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All transit ASs must be able to carry traffic that originates from locations outside of that AS, is destined to locations outside of that AS, or both. This requires a certain degree of interaction and coordination between BGP and the Interior Gateway Protocol (IGP) that the particular AS uses. In general, traffic that originates outside of a given AS passes through both interior gateway (that support the IGP only) and border gateway (that support both the IGP and BGP). All interior gateway receive information about external routes from one or more of the border gateway of the AS that uses the IGP.
Depending on the mechanism used to propagate BGP information within a given AS, take special care to ensure consistency between BGP and the IGP, since changes in state are likely to propagate at different rates across the AS. A time window might occur between the moment when some border gateway (A) receives new BGP routing information (which was originated from another border gateway (B) within the same AS) and the moment the IGP within this AS can route transit traffic to the border gateway (B). During that time window, either incorrect routing or black holes can occur.
To minimize such routing problems, border gateway (A) should not advertise to any of its external peers a route to some set of exterior destinations associated with a given address prefix using border gateway (B) until all the interior gateway within the AS are ready to route traffic destined to these destinations by using the correct exit border gateway (B). Interior routing should converge on the proper exit gateway before advertising routes that use the exit gateway to external peers.
If all routers in an AS are BGP speakers, no interaction is necessary between BGP and an IGP. In such cases, all routers in the AS already have full knowledge of all BGP routes. The IGP is then only used for routing within the AS, and no BGP routes are imported into the IGP. The user can perform a recursive lookup in the routing table. The first lookup uses a BGP route to establish the exit router, while the second lookup determines the IGP path to the exit router.
