Configuring Graceful Restart When Bgp And Ldp Are Configured; Routing Around The Restarting Router To Minimize Network Instability - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - SERVICE AVAILABILITY CONFIGURATION GUIDE 2010-10-08 Configuration Manual

JunosE 11.3.x Service Availability Configuration Guide

Configuring Graceful Restart When BGP and LDP Are Configured

Routing Around the Restarting Router to Minimize Network Instability

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the upgrade requirements during the initialization phase, it detects whether graceful
restart is configured. If it is not configured, the CLI displays a warning message and
prompts you to proceed or halt. You can stop at this point to configure graceful restart.
If instead you proceed, the unified in-service software upgrade can complete successfully,
but the IS-IS neighbors are likely to break the adjacencies with the upgrading router and
consider that routes formerly reached through this router are now unreachable. When
the unified in-service software upgrade completes and the routing protocols restart, the
IS-IS neighbors can relearn the routes through the router.
When you issue the issu start command, IS-IS lengthens its hello timer values and sends
LSPs with the new values. The upgrade proceeds when the IS-IS neighbors have
acknowledged the new values.
When BGP, IS-IS, and LDP are all configured on a router on which you want to perform
a unified in-service software upgrade, ensure that the IS-IS graceful restart timeout is
longer than the LDP graceful restart timeout. The IS-IS graceful restart does not complete
when the LDP graceful restart timeout is longer than the IS-IS graceful restart timeout.
Configure IS-IS graceful timeout with the nsf t3 command. Configure LDP graceful restart
timeout with the mpls ldp graceful-restart timers max-recovery command.
NOTE: The situation described in this section is very uncommon. This rare
circumstance arises when you have redundant uplinks to the core and network
topology changes cause routes to go through the upgrading router. In a typical
network design, this is not an issue and you do not need to route peers around
the upgrading router.
During the unified ISSU upgrade phase, network instability can result if the restarting
router goes into an unstable state after the unified ISSU process fails. Some IS-IS traffic
loss occurs during the resulting line module resets. For those reasons, you might want
IS-IS peers to route around the router that is being upgraded.
You can use the overload advertise-high-metric issu command to cause the router to
advertise a high metric to its neighbors so that they route around the upgrading router.
When you issue the issu start command, the router raises the metric to the maximum
link cost on all interfaces running IS-IS. The maximum value depends on the metric type.
IS-IS neighbors then choose a path with lower metrics to reach any destination that was
previously reached through the upgrading router. When unified ISSU is completed, IS-IS
reverts the metrics back to the values that were configured before the unified in-service
software upgrade.
When traffic engineering has been configured, the traffic engineering metrics are also
increased. New tunnels are not established through the upgrading router and any tunnels
undergoing re-optimization in other routers go around the upgrading router.
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