Label Distribution Protocol LDPitle

Label Distribution Protocol (LDP) is a protocol in which two Label Switch Routers (LSR) exchange label mapping information. The two LSRs are called LDP peers and the exchange of information is bi-directional. LDP is used to build and maintain LSR databases that are used to forward traffic through MPLS networks.

LDP can be used to distribute the inner label (VC/VPN/service label) and outer label (path label) in MPLS. For inner label distribution targeted LDP (tLDP) is used. LDP and tLDP discovery runs on UDP port 646 and the session is built on TCP port 646. During the discovery phase hello packets are sent on UDP port 646 to the ‘all routers on this subnet’ group multicast address (224.0.0.2). However, tLDP unicasts the hello packets to the targeted neighbor’s address.

Configuration

Label Distribution Protocol (LDP) is used between MPLS’s nodes to establish and maintain the label bindings. For MPLS to operate correctly, label distribution information needs to be transmitted reliably, and the label distribution protocol message pertaining to a particular FEC need to be transmitted in sequence. Flow control is also desirable. One way to meet these goals is to use TCP as the underlying transport, as is done in MPLS-LDP and MPLS-BGP. More than one LDP can be used.

In traffic engineering applications, it is desirable to set up explicit routed paths, from ingress to egress, including also resource reservation along that path. This can be done by:

Starting with an existing protocol used for setting up resource reservations, and extend it to support explicit routing and label distribution. This approach is specified in the protocol [MPLS-RSVP-TUNNELS].

Starting with an existing protocol used for label distribution, and extend it to support explicit routing and resource reservation. This approach is specified in [MPLS-CR-LDP].

CR-LDP is an extension of LDP that adds signaling capabilities almost identical to those of RSVP-TE. It isn’t in common use – not because there’s anything wrong with the protocol, but simply because the two MPLS vendors dominating the market, Cisco and Juniper, chose to support RSVP-TE exclusively. I won’t write more about CR-LDP unless it comes back into vogue, which is highly unlikely.

MPLS routers build a Label Information Base (LIB) and a Label Forwarding Information Base (LFIB) on each router and exchange information via LDP to determine which path to forward labeled traffic across.

Label Distribution Protocol

R2(config)# mpls label protocol ldp

Use to configure the use of LDP on all interfaces.

The global mpls label protocol ldp command sets the default label distribution protocol for all interfaces to be LDP.

Verification

# show mpls ldp neighbor

Use to display the status of Label Distribution Protocol (LDP) sessions Use this command in privileged EXEC mode. Use the keyword address to identify the neighbor with this IP address. Use the keyword interface to define the LDP neighbors accessible over this interface. This command displays information about all LDP neighbors if no optional keywords or arguments are specified.

# show mpls ldp bindings

Use to display the contents of the label information base (LIB). Use this command in privileged EXEC mode. A request can specify that the entire database be displayed, or that the display be limited to a subset of entries according to the Prefix, Input or output label values or ranges and Neighbor advertising the label. If no optional keyword or argument is supplied, the command displays the entire LIB.

Troubleshoot

# debug mpls ldp peer state-machine

Use to display information about state transitions for Label Distribution Protocol (LDP) sessions.

Use this command in privileged EXEC mode. T o disable debugging output, use the no form of this command. LDP manages peer sessions by means of the following two coupled state machines: o A low-level state machine that deals with session establishment and shutdown.  o A high-level state machine that deals with setting up and shutting down label advertisement

# debug mpls ldp messages sent

Use to display LDP messages sent to LDP peers permitted by the access control list. Use this command in privileged EXEC mode. To disable debugging output, use the no form of this command. All messages sent (for the sent keyword) or received (for the received keyword) are displayed except for periodic keep alive messages.

R2 conf t

# mpls label protocol ldp

# interface e0/1

# mpls label protocol tdp

R3 conf t

# interface e0/2

# mpls label protocol tdp