MPLS Concepts

The two major elements of MPLS architecture are the control plane and the data plane.

* The control plane exchanges routing information (with routing protocols such as OSPF) and labels with protocols such as LDP or TDP
* The data plane is the forwarding engine

MPLS labels maintain how to forward information. They function differently depending on whether MPLS is functioning in frame-mode or cell-mode.

* In frame-mode MPLS labels are 32-bit fields inserted between the Layer 2 and Layer 3 headers. These are broken into the following
o 20-bit label
o 3-bit experimental field
o 1 bit bottom-of-stack indicator
o 8-bit TTL field
* In cell-mode the ATM header is the label

A label switch router (LSR) is a device that forwards based on labels.

An edge LSR labels and removes labels from packets.

LSRs that perform cell-mode MPLS are divided into the following categories:

* ATM LSRs if they are ATM switches. All interfaces are enabled for MPLS, and forwarding is done based only on labels.
* ATM edge LSRs if they are routers connected to an MPLS-enabled ATM network.

Forwarding equivalence class (FEC) describes the forwarding characteristic of a packet, such as the destination IP.

MPLS is used for the following applications:

* Unicast IP routing
* Multicast IP routing
* MPLS traffic engineering provides more efficient link use
* Differentiated Quality of Service
* MPLS VPNs - Separate customer routing information across the MPLS backbone
* Any Transport over MPLS - Transport Layer 2 packets over an MPLS backbone