Problem Multiple Exit Points Exist but Traffic Goes Out Through One or Few Exit Routers Cause BGP Policy Definition Causes Traffic to Exit from One Place

This problem commonly is seen when an AS receives the same prefix announcements from mul-tiple EBGP connections but traffic destined to those prefixes prefers only one or two exit points.

As illustrated by Figure 15-34, AS 109 has multiple connections to other autonomous systems. AS 109 has three EBGP connections with AS 110, two with AS 111, and one with AS 112. AS 111 is peering with AS 110 and AS 112.

Figure 15-34. Autonomous System with Multiple Connections to Other Autonomous Systems with Multiple Exit Points

Figure 15-34. Autonomous System with Multiple Connections to Other Autonomous Systems with Multiple Exit Points

Prefixes P1, P2, and P3 are originated by AS 110 and are advertised to EBGP neighboring autonomous systems 109, 111, and 112. AS 109 receive updates for these prefixes from multiple locations? three updates from AS 110, two from AS 111, and one from AS 112. Even with such redundant BGP advertisements for Prefixes P1, P2, and P3, all the traffic for these prefixes from AS 110 might take only one or two exit points. The rest of the connections are underutilized. Such a scenario typically results in overutilized links because traffic tends to exit from one or two preferred paths, as governed by BGP policy of AS 109.

Figure 15-35 shows the flowchart to follow to resolve this problem.

Figure 15-35. Problem-Resolution Flowchart

Figure 15-35. Problem-Resolution Flowchart

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