Count to Infinity Problem

Count to Infinity Problem


	In Figure 5-9, router A has lost connectivity to network 1. Router A adjusts the metric in the routing table for network 1 to 16. Assume router B transmits its routing table before router A.


	The message from router B contains a route to network 1 with a hop count of two. This is better than the route currently in router A’s routing table, so the route is installed. Router A now advertises that it can reach network 1 with a hop count of three. Because router B receives this information on the same interface as the route currently in the table, it installs the route with a hop count of four. Router B now advertises to router A a hop count of four for network 1 and router A installs it with a hop count of five and so on ad infinitum (or at least to 16).


	Figure 5-9: Rip Count to Infinity Problem.


	While the routers are counting to 16, we have a routing loop. Packets that A has to send to network 1 are sent to router B and router B sends them to router A and so on. The routing loop will be broken when the routers finally count to 16, but with 30-second updates this could take some time. Meanwhile, the network is being flooded with packets essentially making the network unusable.


	Split Horizon Split horizon is a technique used to solve the counting to infinity problem. With split horizon, a router does not advertise a route over the interface from which it learned the route. This prevents router B from advertising the route to network 1 back to router A. Within 30 seconds, router A would advertise a hop count of 16 to network 1, alerting the network that the network is unreachable.


	Split Horizon with Poison-Reverse This technique allows a router to send updates about routes over the interface that they were learned from, but the hop count is set to 16. For our example, router B would advertise a route to network 1 with a hop count of 16, preventing routing A from placing it in the routing table. DVMRP uses a modified version of poison-reverse for determining downstream dependencies.


	Hold Down Hold down causes a router to ignore routing updates about a route for a period of time after the route has been declared unreachable. For our example, router A determines that network 1 is unreachable. With hold down, router A will ignore advertisements about network 1 from routers B and E during the hold down period, which will allow router A to transmit its routing table, informing the network that network 1 is unreachable.

Triggered Updates Although split horizon solves the routing loop problem between two routers, a situation could occur when three or more routers form a routing loop. Split horizon cannot prevent this from happening. Triggered updates require a router to immediately transmit the routing table when a change occurs, which speeds up the convergence of the network but has the potential for creating broadcast storms. Another situation could arise where a router receives a triggered update and then a regular update from another router reinstalling the route. In short, this is not a technique that solves all the convergence problems of RIP, although the ones mentioned do add a measure of stability to a RIP network.

Count to Infinity Problem

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