Understanding MPLS

MPLS is a packet-forwarding technology which uses labels to make data forwarding decisions. With MPLS, the Layer 3 header analysis is done just once (when the packet enters the MPLS domain). Label inspection drives subsequent packet forwarding. MPLS provides these beneficial applications:

Virtual Private Networking (VPN)
Traffic Engineering (TE)
Quality of Service (QoS)
Any Transport over MPLS (AToM)

Additionally, it decreases the forwarding overhead on the core routers. MPLS technologies are applicable to any network layer protocol.

In MPLS network, routers are catagorized as Label Edge Routers(LERs) and Label Switched Routers(LSRs). The LERs are the endpoints of the MPLS tunnels, known as Label Switched Paths(LSPs)

and are normally at the edge of the network. The LSRs are at the core of the network and provide the connectivity between the LERs.

The purpose of MPLS is to provide a tunneling service to forward customer packets across the provider network based on information other than simply IP address destination. In order to provide the service, MPLS will mark packets inbound from the customer devices(CE) with a special header called label. The label is simply an additional header that is packets inbound to the provider network.

A label is a short, four-byte, fixed-length, locally-significant identifier which is used to identify a Forwarding Equivalence Class (FEC). The label which is put on a particular packet represents the FEC to which that packet is assigned.

                         

Label—Label Value (Unstructured), 20 bits
Exp—Experimental Use, 3 bits; currently used as a Class of Service (CoS) field.
S—Bottom of Stack, 1 bit
TTL—Time to Live, 8 bits

These labels are used to make forwarding decisions rather than the IP address. This basically turns the Layer 3 routed network into a switched network. This provides several advantages over traditional IP routing.

LERs mark the packets with label information based on provider configured policies. By marking the packets, the provider network can differentiate customer traffic based on the labels, for example, by forwarding differently labeled packets over a preferred path.

The MPLS enabled routers(LERs and LSPs) use a signalling protocol to distribute labeled packets across the network. The method for distributing labels through the network depends on the signalling protocol being used, either Label Distribution Protocol(LDP) or Resource Reservation Protocol(RSVP).