Cisco ASR 9000 Series Routing Configuration Guide

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.
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.
Note
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.
When you enable multicast-intact on an IGP, the IGP publishes a parallel or alternate set of equal-cost next-hops
for use by PIM. These next-hops are called mcast-intact next-hops. The mcast-intact next-hops have the
following attributes:
•
They are guaranteed not to contain any IGP shortcuts.
•
They are not used for unicast routing but are used only by PIM to look up an IPv4 next-hop to a PIM
source.
•
They are not published to the FIB.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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Implementing IS-IS
Attached Bit on an IS-IS Instance

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English