MPLS LDP—Local Label Allocation Filtering

This feature introduces command-line interface (CLI) commands to modify the way in which Multiprotocol Label Switching (MPLS) Label Distribution Protocol (LDP) handles local label allocation. This MPLS LDP feature enhancement enables the configuration of filtering policies for selective local label binding assignments by LDP to improve LDP scalability and convergence.

Restrictions for MPLS LDP—Local Label Allocation Filtering

The MPLS LDP—Local Label Allocation Filtering feature does not support access lists. This feature
supports prefix lists.
Restrictions for the MPLS LDP—Local Label Allocation Filtering feature in Cisco IOS
Releases 12.2(33)SRC and 12.2(33)SB:

- LDP local label allocation configuration for prefix list or host routes is supported only in the global routing table.
- LDP and RIB restart handling supported in Cisco IOX software does not apply.
- Wildcard Forwarding Equalence Class (FEC) requests are not supported.
- Remote bindings are retained for LDP table entries that are filtered.


Overview


LDP allocates a local label for every route learned from the Interior Gateway Protocol (IGP). In the absence of inbound and outbound label filtering, these local labels are advertised to and learned by all peers.

In most Layer 3 Virtual Private Network (VPN) configurations only the label switched paths (LSPs) created to reach the /32 host routes or Border Gateway Protocol (BGP) next hops between the provider edge (PE) routers carry traffic and are relevant to the Layer 3 VPNs. LSPs between the PE routers that are not members of a VPN use more memory and create additional processing in LDP across the core.

With the load increases in the service provider domain in the last decade (1997–2007), scalability has become more important in the service provider networks. Controlling the local label allocation could off-load LDP processing of non-VPN LSPs in the service provider network core devices.

The MPLS LDP—Local Label Allocation Filtering feature introduces the mpls ldp label and allocate commands that allow you to configure LDP to selectively allocate local labels for a subset of the prefixes learned from the IGP. You can select that LDP allocate local labels for prefixes configured in a prefix list in the global table or for host routes in the global table.

Local label allocation filtering reduces the number of local labels allocated and therefore the number of messages exchanged with peers. This improves LDP scalability and convergence.


Router> enable
Router# configure terminal
Router(config)# ip prefix-list list1 permit 192.168.0.0/16 le 20

Router(config)# mpls ldp label
Router(config-ldp-lbl)# allocate global prefix-list list1
Router(config-ldp-lbl)# allocate global host-routes

/* The host-routes keyword specifies that local label allocation be done for host routes only. */