Multicast Topology Support Using Is-Is; Mpls Label Distribution Protocol Igp Synchronization; Mpls Ldp-Igp Synchronization Compatibility With Ldp Graceful Restart - Cisco ASR 9000 Series Routing Configuration Manual

Implementing IS-IS
• When multicast-intact is enabled on an IGP, all IPv4 destinations that were learned through link-state
• In IS-IS, the max-paths limit is applied by counting both the native and mcast-intact next-hops together.

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
• When an LDP session closes, the router continues to forward traffic using the link associated with the
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.
In IS-IS, a link with a maximum wide metric (0xFFFFFF) is not considered for shortest path first (SPF).
Note
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.

MPLS LDP-IGP Synchronization Compatibility with LDP Graceful Restart

LDP graceful restart protects traffic when an LDP session is lost. If a graceful restart-enabled LDP session
fails, MPLS LDP IS-IS synchronization is still achieved on the interface while it is protected by graceful
restart. MPLS LDP IGP synchronization is eventually lost under the following circumstances:
• LDP fails to restart before the LDP graceful restart reconnect timer expires.
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 OSPFv2, the behavior is slightly different.)
before LDP has exchanged labels with peers on that link.
LDP peer rather than using an alternate path with an established LDP session.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
Multicast Topology Support Using IS-IS
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