This chapter contains LDP (Label Distribution Protocol) configuration examples.

The Label Distribution Protocol (LDP) is a routing protocol used in MPLS technology. The LDP daemon (ldpd) uses NSM services to obtain routing information. Routers send Hello packets to establish Hello Adjacencies with other nearby routers. This opens the way for sessions between routers to be established during which routers exchange labels in preparation for forwarding packets.

LDP generates labels for and exchanges labels between peer routers. It works in with other routing protocols (RIP, OSPF and BGP) to create label-switched paths (LSP) used when forwarding packets. A label-switched path is the path taken by all packets that belong to the Forwarding Equivalence Class (FEC) corresponding to that LSP. This is analogous to establishing a virtual circuit in ATM (Asynchronous Transfer Mechanism). In this way, OcNOS LDP assigns labels to every destination address and destination prefix provided by OcNOS. The LDP interface to the MPLS forwarder adds labels to, and deletes labels from, the forwarding tables.

LDP defines a mechanism for discovering adjacent, LDP capable Label Switching Routers (LSR) that participate in label switching (adjacencies). Whenever a new router comes up it sends out a hello packet to a specified, multicast address announcing itself to the network. Every router directly connected to the network receives the packet. Receipt of a hello packet from another LSR creates a Hello Adjacency with that LSR. To create a Hello Adjacency with an LSR that cannot send/receive multicast packets, LDP allows a router to be manually configured to send unicast Hello packets to non-multicast LSRs . This non-multicast LSR is a targeted peer. Adjacencies are maintained by sending out periodic Hello packets to the multicast group and to all targeted peers. Hello packets are sent using UDP.

LDP capable LSRs establish a session before exchanging label information. All the session messages are sent using TCP to ensure reliable delivery. After the LSRs establish a session and negotiate options, a given pair of routers may exchange label information. The labels exchanged over a session are valid only during the lifetime of the session and routers release them when session is closed.

A Forwarding Equivalence Class (FEC) section defines a set of packets that are forwarded on the same path by the MPLS network. Two common methods to define FEC are by advertising the IPv4 routes using:

An LDP Label is a 20-bit number the LSR uses to forward a packet to its destination. When an LSR creates a new FEC, the router generates new labels and distributes them to its peers. A router keeps both incoming and outgoing labels in its database.

The OcNOS LDP implementation supports two label distribution modes:

The OcNOS LDP implementation supports two label retention modes:

LSPs can be set up in the following two ways:

Loop detection can be enabled to detect routing loops in LSPs. There are two methods supported for the loop detection mechanism:

The use of these messages ensures that a loop is detected while establishing a label switched path and before any data is passed over that LSP.

The enable-ldp ipv4 command is used to enable LDP for IPv4 on a specified interface, as follows:

For the examples covered in this section, the command enable-ldp ipv4 is used.

Running LDP on a system requires the following tasks:

LDP MD5 configuration enables LDP MD5 password authentication on a per-peer basis.

In this example, MD5 authentication is configured for a direct LDP session.

In this example, MD5 authentication is configured for the targeted LDP session established between R1 and R3.

This example shows removing the MD5 authentication configuration from an LDP session.

This example shows the number of configured LDP basic neighbors and targeted neighbors count.

This example shows forwarding table entries, SWAP entries and POP entries for IPV4 and IPV6 prefixes.

This example shows MPLS ping and trace route for LDP

LDP Session Protection is an optimization feature. It is used when directly connected LDP peer sessions (via multicast) become unavailable but still have IP reachability over a different path. LDP bindings are kept in the LIB to save time from full synchronization when the direct connections comes back up.

There are two types of LDP connections:

By default if the directly connected LDP session loses connectivity to its peer, all bindings are flushed from the LIB. When interfaces come up and LDP sessions are re-established, LDP has to synchronize its label bindings.

Enable session protection for indirect link failures to protect the labels until the session is active when multicast adjacency gets deleted. When multicast and TLDP connection exists between the same peer, the interface down notification is received only on the router with the link failure but not in the peer. When the interface goes down the multicast adjacency and the labels get deleted without notifying the peer device. When the interface comes up before the adjacency hold timer expires in the peer node, multicast adjacency is added and advertises the labels to the peer but does not receive any labels from the peer as there is no change in the adjacency.

LDP Session Protection is an optimization, when enabled, will not flush the LIB when direct LDP sessions go down. As long as there exists another path to the LDP Peer, it will maintain the LIB synchronized using Targeted LDP Session. IGP will cause a reroute, but the label bindings will still be present from the old peer. When interfaces come back up, LDP will not need to synchronize since it maintains the state using the targeted sessions.

Verify that session protection status is not shown when session protection not enabled.

Configure session protection under LDP in both nodes.

Verify that session protection status shown once session protection enabled in both peer nodes.

Perform Link failure and check labels are retained until peer is reachable through alternate path.

Bring up the link and check same labels reused.