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The Function of Labels in MPLS Networks

Types of Label Switching Routers

Multi-Protocol Label Switching (MPLS) converts your routed network to something closer to a switched network. Instead of forwarding packets on a hop-by-hop basis, paths are established for particular source-destination pairs. These predetermined paths are called label-switched paths (LSPs).

The routers that make up a label-switched network are called label-switching routers (LSRs), and they come in a few flavors:

  • Ingress router: The router at the entry point of an LSP. The ingress router is the only place where normal IP traffic can flow into an MPLS LSP. The inbound router receives IP traffic. When it determines that to reach its destination it must go thru an LSP, the inbound router encapsulates the traffic with an MPLS header and forwards it to the next hop in the LSP.

  • Transit router: Any router in the middle of an LSP. Transit routers simply switch MPLS packets to the next hop in the LSP, using the incoming interface where the packet came in from as well as the MPLS header to determine where to send the packet.

  • Penultimate router: The second-to-last router in the LSP. The penultimate router is the router before the last hop in an LSP. Because the last hop in an LSP doesn’t need to switch the packet forward to another transit router, it has no need for the MPLS headers.

    It’s the responsibility of the penultimate router to remove the MPLS header before sending it on to the last hop in the LSP. Note that having the penultimate router remove the MPLS label before sending it on to the egress router is optional.

  • Egress router: The exit point for the LSP. The egress router receives IP traffic from the penultimate router. It does a normal IP lookup, and it forwards the traffic using normal IP routing.

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Note that the traffic on the LSP from router 1 to router 9 doesn’t have to originate at router 1. Imagine that router 1 is connected to a server. That server is running an application that is being used by someone accessing the network somewhere beyond router 9. Just because the entire traffic flow extends beyond the two endpoints of the LSP doesn’t mean that the traffic doesn’t use the LSP.

In this case, normal IP routing is used to pass the traffic to router 1. Router 1 does a normal lookup as though the packet were a normal IP packet. The lookup reveals that the destination for this traffic is router 9, and that destination is associated with an LSP.

In other words, the next hop is the whole LSP, not just the next hop router. Router 1 then forwards the packet along as per the LSP definition, and each subsequent router treats the packet as an LSP packet. In this case, router 1 represents the starting point for the LSP. As such, router1 is the ingress router.

Examining the path again, router 9 is the last router in the LSP. So when the packet arrives at router 9, there is no LSP to follow. Therefore, router 9 does a normal IP lookup on the packet, and it forwards the packet as an IP packet. And because router 9 is the last router in the LSP, it’s the egress router.

All the routers between router 1 and router 8 are transit routers. They’re responsible for ushering the MPLS traffic along to the next hop in the LSP. The second-to-last router in the LSP (router 8, in this example) is the penultimate router. The penultimate router is typically responsible for stripping the MPLS headers off the packets (known as penultimate hop popping, or PHP).

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