MPLS VPN configuration / basic principle

Brief introduction of MPLS

MPLS (mulTI-protocollabelswitch) is the latest development of Internet core multi-layer switching computing. Here we mainly describe the generation process of the label forwarding table and how IP packets are forwarded through MPLS .

MPLS label stack header

Figure 1 MPLS label stack header

The 32-bit MPLS stack header includes the following areas (as shown in Figure 1):

The label area (20 bits) that carries the actual value of the MPLS label;

CoS area (3 bits), used for queuing and discarding algorithms applied to packets as they pass through the network;

Stack area (S area, 1 bit), used to support the marking stack sequence;

TTL area (8 bits), providing traditional IPTTL function.

Label forwarding table generation process

1. A routing table is generated between routers through a routing protocol or static routing.

Figure 2 Routing tables are generated between routers through routing protocols or static routes

As shown in Figure 2, suppose that the OSPF protocol is running between the four routers A, B, C, and D on the way. Router A learns the route of the network segment 211.91.168.0/24 of router D.

2. The router running MPLS assigns labels to the routes in the routing table.

Figure 3 Routers running MPLS assign labels to routes in the routing table

The routing tables of the four routers in Figure 3A, B, C, and D all have routes for the 211.91.168.0/24 network segment. Assuming that each router has run the MPLS protocol, each router will assign a label to the route.

3. Discover its MPLS neighbors through LDP / RSVP protocol.

Assume that the LDP protocol is enabled on each router interface. Through the LDP discovery protocol, Router A knows that Router B is its MPLS neighbor, Router B knows that A and C are its MPLS neighbors, Router C knows that B and D are its MPLS neighbors, and D's MPLS neighbor is C.

4. Advertise the labeled routes to its MPLS neighbors.

Each router advertises its labeled route to its MPLS neighbor, regardless of whether it has learned the label of the route from its neighbor. In this way, the label of route 211.911.68.0/24 in each router is shown in Figure 4.

Figure 4 Labels in each router

5. The router adds the label advertised by its next-hop router to its forwarding table. Usually, in practical applications, routers forward IP packets whose destination is not local to their next hop. Therefore, in MPLS, the router only adds the label advertised by its next-hop router to its forwarding table. The corresponding forwarding for the 211.911.68.0/24 network segment is shown in Figure 5.

Figure 5 The router adds the label advertised by its next-hop router to its forwarding table