MPLS Principle Learning

Before learning MPLS, you should know that MPLS has two modes of operation.

1, Frame mode--> in the third-level group header (such as IP packet header) before the tag to forward.

2.---> in the MPLS network composed of ATM LSR, MPLS is used to exchange Vpi/vci information in the control plane instead of using ATM signaling to forward.

1, forwarding equivalence class. Fec

If the portal router receives the same subnet for the grouping, those groupings belong to the same class. This is determined according to the longest matching lookup in the fib. For example, suppose that there is an item in the FIB table with the address of 1.1.1.0/24, and the next hop is 2.2.2.2. If I receive two IP addresses that are 1.1.1.1 and 1.1.1.2, the two groupings are forwarded to the same next, 2.2.2.2, So this time these two messages belong to the same FEC.

This column more highlights: http://www.bianceng.cn/Network/basis/

As you can see here, for the 6.1.1.0/24 network segment, regardless of the rear main seat, the distribution of the label, 20, are the same, in fact, by the downstream routers to the outgoing label distribution of 20, are the same. So 6.1.1.0 is an equivalent forwarding class.

This is the same as the IP-level routing table.

For the IP level, to the same destination, go the same route.

For the MPLS level, for the same purpose, distribution of the same label, that is the same path to go.

2, label Exchange.

There are three procedures for label swapping:

Search according to the tag

Locate the outbound interface and the label for the group.

Make a label swap and then send the packet back out of the interface.

The MPLS architecture is divided into two separate units: Control plane and forward forwarding, as shown in the following illustration: