79 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright © 2004, Cisco Systems, Inc.
Figure [1]: Routing Update
Figure [2]: Split Horizon
Split horizon is another mechanism to help prevent routing loops. Split horizon does not allow
the originator of network information to receive updates about the network from another router.
This prevents the originator of correct information from being influenced by the incorrect
information of another router.
Use the figure in this section for reference. If Router 2 sends out an update to Router 1 about
the status of Network A, it cannot receive a report back from Router 1 about Network A.
From the description in the curriculum, which is included above, if split horizon were in use in
Step 2, Router A would have ignored information about Network 1 from Router B. More
specifically, Router B would not have attempted to update Router A about that particular
network in the first place because Router A originally informed Router B about the status of
Network 1. Refer to Figure [1] Routing Update for a graphical representation of this process.