MPLS – Part 7

We’ve already discussed how the P (Provider) routers swap or exchange labels at each hop within the WAN cloud. The sequence of routers and labels used for a particular path is referred to as the LSP (Label-Switched Path). In general, the LSP going between the sites in the reverse direction does not use the same label values. In fact, unlike a Frame Relay PVC, with MPLS there isn’t even a requirement that the same physical path be used in both directions. In other words, an MPLS LSP is unidirectional, whereas a Frame Relay PVC is bidirectional.

You might be wondering how the PE (Provider Edge) and P routers know which label values to use when doing a “push” or a “swap”. There are three protocols that can be used to advertise LSP labels between routers (TDP, LDP and RSVP), and we’ll discuss them in a later post.

Congratulations … you’re now doing MPLS, or Multi-Protocol Label Switching!

It gets its name from the fact that the P routers are “Label Switching”, and therefore don’t care about the “Multi-Protocols” used by the customer (and therefore should support any routed protocols). The PE  routers only need to know the routes for customers to which they are directly attached, and the P routers do not need to know any customer routes, for any protocol. Finally, the CE (Customer Edge) routers know nothing about labels at all, because they never see one.

Now that we have an idea of how MPLS works, we can define some additional terms. We know that a CE router is located at a customer site, and thus is CPE (Customer Premises Equipment). A CE generally deals with unlabeled packets, sending to and receiving from, a PE router.

A PE is located at one of the provider’s POPs (Points of Presence). A PE pushes labels onto packets it receives from a CE before forwarding the packets to a P router, and “pops” labels from packets received from a P router before forwarding the packets to a CE.

The P routers are located within the core of the provider’s cloud. Because P routers primarily do label swaps, a P router can also be referred to as a LSR (Label-Switch Router). Likewise, a PE can be called an Edge LSR, or LER (Label Edge Router).

Here’s a summary of the terminology when it comes to the provider routers involved with MPLS:

  • PE = POP = Edge LSR = LER — they “push” and “pop” labels
  • P = LSR = Core — they “swap” labels

Next time, we’ll discuss MPLS in more detail, and see how it deals with LSP labels.

Author: Al Friebe

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