BGP Split Horizon

Result: Full mesh of IBGP sessions is required for proper IBGP update propagation.

© 2004 Cisco Systems, Inc. All rights re

All routers within an AS must make routing decisions in a consistent way. They must have access to the same routing information with the same attributes in order to come to the same conclusion about which exit point of the AS to use. In other words, the BGP attributes should not be changed within the AS.

The AS-path attribute is not changed over an IBGP session, because the BGP update has not crossed the AS boundary. However, the AS-path attribute is the primary means of detecting routing information loops—a BGP router that encounters its own AS in the AS path of an incoming BGP update silently ignores the information. Because the AS path is modified by BGP-speaking routers only on EBGP sessions, this loop-preventing mechanism is useful between autonomous systems only, not within them.

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.

2-14 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

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