advertise-external

Syntax

Hierarchy Level

Description

Specify BGP to advertise the best external route into an IBGP mesh group, a route reflector cluster, or an AS confederation even if the best route is an internal route.

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.

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.

In a confederation, when advertising a route to a confederation border router, any route from a different confederation sub-AS is considered external. When configuring the advertise-external statement for an AS confederation, it is recommended that EBGP peers belonging to different autonomous systems are configured in a separate EBGP peer group. This ensures consistency while BGP sends the best external route to peers in the configured peer group.

To configure the advertise-external statement on a route reflector, you must disable intracluster reflection with the no-client-reflect statement.

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

The conditional option causes BGP to advertise the external route only 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 longer than that of the active path is not advertised.

Junos OS 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.

Default

BGP does not advertise the external route with the highest local preference value to internal peers unless it is the best route.

Options

conditional—(Optional) Advertise the best external path only if the route selection process reaches the point at which the multiple exit discriminator (MED) metric is evaluated. The conditional option restricts advertisement to when the best external path and the active path are equal until the MED step of the route selection process. This implies that external routes with a longer AS path length than the active path, for instance, are not advertised. The criteria used for selecting the best external path is the same whether or not the conditional option is configured.

Required Privilege Level

routing—To view this statement in the configuration.

routing-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 9.3.