Specifications

Implementing IS-IS on Cisco IOS XR Software
Information About Implementing IS-IS on Cisco IOS XR Software
RC-176
Cisco IOS XR Routing Configuration Guide
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.
Note If the connectivity for the Level 2 instance is lost, the attached bit in the Level 1 instance LSP would
continue sending traffic to the Level 2 instance and cause the traffic to be dropped.
To simulate this behavior when using multiple processes to represent the level-1-2 keyword
functionality, you would manually configure the attached bit on the Level 1 process.
IS-IS Support for Route Tags
The IS-IS Support for route tags feature provides the capability to associate and advertise a tag with an
IS-IS route prefix. Additionally, the feature allows you to prioritize the order of installation of route
prefixes in the RIB based on a tag of a route. Route tags may also be used in route policy to match route
prefixes (for example, to select certain route prefixes for redistribution).
Multicast-Intact Feature
The multicast-intact feature provides the ability to run multicast routing (PIM) when IGP shortcuts are
configured and active on the router. Both OSPFv2 and IS-IS support the multicast-intact feature. MPLS
TE and IP multicast coexistence is supported in Cisco IOS XR software by using the mpls traffic-eng
multicast-intact IS-IS or OSPF router command.
You can enable multicast-intact in the IGP when multicast routing protocols (PIM) are configured and
IGP shortcuts are configured on the router. IGP shortcuts are MPLS tunnels that are exposed to IGP. The
IGPs route the IP traffic over these tunnels to destinations that are downstream from the egress router of
the tunnel (from an SPF perspective). PIM cannot use IGP shortcuts for propagating PIM joins because
reverse path forwarding (RPF) cannot work across a unidirectional tunnel.