This chapter contains LDP inter-area configuration examples.

Provider-based MPLS (Multiprotocol Label Switching) networks are expanding with the success of Layer 3 Virtual Private Networks and the new deployments of Layer 2 VPNs. Service providers MPLS backbones are significantly growing both in terms of density with the addition of Provider Edge (PE) routers to connect new customers and in terms of footprint as traditional Layer 2 aggregation networks may be replaced by IP/MPLS networks. As a consequence, many providers need to introduce IGP areas. Inter-area LSPs (that is, LSPs that traverse at least two IGP areas) are required to ensure MPLS connectivity between PEs located in distinct IGP areas.

On a large MPLS networks, multiple IGP areas need to be configured for flexible network deployment and fast route convergence. When advertising routes between IGP areas, to prevent a large number of routes from consuming too many resources, an Area Border Router (ABR) needs to aggregate the routes in the area and advertises the aggregated route to the neighboring IGP areas. By default, when establishing LSPs, LDP searches the routing table for the route that exactly matches the FEC in the received Label Mapping message.

The LDP inter-area feature provides a longest-match label mapping procedure where a label is used if the Forwarding Equivalence Class (FEC) matches an entry in the Routing Information Base (RIB). Matching is defined by an IP longest-match search and does not mandate an exact match.

The LDP Inter-Area configuration process can be divided into the following tasks:

To configure a summary route on an ABR, configure two networks such R4 and R5 in Figure 12-1, so that two prefixes are summarized. For example, 4.4.4.0/24 is a summary route for the networks 4.4.4.1/32 and 4.4.4.2/32.

Give the area-range command on the routers to summarize the networks. For example: area 2 range 4.4.4.0/24.