Overview
Introduces the concept of best-external paths in BGP and provides guidance on configuring best-external path advertisement to optimize routing decisions.
A best-external path is a BGP route selection mechanism that
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identifies the most optimal external path for a prefix when the current best path is internal (iBGP)
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enables advertisement of both the best and a backup path to peers to improve redundancy, and
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ensures internal routers are prepared with an external alternative if the primary iBGP path fails.
Selection procedure:
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Determine the best path from all available paths for the prefix.
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Remove the current best path.
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Remove all remaining internal paths.
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From the remaining paths, remove those with the same next hop as the current best path.
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Use the BGP best path selection algorithm on the remaining paths to identify the best-external path.
Enhancing failover with the BGP best-external path feature
When a router’s optimal path to a prefix comes from an iBGP peer, the router activates the best-external path feature to select and advertise the next best eBGP route to its internal peers. This function allows internal routers to promptly switch to a valid external route if the iBGP path becomes unavailable. As a result, the network avoids delays due to full path recalculation.