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Upgrade of the forwarding plane for each line module After the routing protocols
reconverge with their peers and rebuild their routing tables, unified ISSU upgrades
the forwarding plane on all line modules simultaneously. This upgrade halts
forwarding through the chassis. This forwarding outage lasts about 15 seconds
for the E120 and E320 routers and about 50 seconds for the ERX1440 router.
If capable, routing protocols temporarily lengthen their timers to survive the outages.
During the initialization phase, unified ISSU checks for timers that are set too short
and whether the protocol enables timer renegotiation. If these checks fail, unified
ISSU generates a warning and enables you to reconfigure the protocols before you
issue the issu start command.
We recommend that you configure timers to be longer than usual for the routing
protocols that cannot renegotiate timers. You can use bidirectional forwarding
detection (BFD) to quickly detect forwarding interruptions.
Table 15 on page 93 describes how individual routing protocols behave during a
unified in-service software upgrade.
Table 15: Behavior of Routing Protocols During a Unified In-Service Software Upgrade
Protocol
Behavior
BFD
BFD renegotiates its timers as needed. Typically, the timers are lengthened
until the SRP module switchover takes place, then shortened for the
forwarding plane upgrade, and finally shortened to the original configured
values.
BGP
The configured BGP timers are typically long enough to survive the
forwarding outages. If, not, unified ISSU generates a warning message
with a recommended timer interval.
BGP sends out keepalive messages immediately before and immediately
after both the SRP module switchover and the forwarding plane restart,
independent of the interval since it last sent them.
IS-IS
If necessary, temporarily lengthens the hello timers.
LDP
Unified ISSU warns you if the hello timers or the keepalive timers are not
long enough to survive the forwarding plane upgrade.
LDP sends out hello messages and keepalive messages immediately before
and immediately after both the SRP module switchover and the forwarding
plane restart, independent of the interval since it last sent them.
Interruption in Traffic Forwarding for Layer 3 Routing Protocols During Unified ISSU
Chapter 4: Configuring a Unified In-Service Software Upgrade
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