LDP

Overview

LDP is used to distribute labels in non-traffic-engineered MPLS applications. Routers can establish LSPs across a network by mapping network-layer routing information directly to the data link layer switched paths. After the LDP distributes the labels to the LSR, the LSR assigns the label to an FEC, and then informs all other LSRs in the path about the label and how the label switches data accordingly.

An FEC is a collection of common actions associated with a class of packets. LDP helps establish the LSP by providing a set of procedures that LSRs can use to distribute labels.

When a service tunnel is configured on managed routers using LDP signaling in an MPLS environment, LDP sessions are set up based on the configured hello and other PDU values. If another service tunnel is created to the same destination, the LDP session is reused.

The LDP sessions between LSRs:

• find and establish LDP peers in the managed network

• exchange label mappings for each LSR

• exchange label bindings

After all the LSRs are LDP-aware and the LSP is created, forwarding can occur as follows:

1. An FEC is associated with the LSP.

2. The FEC maps the packets to the LSP.

3. The next LSR that is part of the LSP splices incoming FEC labels to the outgoing FEC label of the next hop.

There are three types of LDP:

• T-LDP

• LDP DU

• LDP DoD

T-LDP is used to distribute labels for VLL and VPLS. T-LDP allows the targeting of remote devices that are not directly connected as targeted peers.

LDP DU can be used to create tunnels between PEs for IP-VPN services.

LDP DoD allows LDP peers to request label bindings for specific FECs.

The following figure shows an example of LDPs that are used in a simple Layer 2 and Layer 3 service provider network.

Figure 28-2: LDP sample network

• The solid straight lines between the devices indicate IP connectivity. These are directly connected peers.

• The dotted bidirectional curved lines indicate T-LDP sessions. These are targeted peers that are not directly connected.

• The solid bidirectional curved lines indicate LDP DU sessions. This example only shows two instances, between M and Z, and Z and K. If there is IP connectivity between all the devices, then all of the devices have LDP DU sessions.

Provider edge router Z advertises the labels for its address 1.1.1.10 to adjacent link state devices M and K. Routers M and K distribute the labels for that address to the rest of the network.

If provider edge router A wants to send a VPN-labeled packet to router Z, it uses label 61 as the outer label. When the packet reaches router N, outer label 61 is swapped for outer label 52 and the packet continues downstream to router M. Router M then swaps out label 52 for either outer label 31 or 42, depending on the router M label selection algorithm. If outer label 31 is selected, the packet reaches router Z directly, which then continues to route the packet to the customer site based on the VPN label.

If a router receives more than one request for an IP address, and the router does not support ECMP, it selects the first label it receives. If the router supports ECMP, it selects the label with the lowest cost path, or selects both labels if the cost paths are identical.

When a router supports ECMP, LSRs can have multiple equal cost paths to an IP address. ECMP LDP retains all labels it receives from multiple next hop peers. The forwarding plane contains multiple next hops for an FEC and as a result, provides load balancing for LDP-based LSPs.

When a router supports ECMP LDP and a device configured as the next hop is no longer valid, for example a session between peers is lost or the peer withdraws its label, a new valid LDP next hop peer is selected and the forwarding plane is updated.

Load balancing across LSPs for LDP over RSVP is also supported. When ECMP is enabled, all equal cost LSP endpoints are installed in the routing table by the IGP for consideration by LDP. LDP selects the LSP with the lowest LSP metric to determine the next hop. If multiple LSPs are available with equal cost, then ECMP is utilized until the ECMP count is exhausted. An LDP tree is created by sending LSP Trace messages along an ECMP path to downstream LSR NEs.

Weighted ECMP is supported for fine control of load balancing, especially where capacity of local links is unequal or variable.

LDP for P2MP

LDP support for P2MP can be enabled on an LDP interface. You can configure an LDP interface for multicast traffic forwarding towards a downstream NE. LDP configuration allows an exchange of P2MP FEC via an established session to a peer, and the use of next hops over an interface.

LDP configuration is supported on a tunnel interface under a base routing instance and under protocols such as PIM and IGMP. A P2MP ID is used as an index for LDP-based tunneling instead of an LSP ID.

Note: BFD and source redundancy are only supported on an RSVP tunnel interface.