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