Discussion

stops running or a network link goes down but is something you might be able to
respond to and repair quickly. When this happens, the routing protocols stop sending
keepalive messages to their neighbors. After a short while (the actual time
depends on the protocol and how it’s configured), the neighbor will declare that the
local router is down and, because the network topology has changed, will start calculating
new paths to network destinations. This recalculation floods a lot of protocol
traffic, which can disrupt the operation of the network. If you know that the downtime
for the routing protocols will generally be brief, you really don’t want the routing
protocols to recalculate paths, only to recalculate them again once the routing
protocols are back up.
Graceful restart is a way to hide the fact that a routing protocol has restarted and
thus prevent path recalculations. With graceful restart, if the router or routing protocol
has to restart, it informs its adjacent neighbors and requests a grace period from
them. During this grace period, the neighbor acts as a helper, masking the fact that
the local router is down. The restarting router continues to forward traffic during the
restart period, and convergence in the network is not disrupted. The restart is not
visible to the portion of the network that is not communicating directly with the
local router. The neighboring routers are aware of the restart. Also, the restarting
router is not removed from the network topology. Because the network’s topology is
“frozen” during the restart period, you should use graceful restart only when you
know that your topology is stable.
You turn on graceful restart globally, as shown in this recipe. You generally leave
graceful restart running all the time in case of an unplanned failure. The global configuration
applies to all routing protocols, including BGP, IS-IS, OSPF, PIM SM, RIP,
and RIPng, and to all MPLS-related protocols, including RSVP, LDP, CCC, and TCC.
Configuring graceful restart is just a request. The JUNOS software honors the request
only when the network topology is stable and the neighboring routers cooperate.
Now you see that graceful restart settings are different for the different protocols. For
BGP, use the show bgp neighbor command:
aviva@RouterG> show bgp neighbor 10.0.0.2
Peer: 10.0.0.2+2098 AS 64555 Local: 10.0.0.1+179 AS 64550
Type: External State: Established Flags:
Last State: OpenConfirm Last Event: RecvKeepAlive
Last Error: None
Options:
Holdtime: 90 Preference: 170
Number of flaps: 1
Error: 'Cease' Sent: 0 Recv: 1
Peer ID: 10.0.0.1 Local ID: 192.168.19.1 Active Holdtime: 90
Keepalive Interval: 30 Peer index: 0
Local Interface: fe-1/0/1.0
NLRI for restart configured on peer: inet-unicast
NLRI advertised by peer: inet-unicast
NLRI for this session: inet-unicast
278 | Chapter 8: IP Routing
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.