When a network has two or more IGP areas, or instances, inter-area LSPs are required for MPLS connectivity between the PE devices that are located in the distinct IGP areas. To extend LDP across multiple areas of an IGP instance or across multiple IGP instances, the current standard LDP implementation based on RFC 3036, LDP Specification, requires that all /32 prefixes of PEs be leaked between the areas or instances. IGP route leaking is the distribution of the PE loopback addresses across area boundaries. An exact match of the prefix in the routing table (RIB) is required to install the prefix binding in the FIB and set up the LSP.
This behavior is the default behavior for the 7705 SAR when it is configured as an Area Border Router (ABR). However, exact prefix matching causes performance issues for the convergence of IGP on routers deployed in networks where the number of PE nodes scales to thousands of nodes. Exact prefix matching requires the RIB and FIB to contain the IP addresses maintained by every LSR in the domain and requires redistribution of a large number of addresses by the ABRs. Security is a potential issue as well, as host routes leaked between areas can be used in DoS and DDoS attacks and spoofing attacks.
To prevent these performance and security issues, the 7705 SAR can be configured for an optional behavior in which LDP installs a prefix binding in the LDP FIB by performing a longest prefix match with an aggregate prefix in the routing table (RIB). This behavior is described in RFC 5283, LDP Extension for Inter-Area Label Switched Paths. The LDP prefix binding continues to be advertised on a per-individual /32 prefix basis.
When the longest prefix match option is enabled and an LSR receives a FEC-label binding from an LDP neighbor for a prefix-address FEC element, FEC1, it installs the binding in the LDP FIB if:
the routing table (RIB) contains an entry that matches FEC1. Matching can either be a longest IP match of the FEC prefix or an exact match.
the advertising LDP neighbor is the next hop to reach FEC1
When the FEC-label binding has been installed in the LDP FIB, LDP programs an NHLFE entry in the egress data path to forward packets to FEC1. LDP also advertises a new FEC-label binding for FEC1 to all its LDP neighbors.
When a new prefix appears in the RIB, LDP checks the LDP FIB to determine if this prefix is a closer match for any of the installed FEC elements. If a closer match is found, this may mean that the LSR used as the next hop changes; if so, the NHLFE entry for that FEC must be changed.
When a prefix is removed from the RIB, LDP checks the LDP FIB for all FEC elements that matched this prefix to determine if another match exists in the routing table. If another match exists, LDP must use it. This may mean that the LSR used as the next hop changes; if so, the NHLFE entry for that FEC must be changed. If another match does not exist, the LSR removes the FEC binding and sends a label withdraw message to its LDP neighbors.
If the next hop for a routing prefix changes, LDP updates the LDP FIB entry for the FEC elements that matched this prefix. It also updates the NHLFE entry for the FEC elements.