Four-Octet As Numbers; Graceful Restarts - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

JUNOSe 11.1.x BGP and MPLS Configuration Guide

Four-Octet AS Numbers

BGP speakers that support four-octet AS and sub-AS numbers are sometimes referred
to as " new" speakers. The four-octet AS numbers are employed by the AS-path and
aggregator attributes. " Old" speakers are those that do not support the four-octet
numbers.
Two new transitional optional attributes, new-as-path and new-aggregator, are used
to carry the four-octet numbers across the old speakers. A new speaker
communicating with an old speaker will send the new attributes with the four-octet
numbers for locally-originated and propagated routes. The old speaker propagates
the new attributes for received routes. The new speaker also sends the AS-path and
aggregator attributes with two-octet numbers; any AS number greater than 65535
is replaced with a reserved AS number, 23456.

Graceful Restarts

When BGP restarts on a router, all of the router's BGP peers detect that the BGP
session transitioned from up to down. The transition causes a routing flap throughout
the network as the peers recalculate their best routes in light of the loss of routes
from that peering session.
The BGP graceful restart capability reduces the network disruption that normally
results from a peer session going down. If the session is with a peer that had
previously advertised the graceful restart capability, the receiving BGP speaker marks
all routes from that peer in the BGP routing table as stale. BGP keeps these stale
routes for a limited time and continues to use these routes to forward traffic. Any
existing stale routes from that peer are deleted to account for consecutive restarts.
When the restarting peer reestablishes the session, the receiving BGP speaker replaces
the stale routes with the fresh routes it receives from the peer. The restarted peer
sends an End-of-RIB marker to signal when it has finished sending all its routes to
the BGP speaker. Until this point, BGP has still been using the stale routes to forward
traffic. Upon receipt of the End-of-RIB marker, the BGP speaker flushes any remaining
stale routes from the restarted peer.
The End-of-RIB marker is an update message that contains no advertised or withdrawn
prefixes; it is sent only to BGP speakers that have previously advertised the graceful
restart capability.
The receiving speaker also sends its own routes to the restarted speaker, and sends
an End-of-RIB marker when it completes the update. The restarted peer defers
reinitiating the BGP best-path selection process until it has received this marker from
all peers with which it had a session in the established state and from which it had
received an End-of-RIB marker before it restarted.
After running the selection process to pick the best route to all prefixes using the
fresh routes, BGP then installs the best routes in the IP routing table on the restarted
peer. Any of these that are best overall routes to a prefix are then pushed by the
router to the forwarding tables on the line modules.
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Selecting the Best Path