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Implementing IS-IS on Cisco IOS XR Software
Information About Implementing IS-IS on Cisco IOS XR Software
RC-90
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 map. You can use the
route map to identify the level into which the default route is to be announced, and you can specify other
filtering options configurable under a route map. You can use a route map 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. 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.
Caution 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.
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.
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.
• When multicast-intact is enabled on an IGP, all IPv4 destinations that were learned through
link-state advertisements are published with a set equal-cost mcast-intact next-hops to the RIB. This
attribute applies even when the native next-hops have no IGP shortcuts.
• In IS-IS, the max-paths limit is applied by counting both the native and mcast-intact next-hops
together. (In OSPFv2, the behavior is slightly different.)