Overload Bit Configuration During Multitopology Operation; Is-Is Overload Bit Avoidance; Default Routes - Cisco ASR 9000 Series Routing Configuration Manual

Implementing IS-IS
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.

Overload Bit Configuration During Multitopology Operation

Because the overload bit applies to forwarding for a single topology, it may be configured and cleared
independently for IPv4 and IPv6 during multitopology operation. For this reason, the overload is set from the
router address family configuration mode. If the IPv4 overload bit is set, all routers in the area do not use the
router for IPv4 transit traffic. However, they can still use the router for IPv6 transit traffic.

IS-IS Overload Bit Avoidance

The IS-IS overload bit avoidance feature allows network administrators to prevent label switched paths (LSPs)
from being disabled when a router in that path has its Intermediate System-to-Intermediate System (IS-IS)
overload bit set.
When the IS-IS overload bit avoidance feature is activated, all nodes with the overload bit set, including head
nodes, mid nodes, and tail nodes, are ignored, which means that they are still available for use with label
switched paths (LSPs).
Note The IS-IS overload bit avoidance feature does not change the default behavior on nodes that have their
overload bit set if those nodes are not included in the path calculation (PCALC).
The IS-IS overload bit avoidance feature is activated using the following command:
mpls traffic-eng path-selection ignore overload
The IS-IS overload bit avoidance feature is deactivated using the no form of this command:
no mpls traffic-eng path-selection ignore overload
When the IS-IS overload bit avoidance feature is deactivated, nodes with the overload bit set cannot be used
as nodes of last resort.

Default Routes

You can force a default route into an IS-IS routing domain. Whenever you specifically configure redistribution
of routes into an IS-IS routing domain, the Cisco IOS XR software does not, by default, redistribute the default
route into the IS-IS routing domain. The default-information originate command generates a default route
into IS-IS, which can be controlled by a route policy. You can use the route policy to identify the level into
which the default route is to be announced, and you can specify other filtering options configurable under a
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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