NetworkTalk & BGP

A.5.b) Ibgp loop prevention: “split-horizon” mechanism

1. A local router propagates only the routes that are selected as best to the Ebgp and Ibgp neighbors. However, the router never sends a route back on the same BGP session upon which it was received.
When it picks a neighbor as the best next-hop, the router makes sure that the neighbor is not pointing back to the local router. In order to make this backward destination unreachable, a withdraw message is sent to that neighbor. The process of preventing routing information from being sent back to the source of information is called “split-horizon.”

2. In the case of Ibgp session, routing information loops within the AS are prevented by Ibgp split-horizon: routing information that is received through an Ibgp session is never forwarded to another Ibgp neighbor, only toward Ebgp neighbors.
Because of BGP split-horizon, no router can relay Ibgp information within the AS: all routers must be directly updated from the border router that received the Ebgp update.
To summarize, in order to prevent loops, incoming Ibgp update is never propagated to other Ibgp peers.

3. Full mesh of Ibgp sessions has to be established between all Ibgp speakers within the AS for proper routing information propagation.
The Ibgp full mesh is a logical mesh of TCP sessions only; physical full mesh is not required.
Thus, common BGP practices ask you to build your Ibgp network (ISIS, for instance) on passive loopback interfaces, since they are always up and never flap: their role of reachability as best next-hop is constantly guaranteed.

4. Below the illustrations shows a network which is not entirely meshed and the same fully meshed. During an operation of IP transit, AS 1000 is unable to send the BGP table received from AS 300, to AS 2000. Because the interior router who does not share an Ebgp session cannot update their Ibgp neighbors.
See below Figure A.9: BGP split horizon mechanism within a not-fully meshed network.
Ibgp updates can flow from AS 300 to AS 2000, through AS 1000 only when the network is fully interconnected logically.
See below Figure A.10: BGP split horizon mechanism within a full meshed network

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