MPLS Label Switching 15

Label-Switched Paths

Now let’s take a look at the label-switched paths. A label-switched path (LSP) is a unidirectional set of LSRs that the labeled packet must flow through in order to get to a particular destination.

Let’s say that the user on PE1 wants to ping the loopback address of PE2. So, the user types ping 192.168.1.4.

By looking at the labels in the following output of PE1, you can see the outbound label that will be used is 28 and it will be sent out Serial 0/0:

If a labeled packet of 28 arrives on P1, it will be sent out Serial 0/1 with an outbound label of 27, as the following output shows:

If a labeled packet of 27 arrives on P2, it will be sent out Serial 0/1 unlabeled. The Pop tag, which you can see from the show mpls forwardingtable command on P2, means, “Don’t send this traffic as labeled, but instead send it as unlabeled IP traffic.” You can think of Pop tag as meaning, “The next hop router needs to do a Layer 3 lookup on the packet” or “The next hop router is the destination network or has a connected interface that is in the destination network.” The official name for this process is called penultimate hop popping.

16 Chapter 1 An Introduction to MPLS

The word penultimate means “next to last.” With penultimate hop popping, the penultimate router in an LSP pops the label and forwards the packet as unlabeled IP to the next hop router.

In this example, the next-to-last router (P2) in the LSP pops the label and forwards the unlabeled packet to its ultimate destination (PE2), as the following output demonstrates:

Figure 1.9 shows the LSP from PE1 to PE2.

F I G U R E 1 . 9 The LSP from PE1 to PE2

Now let’s now see what happens when a user on PE1 wants to ping the loopback address of PE2. The user types ping 192.168.1.3.

By looking at the labels of PE1 in the following output, you can see the outbound label that will be used is 29, and it will be sent out Serial 0/0: