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2.1. MPLS-TE with PCE

2.1. MPLS-TE with PCE​

As described in [RFC4655], a PCE can be used to compute MPLS-TE paths within a "domain" (such as an IGP area) or across multiple domains (such as a multi-area AS or multiple ASes).

  • Within a single area, the PCE offers enhanced computational power that may not be available on individual routers, sophisticated policy control and algorithms, and coordination of computation across the whole area.

  • If a router wants to compute a MPLS-TE path across IGP areas, then its own TED lacks visibility of the complete topology. That means that the router cannot determine the end-to-end path and cannot even select the right exit router (Area Border Router (ABR)) for an optimal path. This is an issue for large-scale networks that need to segment their core networks into distinct areas but still want to take advantage of MPLS-TE.

Previous solutions used per-domain path computation [RFC5152]. The source router could only compute the path for the first area because the router only has full topological visibility for the first area along the path, but not for subsequent areas. Per-domain path computation uses a technique called "loose-hop-expansion" [RFC3209] and selects the exit ABR and other ABRs or AS Border Routers (ASBRs) using the IGP-computed shortest path topology for the remainder of the path. This may lead to sub-optimal paths, makes alternate/back-up path computation hard, and might result in no TE path being found when one really does exist.

The PCE presents a computation server that may have visibility into more than one IGP area or AS, or may cooperate with other PCEs to perform distributed path computation. The PCE obviously needs access to the TED for the area(s) it serves, but [RFC4655] does not describe how this is achieved. Many implementations make the PCE a passive participant in the IGP so that it can learn the latest state of the network, but this may be sub-optimal when the network is subject to a high degree of churn or when the PCE is responsible for multiple areas.

The following figure shows how a PCE can get its TED information using the mechanism described in this document.

            +----------+                           +---------+
            |  -----   |                           |   BGP   |
            | | TED |<-+-------------------------->| Speaker |
            |  -----   |   TED synchronization     |         |
            |    |     |        mechanism:         +---------+
            |    |     | BGP with Link-State NLRI
            |    v     |
            |  -----   |
            | | PCE |  |
            |  -----   |
            +----------+
                 ^
                 | Request/
                 | Response
                 v
   Service  +----------+   Signaling  +----------+
   Request  | Head-End |   Protocol   | Adjacent |
   -------->|  Node    |<------------>|   Node   |
            +----------+              +----------+

 Figure 2: External PCE Node Using a TED Synchronization Mechanism

The mechanism in this document allows the necessary TED information to be collected from the IGP within the network, filtered according to configurable policy, and distributed to the PCE as necessary.

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