Cisco Systems CCNA 2 manual Routing Update

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.

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