Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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that were advertised. In other words, BGP stores the attributes prior to the application
of any outbound policy.
For peer groups, BGP stores the full set of attributes associated with the route after the
application of any outbound policy; that is, it stores the attributes as they will be
advertised. BGP does not store a bit to track whether a route was advertised to the peer
group. Storing the full attribute set for each peer group route is memory intensive but
acceptable for peer groups, because the number of peer groups is relatively small. An
advantage of enabling rib-out for peer groups is that convergence is accelerated because
the attributes for each route are already determined for all routes to be advertised to the
peer group. BGP has to apply outbound policy only once for each route rather than once
for each peer for each route.
After enabling rib-out for a peer group, you can issue the show ip bgp advertised-routes
command to display the routes that will be advertised to the peer group and the attributes
(after the application of any outbound policy) that will be advertised with the routes.
When you have enabled rib-out for individual peers or a peer group, before sending an
advertisement or withdrawal the router compares the route it is about to send with the
last route sent for the same prefix (and stored in the Adj-RIBs-Out table for the peer or
peer group) and sends the update message only if the new information is different from
the old.
The comparison prevents the sending of unnecessary withdrawals for both peers and
peer groups, because the BGP speaker will not send a withdrawal if the table indicates
it has not previously advertised that route to the peer. However, because the route
attributes are no longer stored with the routes in peer Adj-RIBs-Out tables, BGP cannot
compare them with the attributes in the new update message. Consequently, BGP cannot
determine whether the update contains new attributes or the same attributes as those
previously advertised, and might send superfluous advertisements to peers. This
circumstance does not happen for peer groups, because their Adj-RIBs-Out tables store
the full attribute set.
Effects of Changing Outbound Policies
After you change the outbound policy for a peer or peer group, the policy changes do not
take effect until you issue either a hard clear or an outbound soft clear. (See “Resetting
a BGP Connection” on page 96 for information about performing clears with the
clear ip
bgp command.) The clear action causes BGP to reapply the outbound policy of the peer
or peer group to each route in the BGP routing table. BGP then stores the results in the
Adj-RIBs-Out table for that peer or peer group. The BGP session with each peer or peer
group member takes the routes from the appropriate Adj-RIBs-Out table and sends them
in update messages to the peer or peer group member.
NOTE: You cannot change outbound policy for an individual peer group member. You
can change outbound policy only for a peer group as a whole or for peers that are not
members of a peer group.
neighbor rib-out disable
Copyright © 2010, Juniper Networks, Inc. 42
JunosE 11.2.x BGP and MPLS Configuration Guide