Routing Protocol Basics

• A procedure for passing reachability information about networks to other routers.

• A procedure for receiving reachability information from other routers

• A procedure for determining optimal routes based on the reachability information it has and for recording this information in a route table

• A procedure for reacting to, compensating for, and advertising topology changes in an internetwork

A few issues common to any routing protocol are path determination, metrics, convergence, and load balancing.

Path Determination

All networks within an internetwork must be connected to a router, and wherever a router has an interface on a network that interface must have an address on the network. This address is the originating point for reachability information.

Figure 4.1 shows a simple three-router internetwork. Router A knows about networks 192.168.1.0, 192.168.2.0, and 192.168.3.0 because it has interfaces on those networks with corresponding addresses and appropriate address masks. Likewise, router B knows about 192.168.3.0, 192.168.4.0, 192.168.5.0, and 192.186.6.0; router C knows about 192.168.6.0, 192.168.7.0, and 198.168.1.0. Each interface implements the data link and physical protocols of the network to which it is attached, so the router also knows the state of the network (up or down).

At first glance, the information-sharing procedure seems simple. Look at router A:

This procedure does seem quite simple. So why are routing protocols so much more complicated than this? Look again at Figure 4.1.

• What should router A do with the updates from B and C after it has recorded the information in the route table? Should it, for instance, pass B's routing information packet to C and pass C's packet to B?

• If router A does not forward the updates, information sharing may not be complete. For instance, if the link between B and C does not exist, those two routers would not know about each other's networks. Router A must forward the update information, but this step opens a whole new set of problems.

• If router A hears about network 192.168.4.0 from both router B and router C, which router should be used to reach that network? Are they both valid? Which one is the best path?

• What mechanism will be used to ensure that all routers receive all routing information while preventing update packets from circulating endlessly through the internetwork?

• The routers share certain directly connected networks (192.168.1.0, 192.168.3.0, and 192.168.6.0). Should the routers still advertise these networks?

These questions are almost as simplistic as the preceding preliminary explanation of routing protocols, but they should give you a feel for some of the issues that contribute to the complexity of the protocols. Each routing protocol addresses these questions one way or another, as will become clear in following sections and chapters.

Metrics

When there are multiple routes to the same destination, a router must have a mechanism for calculating the best path. A metric is a variable assigned to routes as a means of ranking them from best to worst or from most preferred to least preferred. Consider the following example of why metrics are needed.

Assuming that information sharing has properly occurred in the internetwork of Figure 4.1, router A might have a route table that looks like Table 4.1.

Table 4.1. A rudimentary route table for router A of Figure 4.1.

Network	Next-Hop Router
192.168.1.0	Directly connected
192.168.2.0	Directly connected
192.168.3.0	Directly connected
192.168.4.0	B, C
192.168.5.0	B, C
192.168.6.0	B, C
192.168.7.0	B, C

This route table says that the first three networks are directly connected and that no routing is needed from router A to reach them, which is correct. The last four networks, according to this table, can be reached via router B or router C. This information is also correct. But if network 192.168.7.0 can be reached via either router B or router C, which path is the preferable path? Metrics are needed to rank the alternatives.

Different routing protocols use different, and sometimes multiple, metrics. For example, RIP defines the "best" route as the one with the least number of router hops; IGRP defines the "best" route based on a combination of the lowest bandwidth along the route and the total delay of the route. The following sections provide basic definitions of these and other commonly used metrics. Further complexities—such as how some routing protocols use multiple metrics and deal with routes that have identical metric values—are covered later, in the protocol-specific chapters of this book.

Hop Count

A hop count metric simply counts router hops. For instance, from router A it is 1 hop to network 192.168.5.0 if packets are sent out interface 192.168.3.1 (through router B) and 2 hops if packets are sent out 192.168.1.1 (through routers C and B). Assuming hop count is the only metric being applied, the best route is the one with the fewest hops, in this case, A-B.

But is the A-B link really the best path? If the A-B link is a DS-0 link and the A-C and C-B links are T-1 links, the 2-hop route may actually be best because bandwidth plays a role in how efficiently traffic travels through the network.

Bandwidth

A bandwidth metric would choose a higher-bandwidth path over a lower-bandwidth link. However, bandwidth by itself still may not be a good metric. What if one or both of the T1 links are heavily loaded with other traffic and the 56K link is lightly loaded? Or what if the higher-bandwidth link also has a higher delay?

Load

This metric reflects the amount of traffic utilizing the links along the path. The best path is the one with the lowest load.

Unlike hop count and bandwidth, the load on a route changes, and therefore the metric will change. Care must be taken here. If the metric changes too frequently, route flapping—the frequent change of preferred routes—may occur. Route flaps can have adverse effects on the router's CPU, the bandwidth of the data links, and the overall stability of the network.

Delay

Delay is a measure of the time a packet takes to traverse a route. A routing protocol using delay as a metric would choose the path with the least delay as the best path. There may be many ways to measure delay. Delay may take into account not only the delay of the links along the route but also such factors as router latency and queuing delay. On the other hand, the delay of a route may be not measured at all; it may be a sum of static quantities defined for each interface along the path. Each individual delay quantity would be an estimate based on the type of link to which the interface is connected.

Reliability

Reliability measures the likelihood that the link will fail in some way and can be either variable or fixed. Examples of variable-reliability metrics are the number of times a link has failed or the number of errors it has received within a certain time period. Fixed-reliability metrics are based on known qualities of a link as determined by the network administrator. The path with highest reliability would be selected as best.

Cost

This metric is configured by a network administrator to reflect more- or less-preferred routes. Cost may be defined by any policy or link characteristic or may reflect the arbitrary judgment of the network administrator.

The term cost is often used as a generic term when speaking of route choices. For example, "RIP chooses the lowest-cost path based on hop count." Another generic term is shortest, as in "RIP chooses the shortest path based on hop count." When used in this context, either low est-cost (or highest-cost) and shortest (or longest) merely refer to a routing protocol's view of paths based on its specific metrics.

Convergence

A dynamic routing protocol must include a set of procedures for a router to inform other routers about its directly connected networks, to receive and process the same information from other routers, and to pass along the information it receives from other routers. Further, a routing protocol must define a metric by which best paths may be determined.

A further criteria for routing protocols is that the reachability information in the route tables of all routers in the internetwork must be consistent. If router A in Figure 4.1 determines that the best path to network 192.168.5.0 is via router C and if router C determines that the best path to the same network is through router A, router A will send packets destined for 192.168.5.0 to C, C will send them back to A, A will again send them to C, and so on. This continuous circling of traffic between two or more destinations is referred to as a routing loop.

The process of bringing all route tables to a state of consistency is called convergence. The time it takes to share information across an internetwork and for all routers to calculate best paths is the convergence time.

Figure 4.2 shows an internetwork that was converged, but now a topology change has occurred. The link between the two left-most routers has failed; both routers, being directly connected, know about the failure from the data link protocol and proceed to inform their neighbors of the unavailable link. The neighbors update their route tables accordingly and inform their neighbors, and the process continues until all routers know about the change.

Notice that at time t₂ the three left-most routers know about the topology change but the three right-most routers have not yet heard the news. Those three have old information and will continue to switch packets accordingly. It is during this intermediate time, when the internetwork is in an unconverged state, that routing errors may occur. Therefore convergence time is an important factor in any routing protocol. The faster a network can reconverge after a topology change, the better.

Load Balancing

Recall from Chapter 3, "Static Routing," that load balancing is the practice of distributing traffic among multiple paths to the same destination in order to use bandwidth efficiently. As an example of the usefulness of load balancing, consider Figure 4.1 again. All the networks in Figure 4.1 are reachable from two paths. If a device on 192.168.2.0 sends a stream of packets to a device on 192.168.6.0, router A may send them all via router B or router C. In both cases, the network is 1 hop away. However, sending all packets on a single route probably is not the most efficient use of available bandwidth. Instead, load balancing should be implemented to alternate traffic between the two paths. As noted in Chapter 3, load balancing can be equal cost or unequal cost and per packet or per destination .