Specifications

Implementing IS-IS on Cisco IOS XR Software
Information About Implementing IS-IS on Cisco IOS XR Software
RC-177
Cisco IOS XR Routing Configuration Guide
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.)
Multicast Topology Support Using IS-IS
Multicast topology support allows for the configuration of IS-IS multicast topologies for IPv4 or IPv6
routing. IS-IS maintains a separate topology for multicast and runs a separate Shortest Path First (SPF)
over the multicast topology. IS-IS multicast inserts routes from the IS-IS multicast topology into the
multicast-unicast Routing Information Base (muRIB) table in the RIB for the corresponding address
family. Since PIM uses the muRIB, PIM uses routes from the multicast topology instead of routes from
the unicast topology.
MPLS Label Distribution Protocol IGP Synchronization
Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) Interior Gateway Protocol
(IGP) Synchronization ensures that LDP has completed label exchange before the IGP path is used for
switching. MPLS traffic loss can occur in the following two situations:
• When an IGP adjacency is established, the router begins forwarding packets using the new
adjacency before LDP has exchanged labels with peers on that link.
• When an LDP session closes, the router continues to forward traffic using the link associated with
the LDP peer rather than using an alternate path with an established LDP session.
This feature provides a mechanism to synchronize LDP and IS-IS to minimize MPLS packet loss. The
synchronization is accomplished by changing the link metric for a neighbor IS-IS link-state packet
(LSP), based on the state of the LDP session.
When an IS-IS adjacency is established on a link but the LDP session is lost or LDP has not yet
completed exchanging labels, IS-IS advertises the maximum metric on that link. In this instance, LDP
IS-IS synchronization is not yet achieved.
Note In IS-IS, a link with a maximum wide metric (0xFFFFFF) is not considered for shortest path first (SPF).
Therefore, the maximum wide metric of -1 (0XFFFFFE) is used with MPLS LDP IGP synchronization.
When LDP IS-IS synchronization is achieved, IS-IS advertises a regular (configured or default) metric
on that link.