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This chapter describes the commands to configure and display the Label Distribution Protocol (LDP) on an Avici router.
About LDP
LDP is a label distribution protocol that enables one Label Switched Router (LSR) to inform another of the meaning of labels used to forward traffic between and through them.
LSRs establish Label Switched Paths (LSPs) through a network by mapping network-layer routing information directly to data-link layer switched paths. These LSPs may have an out-segment to a directly attached neighbor (comparable to IP hop-by-hop forwarding), or to a network egress node, enabling switching via all intermediary nodes.
LDP associates a Forwarding Equivalence Class (FEC) with each LSP it creates. The FEC associated with an LSP specifies which packets are "mapped" to that LSP. LSPs are extended through a network as each LSR "splices" incoming labels for a FEC to the outgoing label assigned to the next hop for the given FEC.
LDP-ECMP supports up to eight next-hop LSRs for LDP routes.
LDP over parallel links provides the ability to form multiple LDP sessions with the same LSR across different interfaces by configuring unique label-space-ids per interface.
LDP Peers
Two adjacent routers running LDP become LDP peers. There are two kinds of LDP peers:
- Link peers are directly connected to the local LSR and are discovered using LDP Link Hellos sent to the well-known LDP discovery port for the "all routers on this subnet" group multicast address.
- Targeted peers are non-directly connected to the local LSR and are discovered using LDP Targeted Hellos sent to a specific address.
FECs
A Forwarding Equivalence Class (FEC) is an IP prefix and CIDR mask used to match IP packet destination addresses to specific Label Switch Path (LSP) admission. From a forwarding perspective, packets within a FEC are all sent to the same next hop, even if their final destinations are very disparate. In other words, the mapping from the network header layer to the forwarding table is many-to-one.
For example, a set of unicast packets whose destination addresses match a specific IP address prefix can be routed as a FEC.
One of the elements for each entry in the forwarding table is the address of the next hop router. Packets associated with a particular FEC that is in turn associated with a particular forwarding entry are forwarded to the next hop router specified by the entry.
FECs permit a wide range of granularity in forwarding. On the "coarse" end of the range, a FEC could include all packets whose network layer destination address matches a particular address prefix. At the "fine" end of the range, a FEC could include only those packets with the same transport layer port numbers (identify a specific application) as well as the same source and destination network layer addresses.
Compliance
For this release, the Avici router functions only as a midpoint for LDP signalled LSPs.
This implementation of LDP complies with RFC 3036 LDP Specification.
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Copyright © 2004
Avici Systems Inc.
Avici® and TSR®
is a registered trademark of Avici Systems Inc.
IPriori™, Composite Links™, SSR™, QSR, and NSR® are
trademarks of Avici Systems Inc.
Source
File Name: LDP%20Commands.fm
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Last Updated: 12/19/04 at 15:00:33