Traffic Management - CCIE Routing and Switching Exam Certification Guide - page 523

the packet leaves the MPLS network. The MPLS network applies services and forwards packets
that are based on the information contained in the label.

MPLS Label
In a packet environment, the MPLS label is inserted between the Layer-2 header and the
Layer-3 header in a Layer-2 frame. This applies for Packet over Synchronous Optical Network
(SONET) (PoS), Ethernet, Frame Relay, and label over ATM. In ATM networks with label
switching, the label is mapped into the virtual path identifier/virtual channel identifier (VPI/
VCI) fields of the ATM header. The MPLS label field is 32 bits in length, with the actual label
(tag) being 20 bits.

MPLS Label Switch Routers (LSRs)
All routers within a MPLS network are LSRs, which forward based on the label and not based
on routing protocols. If the MPLS network uses ATM, the LSRs are called ATM LSRs. The
Edge Label Switch Router (Edge LSR) is responsible for adding or removing the label to the
packet.

Figure 11-3 shows a diagram of these routers.

MPLS LSRs

Figure 11-3

MPLS Network

Edge LSR non LSR

LSRs

Edge LSR

Edge LSR

non LSR

non LSR