Transit Network Using Edge Routers as Next Hops

This topic describes how to configure edge routers to announce themselves as the next hop in IBGP updates.

The next-hop attribute is usually not modified by an IBGP peer when the BGP update is propagated across IBGP sessions. However, you could configure the BGP router to have a different behavior and set its IP address as the next-hop address even when the BGP updates are sent across IBGP sessions (emulating behavior on EBGP sessions). If you do configure an IBGP router to emulate the behavior of EBGP sessions on the IBGP sessions of the border routers, the BGP updates that are received on the EBGP sessions will be forwarded on the IBGP sessions and the next-hop attribute will be set to the IP address that is used on the local side of the IBGP session. The original next hop, set by the far end of the EBGP session, will be lost.

The receiver of the IBGP information will do recursive routing in the normal way. But the next-hop address that is used will be the IP address of the far end of the IBGP session, because the border router has changed it. The IP address of the far-end IBGP peer is always known in the routing table; otherwise, the IBGP session would not have been established. There is no need for the receiver of the IBGP information to have knowledge of how to reach the far end of the EBGP session, because that IP address is no longer set as the next hop.

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