BGP Split-Horizon Rule
The BGP split-horizon rule states that a BGP router that receives a BGP route via an iBGP peering shall not advertise that route to another router that is an iBGP peer. It is important that you commit this rule to memory. By doing so, you will be able to recognize when this is the reason for missing routes. Figure 18-4 shows the current BGP peerings. Notice that R2 has an iBGP peering with R4 and that R4 has an iBGP peering with R5. When R2 advertises the 10.1.1.0/26 network (as an example) to R4, it is via an iBGP peering. Because R4 and R5 are iBGP peers, R4 will not advertise the 10.1.1.0/26 network to R5 because of the BGP split-horizon rule.
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