LDP Extensions for Inter-Area LSPs

Background

On a large-scale network, multiple IGP areas are often configured for flexible network deployment and fast route convergence. To reduce the number of routes and conserve resources, area border routers (ABRs) summarize the routes in their areas and advertise the summarized routes to neighboring IGP areas. However, LDP follows the exact match rule when establishing LSPs. LDP searches for the route exactly matching a forwarding equivalence class (FEC) in the received Label Mapping message. If only summarized routes are available, LDP supports only liberal LSPs and cannot set up inter-area LSPs. LDP extensions are available to help set up inter-area LDP LSPs.

A liberal LSP is an LSP that has been assigned labels but fails to be established.

Implementation

The network shown in Figure 1 has two IGP areas, Area 10 and Area 20. LSR_2 at the border of Area 10 has two host routes to LSR_3 and LSR_4. To reduce the resources consumed by routes, LSR_2 can run IS-IS to summarize the two routes to one route 1.3.0.0/24 and advertise this route to Area 20.

Figure 1 Networking topology for LDP extensions for inter-area LSPs

When establishing an LSP, LDP searches the routing table for the route that exactly matches the FEC in the received Label Mapping message. In Figure 1, LSR_1 has only a summarized route (1.3.0.0/24) but not 32-bit host routes in its routing table. Table 1 lists the route of LSR_1 and routes carried in the FEC.

If only summarized routes are available, LDP supports only liberal LSPs and cannot set up inter-area LDP LSPs. In this situation, tunnels cannot be set up on the backbone network.

To set up an LSP, LSR_1 must follow the longest match rule to find the route. There is a summarized route 1.3.0.0/24 in the routing table of LSR_1. When LSR_1 receives a Label Mapping message (for example, a message carrying FEC 1.3.0.1/32) from Area 10, LSR_1 finds the summarized route 1.3.0.0/24 according to the longest match rule. Then LSR_1 applies the outbound interface and next hop of the summarized route to the route 1.3.0.1/32. An inter-area LDP LSP is established.