If you configure IETF NSF on the Cisco IOS XR router and a neighbor router does not support IETF NSF,
the affected adjacencies flap, but nonstop forwarding is maintained to all neighbors that do support IETF
NSF. A restart reverts to a cold start if no neighbors support IETF NSF.
Note
Multi-Instance IS-IS
You can configure up to eight IS-IS instances. MPLS can run on multiple IS-IS processes as long as the
processes run on different sets of interfaces. Each interface may be associated with only a single IS-IS instance.
Cisco IOS XR software prevents the double-booking of an interface by two instances at configuration time—two
instances of MPLS configuration causes an error.
Because the Routing Information Base (RIB) treats each of the IS-IS instances as equal routing clients, you
must be careful when redistributing routes between IS-IS instances. The RIB does not know to prefer Level
1 routes over Level 2 routes. For this reason, if you are running Level 1 and Level 2 instances, you must
enforce the preference by configuring different administrative distances for the two instances.
Multiprotocol Label Switching Traffic Engineering
The MPLS TE feature enables an MPLS backbone to replicate and expand the traffic engineering capabilities
of Layer 2 ATM and Frame Relay networks. MPLS is an integration of Layer 2 and Layer 3 technologies.
For IS-IS, MPLS TE automatically establishes and maintains MPLS TE label-switched paths across the
backbone by using Resource Reservation Protocol (RSVP). The route that a label-switched path uses is
determined by the label-switched paths resource requirements and network resources, such as bandwidth.
Available resources are flooded by using special IS-IS TLV extensions in the IS-IS. The label-switched paths
are explicit routes and are referred to as traffic engineering (TE) tunnels.
Overload Bit on Router
The overload bit is a special bit of state information that is included in an LSP of the router. If the bit is set
on the router, it notifies routers in the area that the router is not available for transit traffic. This capability is
useful in four situations:
1
During a serious but nonfatal error, such as limited memory.
2
During the startup and restart of the process. The overload bit can be set until the routing protocol has
converged. However, it is not employed during a normal NSF restart or failover because doing so causes
a routing flap.
3
During a trial deployment of a new router. The overload bit can be set until deployment is verified, then
cleared.
4
During the shutdown of a router. The overload bit can be set to remove the router from the topology before
the router is removed from service.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
280 OL-30423-03
Implementing IS-IS
Multi-Instance IS-IS
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