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Configuring BGP to Advertise the Best External Route to Internal Peers

In general, deployed BGP implementations do not advertise the external route with the highest local preference value to internal peers unless it is the best route. Although this behavior was required by an earlier version of the BGP version 4 specification, RFC 1771, it was typically not followed in order to minimize the amount of advertised information and to prevent routing loops. However, there are scenarios in which advertising the best external route is beneficial, in particular, situations that can result in IBGP route oscillation.

Beginning with JUNOS Release 9.3, you can configure BGP to advertise the best external route into an IBGP mesh group, a route reflector cluster, or an AS confederation, even when the best route is an internal route.

Note: In order to configure the advertise-external statement on a route reflector, you must disable intracluster reflection with the no-client-reflect statement.

When a router is configured as a route reflector for a cluster, a route advertised by the route reflector is considered internal only if it is received from an internal peer with the same cluster identifier or no cluster identifier. A route received from an internal peer that belongs to a another cluster, that is, with a different cluster identifier, is considered external.

In a confederation, when advertising a route to a confederation border router, any route from a different confederation sub-AS is considered internal.

You can also configure BGP only to advertise the external route if the route selection process reaches the point where the multiple exit discriminator (MED) metric is evaluated. As a result, an external route with an AS path worse (that is, longer) than that of the active path is not advertised.

The JUNOS software also provides support for configuring a BGP export policy that matches on the state of an advertised route. You can match on either active or inactive routes. For more information, see the JUNOS Policy Framework Configuration Guide.

To configure BGP to advertise the best external path to internal peers, include the advertise-external statement:




advertise-external;

Note: The advertise-external statement is supported at both the group and neighbor level. If you configure the statement at the neighbor level, you must configure it for all neighbors in a group. Otherwise, the group is automatically split into different groups.

For a complete list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

To configure BGP to advertise the best external path only if the route selection process reaches the point where the MED is evaluated, include the conditional statement:



advertise-external {
conditional;
}

For a complete list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

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