Data augmentation

Left alone, routing protocols decide what the best path network traffic will take based on network topology metrics such as bandwidth and router hops. But organizations often want to have more control over the path the traffic takes to get its final destination. In this section, we'll talk about implementing route preferences driven by an organization's business goals. Why would an organization choose traffic preferences different from the ones selected by routing metrics? I discussed a number of the reasons in the scenarios described in Chapter 1. Some paths are more secure than others. Paths over internal networks tend to be more secure than paths over the open Internet. Some paths may be cheaper than others, while some may have more bandwidth and better performance. Whatever the reason, path selection and failover preferences are completely up to the organization, and it falls on network administrators to enforce those policies.

You can picture the general problem of route preferences and business goals as a path selection problem. Figure 4.5 shows two networks, Network 1 and Network 2. There are several ways to get between the two networks through a mesh of routers—Path A is one way, Path B is another, and so on.

Figure 4.5. Path selection and business goals

While routing metrics may indicate that traffic between Network 1 and Network 2 should use one path, an organization may prefer that traffic go in another direction. For example, let's say that routing metrics prefer that traffic from Network 1 to Network 2 go through Path A. If Path A is unavailable, traffic will go through Path B. If Path B is unavailable, traffic will go through Path C, and so on for Paths D and E. The failover sequence is (A, B, C, D, E). Alternatively, the organization may prefer traffic from Network 1 and Network 2 to flow through Path B. It may prefer that the failover sequence be (A, C, D, E, B) or even (B, D, E), which does not allow Paths A or C to be used at all. The organization, in its wisdom, may even decide that no traffic whatsoever should pass between Networks 1 and 2.

I mentioned earlier in the chapter that routers have four possible actions to control routing information: reject routing information, accept but not forward routing information, modify incoming routing information, or modify outgoing routing information. Route preferences are most commonly implemented by rejecting routes or modifying incoming or outgoing routing information.

I'll start with the first method, rejecting routing information. As in our previous example, we want to specify how traffic flows between Network 2 to Network 1. Figure 4.6 shows routing information for Network 1 coming into a router with a path to Network 2.

Figure 4.6. Rejecting routes to implement traffic preferences

There are five possible paths, A, B, C, D, and E, from Router X to Network 1. We want traffic to go over Path A, C, or E, but not B or D. To do this, Router X rejects routing information about routes going over Paths B and D. Router X will have no information about Paths B and D at all.

The next two options for route preferences modify routing information after it reaches the router. We can do this as routing information comes in or goes out. Figure 4.7 shows how to do this by modifying incoming route information.

Figure 4.7. Modifying incoming routing information

Here, we are trying to control the path that traffic from Network 2 takes to Network 1. In our example, routing metrics in all of the incoming routing updates, before we modify them in Router X, are numerical value a. This indicates that the path from Router X to Network 1 can take either Path A, B, or C with equal preference. We want to change the path selection preference so that traffic to Network 1 first goes through Path A. If Path A is unavailable, traffic should go through Path B, and then if B is unavailable, through Path C.

To make this happen, we alter the routing metric values of each routing update after each comes into Router X. The metric for the route to Path B is increased by 1. The metric for the route to Path C is increased by 2. Since the original unaltered metrics for the three routes were equal, the metric for the route through A is the lowest, followed by the metric for the route via B and finally by the metric for the route through C. Since routing protocols take the path with the lowest metric, the most preferred Path is A, followed by B and then C.

Similarly, we can alter routing metrics as they go out of the router. This is shown in Figure 4.8.

Figure 4.8. Modifying outgoing route information

In this figure, we manipulate Routers L, M, and N, and not Router X. As in Figure 4.7, the metrics advertised to Router X for the path to Network 1 are the same for Paths A, B, and C. We want to make traffic prefer routes in the order A, B, then C. To do this, we add 1 to the routing metric of the routing updates from router M, which has Path B to Network 1. We add 2 to the routing metric of the routing updates from router N, which has Path C to Network 1. When the routing information to Network 1 arrive at router X, the path through A has the lowest metric, followed by B then C. Traffic to Network 1 then prefers Path A, B, and then C. When should you use these different techniques for implementing route preferences? It usually depends on what you want to achieve and what routers you administer. The technique of rejecting incoming routes is best used when you want to completely reject some routes and allow others. It does not allow for explicitly picking failover preferences. Modifying route information must be done when you have to set up a specific order of preferences for route failover. For instance, you would modify incoming routing information when you have control over a router receiving routing updates. On the other hand, if you only have control over the routers' advertising paths, then you need to modify routing information that is sent out.