An Introduction to MPLS (continued)

In order to route traffic across the network once labels have been attached, the non-edge routers serve as LSR (Label Switch Routers).  Note that these devices are still routers.  Packet analysis determines whether they serve as MPLS switches or routers.

The function of LSR is to examine incoming packets.  Providing that a label is present, the LSR will look up and follow the label instructions, and then forward the packet according to the instructions.  In general, the LSR performs a label swapping function.  Figure 4 shows LSR within a network.

Figure 4: LSR (Label Switch Routers)

Paths are established between the LER and the LSR.  These paths are called LSP (Label Switch Paths).  The paths are designed for their traffic characteristics; as such, they are very similar to ATM path engineering.  The traffic-handling capability of each path is calculated.  These characteristics can include peak traffic load, inter-packet variation, and dropped packet percentage calculation.

Figure 5 shows the LSP established between MPLS-aware devices.  Because MPLS works as an overlay protocol to IP, the two protocols can co-exist in the same cloud without interference.

Figure 5: LSP (Label Switch Paths)