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Wednesday 11 January 2012



IGP 's as Label Distribution Protocol


No IGP protocol in fact distributes MPLS labels. You run the IGP to have your routing table contain reachable networks withing your topology. Afterwards, each router assigns labels to routes in its routing table and advertises them to its neighbors using Label Distribution Protocol. The labels themselves are not included in advertisements of any IGP, be it RIP, (E)IGRP, OSPF or IS-IS. In a basic MPLS deployment, we run the LDP together with an IGP routing protocol - the IGP protocol itself does not propagate label bindings.


None of the IGPs is actually used for label distribution (including EIGRP). If implementers wanted to make them work for label distribution, then the work would be easier for EIGRP compared to link state protocols. The reason is this: You need to associate a label with a prefix in routing table before passing the label to another router. EIGRP distributes routes, so can easily add allocated label to route sent to neighbors but implementation choices is this: At an early stage modularity is preferred (i.e. separate protocols/software modules) to capture abstractions, because it speeds up/simplifies development while avoiding bugs.


In other hand,  OSPF does not send routing table prefixes to neighbors. It rather sends status of links(LSAs). So, OSPF would have to function both like a link state and a distance vector protocol (i.e. send both link state updates and routing table entries to neighbors.  IGPs would "need to be enhanced in an intrusive way to be able to do this' (i.e. distribute labels for prefixes, even those who are not originators of a prefix). Basically, this is a hypothetical scenario that was not chosen by implementers for the reasons described in this section.
The only routing protocol that is currently able to carry labels along with the destination prefixes is the BGP. 


Although IGP is not involved on the distribution of Labels as this is the function of Label distribution protocols but Still TE(Traffic Engineering) information needs special LSAs and TLVs(Tme,Length, Value) through OSPF and ISIS routing protocols. MPLS TE extensions to OSPF and IS-IS refer to all those changes needed to run a constrained shortest path first (CSPF). So Link state protocols are appropriate for the distribution of TE information but not for label distribution. So we can say, OSPF and IS-IS are used for distribution of topology information and associated link properties/constraints in the TE scenario and RSVP signals the path and is actually responsible for carrying the label in the TE case.

In summary:



  • No IGP can carry label information due to internal architecture.
  • EIGRP was an option but idea was dropped to keep the protocol simple and fast.
  • BGP is the only protocol which was improvise(MPBGP) to carry label information along with routes.
  • In TE scenario  CR-OSPF and CR-IS-IS are used to distribute TE information but RSVP-TE is used to signals the path and responsible for TE label distribution.
This way we can say Link state IGPs with extension (OSPF with Opaque LSAs, IS-IS with  Link State Packets TLV (type, length, value))  keep track  of topology changes propagation only in case of MPLS TE but IGPs are not at all responsible for actual TE label distribution.

Please note that CR-LDP and RSVP-TE are both signaling mechanisms used to support Traffic Engineering across an MPLS backbone


It is also important to note that the label space in the node is a unique pool and that LDP, BGP with labels and RSVP TE picks a label from this unique common space.The label has to be unique. For example if label 30 is used by LDP cannot be used also by RSVP TE on the same router.


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