Unicast IP Routing
Unicast IP Routing |
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RIP for IP, like most distance vector routing protocols, announces its routes in an unsynchronized and unacknowledged manner. This can lead to convergence problems. However, you can enable modifications to the announcement algorithms to reduce convergence time in most situations.
The classic distance vector convergence problem is known as the count-to-infinity problem and is a direct result of the asynchronous announcement scheme. When RIP for IP routers add routes to their routing table, based on routes advertised by other routers, they keep only the best route in the routing table and they update a lower cost route with a higher cost route only if is being announced by the same source as the current lower cost route. In certain situations, as illustrated in Figures 3.1 through 3.5, this causes the count-to-infinity problem.
Assume that the internetwork in Figure 3.1 has converged. For simplicity, assume that the announcements sent by Router 1 on Network 1 and Router 2 on Network 3 are not included.
Figure 3.1 Converged Internetwork
Now assume that the link from Router 2 to Network 3 fails and is sensed by Router 2. As shown in Figure 3.2, Router 2 changes the hop count for the route to Network 3 to indicate that it is unreachable, an infinite distance away. For RIP for IP, infinity is 16.
Figure 3.2 Link to Network 3 Fails
However, before Router 2 can advertise the new hop count to Network 3 in a scheduled announcement, it receives an announcement from Router 1. The Router 1 announcement contains a route to Network 3 which is two hops away. Because two hops away is a better route than 16 hops, Router 2 updates its routing table entry for Network 3, changing it from 16 hops to three hops, as shown in Figure 3.3.
Figure 3.3 Router 2 After Receiving Announcement From Router 1
When Router 2 announces its new routes, Router 1 notes that Network 3 is available three hops away through Router 2. Because the route to Network 3 on Router 1 was originally learned from Router 2, Router 1 updates its route to Network 3 to four hops. (See Figure 3.4.)
Figure 3.4 Router 1 After Receiving Announcement From Router 2
When Router 1 announces its new routes, Router 2 notes that Network 3 is available four hops away through Router 1. Because the route to Network 3 on Router 2 was originally learned from Router 1, Router 2 updates its route to Network 3 to five hops. (See Figure 3.5.)
Figure 3.5 Router 2 After Receiving Another Announcement from Router 1
The two routers continue to announce routes to Network 3 with higher and higher hop counts until infinity (16) is reached. Then, Network 3 is considered unreachable and the route to Network 3 is eventually timed out of the routing table. This is known as the count-to-infinity problem.
The count-to-infinity problem is one of the reasons why the maximum hop count of RIP for IP internetworks is set to 15 (16 for unreachable). Higher maximum hop count values would make the convergence time longer when count-to-infinity occurs. Also note that during the count-to-infinity in the previous example, the route from Router 1 to Network 3 is through Router 2. The route from Router 2 to Network 3 is through Router 1. A routing loop exists between Router 1 and Router 2 for Network 3 for the duration of the count-to-infinity problem.
To help reduce the convergence time of RIP for IP internetworks and to avoid count-to-infinity and routing loops in most situations, you can enable the following modifications to the RIP announcement mechanism:
Split horizon helps reduce convergence time by not allowing routers to advertise networks in the direction from which those networks were learned. The only information sent in RIP announcements are for those networks that are beyond the neighboring router in the opposite direction. Networks learned from the neighboring router are not included.
Split horizon eliminates count-to-infinity and routing loops during convergence in single-path internetworks and reduces the chances of count-to-infinity in multi-path internetworks. Figure 3.6 illustrates how split horizon keeps the RIP router from advertising routes in the direction from which they were learned.
Figure 3.6 Split Horizon
Split horizon with poison reverse differs from simple split horizon because it announces all networks. However, those networks learned in a given direction are announced with a hop count of 16, indicating that the network is unreachable. In a single-path internetwork, split horizon with poison reverse has no benefit beyond split horizon. However, in a multipath internetwork, split horizon with poison reverse greatly reduces count-to-infinity and routing loops. Count-to-infinity can still occur in a multipath internetwork because routes to networks can be learned from multiple sources.
In Figure 3.7, split horizon with poison reverse advertises learned routes as unreachable in the direction from which they are learned. Split horizon with poison reverse does have the disadvantage of additional RIP message overhead because all networks are advertised.
Figure 3.7 Split Horizon with Poison Reverse
Triggered updates allow a RIP router to announce changes in metric values almost immediately rather than waiting for the next periodic announcement. The trigger is a change to a metric in an entry in the routing table. For example, networks that become unavailable can be announced with a hop count of 16 through a triggered update. Note that the update is sent almost immediately, where a time interval to wait is typically specified on the router. If triggered updates were sent by all routers immediately, each triggered update could cause a cascade of broadcast traffic across the IP internetwork.
Triggered updates improve the convergence time of RIP internetworks but at the expense of additional broadcast traffic as the triggered updates are propagated.
