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4: MPLS label distribution Migrations > 4.2 Considerations for MPLS label distr...

4.2 Considerations for MPLS label distribution Migrations

Each label distribution protocol provides different features that may suit some design needs at a given time but can also present some drawbacks.

While RSVP-TE is explicitly a downstream-on-demandprotocol, L-BGP works in a downstream-unsolicitedfashion, meaning that label bindings are distributed as soon as they become available. LDP presents both options with different label request and mapping mechanisms and with control modes within the same protocol definition. Standard transport label distribution in LDP occurs in a downstream-unsolicitedfashion as well, although there are some setups in which it is done on demand, and both can perfectly coexist.

LDP presents information in the form of TLV structures and includes notification and advisory mechanisms, so that future extensibility is guaranteed, with specific bits in each TLV indicating how error handling should be used for unknown TLVs. RSVP-TE expanded the original RSVP standard with additional objects but [RFC3209] states that unknown Label objects or Label Request objects should be handled with ResvErr messages or PathErr messages, the same ones sent by transit LSRs when they do not recognize other objects such us Explicit Route Objects or Record Route objects. Only Hello extensions are defined as backward-compatible and are able to be silently discarded. On the other hand, L-BGP is enforced by standard BGP extensibility with new Capabilities and Families. Initial BGP session negotiation determines, therefore, which capa....


  

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