Label Distribution Protocol
7450 ESS MPLS Guide Page 577
Multi-Area and Multi-Instance Extensions to LDP
In order to extend LDP across multiple areas of an IGP instance or across multiple IGP instances,
the current standard LDP implementation based on RFC 3036 requires that all /32 prefixes of PEs
be leaked between the areas or instances. This is because an exact match of the prefix in the
routing table is required to install the prefix binding in the LDP Forwarding Information Base
(FIB). Although a router will do this by default when configured as Area Border Router (ABR),
this increases the convergence of IGP on routers when the number of PE nodes scales to thousands
of nodes.
Multi-area and multi-instance extensions to LDP provide an optional behavior by which LDP
installs a prefix binding in the LDP FIB by simply performing a longest prefix match with an
aggregate prefix in the routing table (RIB). That way, the ABR will be configured to summarize
the /32 prefixes of PE routers. This method is compliant to RFC 5283, LDP Extension for Inter-
Area Label Switched Paths (LSPs).
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