Overload Bit Configuration During Multitopology Operation; Is-Is Overload Bit Avoidance; Default Routes; Attached Bit On An Is-Is Instance - Cisco ASR 9000 Series Configuration Manual
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- Command reference manual (1138 pages) ,
- Routing configuration manual (702 pages) ,
- Reference manual (696 pages)
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Implementing IS-IS
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
route policy. You can use a route policy to conditionally advertise the default route, depending on the existence
of another route in the routing table of the router.
Attached Bit on an IS-IS Instance
The attached bit is set in a router that is configured with the is-type command and level-1-2 keyword. The
attached bit indicates that the router is connected to other areas (typically through the backbone). This
functionality means that the router can be used by Level 1 routers in the area as the default route to the
backbone. The attached bit is usually set automatically as the router discovers other areas while computing
its Level 2 SPF route. The bit is automatically cleared when the router becomes detached from the backbone.
OL-30423-03
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
Overload Bit Configuration During Multitopology Operation
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