Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 108
Split Horizon Demonstrated

In the slide, there are three routers, labeled A, B, C, and four subnets, labeled W, X, Y, Z. Upon startup, the routers learn of their immediate subnets. Router A learns about subnets Y and Z. Router B learns about subnets X and Y. Router C learns about subnets W and X. The routers may or may not automatically broadcast their tables out after initialization (this is vender independent). Router C transmits its table containing subnets W and X. It will transmit this information out the ports connecting to subnets W and X. Router B will transmit its table containing subnet X and Y out both of its ports, and router A will transmit its table containing subnets Y and Z out both of its ports. All of the costs in these tables are set to 1. So far, so good.

Router C picks up the information that router B transmitted out and makes some decisions. It will add to each entry the cost associated with the port on which it received the information. In this case, that port was assigned a cost of 1. Therefore, it now has two entries in the received table, each with a cost of 2. It then compares it to its table. It already has a entry for subnet X and it has a cost of 1, so it discards that information. The next entry is for subnet Y with a cost of 2. It does not have that entry, so it will add this entry to its table with a cost of 2. Router C figures it is now complete. Eventually, router C will update its table with the entry for subnet Z (propagated by router B after router B received this information). Router C now has the entries in its table of subnet Z, cost 3; subnet Y, cost 2; subnet X, cost 1; and subnet W, cost 1).

Split Horizon Demonstrated

The periodic timer has expired (every 30 seconds) and router C is ready to broadcast its table. Out the port associated with subnet W, it will list the entries for subnets W, X, Y, and Z. However, on the port associated with subnet X, it will only include those entries for subnet X (some routers do not include this entry if they know of another router on this segment) and subnet W. It will not include the entries for subnets Y and Z. This is known as spilt horizon. The rule for split horizon is not to rebroadcast a known route back over the port that the router learned it on. Poisoned reverse allows for the network number to be rebroadcast out, but it will include a 16 in the hop count so no other router can update its tables. This is used to avoid routing loops in oddly looped networks.