An Introduction to MPLS (continued)

In order to designate different classes of service or service priorities, traffic must be marked with special labels as it enters the network.  Special routers called LER (Label Edge Routers) provide this labeling function (Figure 2).  The LER converts IP packets into MPLS packets, and MPLS packets into IP packets.  On the ingress side, the LER examines the incoming packet to determine whether the packet should be labeled.  A special database in the LER matches the destination address to the label.  An MPLS shim header (Figure 3) is attached and the packet is sent on its way.

Figure 2: IP Network with LERs and an IP packet with Shim header attached

To further explain the MPLS shim header, let's look at the OSI model.  Figure 3 (a) shows OSI layers layer 7 through layer 3 (L7-L3) in red and layer 2 (L2) in yellow.  When an IP packet (layers 2-7) is presented to the LER, it pushes the shim header (b) between layers 2 and 3.  Note that the shim header is neither a part of layer 2 or layer 3; however, it provides a means to relate both layer 2 and layer 3 information.

The Shim Header (c) consists of 32 bits in four parts – twenty bits are used for the label, three bits for experimental functions, one bit for stack function, and eight bits for time to live (TTL).  It allows for the marriage of ATM (a layer-2 protocol) and IP (a layer-3 protocol).

Figure 3: The MPLS Shim Header and Format

A shim header is a special header placed between layer two and layer 3 of the OSI model. The shim header contains the label used to forward the MPLS packets.