface command to check the current status of an interface.

Split-horizon is used to prevent routing loops by blocking the advertisement of a route out the interface that it was learned from. Poison reverse is a variation on this concept that sends the route back to the source, but with an illegal metric.

IGRP Metrics

The IGRP metric is one of the most significant advantages for network designers using the protocol. Unlike RIP, which uses hop count as a single metric, IGRP uses two important factors, of six possible metrics, to determine routes. These are presented in Table 4.2.

By default, IGRP considers only two metrics in determining the best route through the network—bandwidth and delay. Under ideal conditions, IGRP will weight bandwidth more heavily for shorter routes (routes with fewer hops) and delay for longer routes. This can provide a more accurate representation of the network’s capacity.