Counting to Infinity

Split horizon will break loops between neighbors, but it will not stop loops in a network such as the one in Figure 4.6. Again, 10.1.5.0 has failed. Router D sends the appropriate updates to its neighbors router C (the dashed arrows) and router B (the solid arrows). Router B marks the route via D as unreachable, but router A is advertising a next-best path to 10.1.5.0, which is 3 hops away. B posts that route in its route table.

Figure 4.6. Split horizon will not prevent routing loops here.

Figure 4.6. Split horizon will not prevent routing loops here.

B now informs D that it has an alternative route to 10.1.5.0. D posts this information and updates C, saying that it has a 4-hop route to the network. C tells A that 10.1.5.0 is 5 hops away. A tells B that the network is now 6 hops away.

"Ah," router B thinks, "router A's path to 10.1.5.0 has increased in length. Nonetheless, it's the only route I've got, so I'll use it!"

B changes the hop count to 7, updates D, and around it goes again. This situation is the counting-to-infinity problem because the hop count to 10.1.5.0 will continue to increase to infinity. All routers are implementing split horizon, but it doesn't help.

0 0

Post a comment