Basic MPLS

How to configure a basic Multiprotocol Label Switching (MPLS) Network and testing it.

An MPLS network is commonly a backbone network comprised of MPLS-enabled routers called Label Switch Routers (LSR). Generally, the network consists of a core LSR with an edge LSR that applies labels to packets.

MPLS Network Setup:

Mechanism

An MPLS network is commonly a backbone network comprised of MPLS-enabled routers called Label Switch Routers (LSR). Generally, the network consists of a core LSR with an edge LSR that applies labels to packets.

This is the setup mechanism of an MPLS network:

1. Routing tables of the different LSRs are computed with an Interior Gateway Protocol (IGP). A link-state protocol, such as Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS), is required if you plan to deploy MPLS TE.
2. A label distribution protocol (LDP) advertises the bindings between routes and labels. These bindings are checked against the routing table. If the route (prefix/mask and next hop) learned from the LDP matches the route learned from IGP in the routing table, an entry is created in the label that forwards information bases (LFIB) on the LSR.

The LSR uses this forwarding mechanism:

1. Once an edge LSR receives an unlabelled packet, the Cisco Express Forwarding table is checked and a label is imposed on the packet if needed. This LSR is called the ingress LSR.
2. Upon the arrival of a labelled packet at the inbound interface of a core LSR, the LFIB provides the outbound interface and the new label that is associated with the outbound packet.
3. The router before the last LSR (the penultimate hop) pops the label and transmits the packet without the label. The last hop is called the egress LSR.