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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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MPLS LDP-IGP Synchronization

A problem with MPLS networks is that LDP and the IGP of the network are not synchronized. Synchronization means that the packet forwarding out of an interface happens only if both the IGP and LDP agree that this is the outgoing link to be used. A common problem with MPLS networks that are running LDP is that when the LDP session is broken on a link, the IGP still has that link as outgoing; thus, packets are still forwarded out of that link. This happens because the IGP installs the best path in the routing table for any prefix. Therefore, traffic for prefixes with a next hop out of a link where LDP is broken becomes unlabeled. This is not a big problem for networks that are running IPv4-over-MPLS only. At the point ...

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