TAILIEUCHUNG - The Complete IS-IS Routing Protocol- P43

The Complete IS-IS Routing Protocol- P43: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | MPLS Signalling Protocols 421 proper label allocation if the interfaces are not listed under this command hierarchy. Not all default decisions are obvious. The neighbour state is verified using the show ldp neighbor command. JUNOS output You can verify the neighbour state using the show ldp neighbor detail operational level output displays the session IP addresses plus the neighbour s link IP address. hannes@Frankfurt show ldp neighbor detail Address Interface Label space ID Hold time so-0 1 0 11 Transport address Configuration sequence 0 Up for 01 33 30 LDP is very much dependent on a working IGP. LDP itself cannot be run in standalone mode. Like BGP it is topology agnostic and cannot assert which label is better over another. LDP picks the label of the outgoing interface based on the best IGP distance. If the LDP topology is non-congruent than the IGP topology then LDP paths might get black holed. One of the most frequent configuration mistakes is that the list of interfaces that run IS-IS and the list of interfaces which run LDP are not the same. Consider Figure . All links in the core network have IS-IS and LDP enabled except the link between Washington and New York which lacks LDP due to a configuration mistake. Paris learns the 32 FEC of the New York router via the London Frankfurt Washington path and selects the path via Washington because it is on the shortest path tree. The traffic gets labelled to Washington where it gets black holed because no valid MPLS labelled switched paths to the FEC of New York are available. Figure . If the IS-IS and LDP topology is non-congruent Washington is black holing traffic 422 14. Traffic Engineering and MPLS If you are troubleshooting an MPLS reachability problem the first thing to check is if the IS-IS adjacencies match the LDP session. It remains problematic why router vendors do not change their default behaviour. LDP should be automatically brought up as soon

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