MPLS control and forwarding planes

Control plane as defined in previous posts is the part of the router architecture that is responsible for collecting and propagating the information that will be used later to forward incoming packets. Routing Protocols and label distribution protocols are parts of control plane.

The forwarding plane is the part of the router architecture used to decide how the packet is going to be switched after being received on the inbound interface. CEF tables are parts of forwarding plane.

Its important to understand and differentiate between control plane and forwarding plane in MPLS networks, specially in troubleshooting problems. I am going to explain this in brief down here:

Control Plane:

The control plane information in MPLS is represented by two main tables which are the RIB and LIB

• Routing protocols are used to build the Routing Information Base (RIB) which represents the routing table. OSPF, EIGRP and BGP are examples.
• Label distribution protocols are used to build the Label Information Base (LIB) table. LDP and TDP are examples.

Forwarding plane:

The forwarding plane information in MPLS is represented by the CEF FIB table and LFIB table.

• The Forwarding Information Base (FIB) is built from the information in the RIB and is used to forward incoming unlabeled IP packets. The egress can be  IP packets or labeled packets.
• The label Forwarding Information Base  is built from the FIB and the LIB. LFIB is used to forward incoming labeled packets to the egress interface. The result can be a labeled packet (label swapping) or normal IP packet (label disposition).

Note: CEF is the only packet switching method that supports MPLS because it is the only method that is capable of forwarding IP packets to labled packets. Hence CEF must be globally enabled on the router.