Advertising Best Routes

By default, BGP selects from its routing table one best route to each destination. If BGP learned that best route from an internal peer, then the BGP speaker does not advertise a route to that destination to the speaker’s internal peers.

In earlier software releases, the default behavior was for BGP to select two best routes to any destination. The best route learned from external (including confederation) peers was advertised to the speaker’s internal peers. The best route learned from all sources was advertised to the speaker’s external peers.

You can issue the bgp advertise-external-to-internal command to cause BGP to revert to advertising two potentially different routes to its peers. See Selecting the Best Path for information about the process BGP uses to determine best routes.

bgp advertise-best-external-to-internal

• Use to cause BGP to select two best routes to every destination as follows:
  • For external peers, BGP selects the best route from the complete set of routes known to BGP.
  • For internal peers, BGP selects the best route from the set of routes BGP has received from external and confederation peers.
• Changes apply automatically whenever BGP subsequently runs the best-path decision process for a destination prefix; that is, whenever a best route is picked for a given prefix.
• The behavior enabled by this command is the default behavior for the E-series router running software releases lower than 5.0.0.
• The command is disabled by default.
• Example

  host2(config-router)#bgp advertise-best-external-to-internal

• Use the no version to restore the default condition, wherein BGP selects one best route for each destination from the complete set of routes; if the best route was received from an internal peer, no route to the destination is advertised to the internal peers.
• See bgp advertise-best-external-to-internal.

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