Practical Applications: Label Distribution | Rick Gallahers MPLS Training Guide: Building Multi Protocol Label Switching Networks

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Hundreds of pages worth of forum comments have been written about label distribution methods, including OSPF-TE and LDP.

The LDP protocol is standardized as detailed in RFC 3036. To obtain detailed vendor explanations and commands, contact Cisco, Juniper, and Riverstone.

In this section, we take a look at how to establish the LDP protocol on a Riverstone router and how to show LDP status. For other vendors, see the related links at the end of the chapter.

Note

With some vendors, RSVP and LDP protocols may not be enabled on the same interfaces.

Configuration Steps

The configuration of LDP will vary in accordance with how a router is configured. If interfaces have already been created with routing protocols, and MPLS is running, the next step is configuring the LDP protocol. The process of doing so is explained in this section.

If you must configure LDP on an interface that is yet to be created, four basic steps are involved. They are outlined here and described in the following sections:

Create and enable the interface.
Create OSPF on the interfaces.
Create and enable MPLS on the new interfaces.
Create and enable LDP.

Enabling the Label Distribution Protocol

LDP, works very differently from RSVP-TE. In the case of LDP, simply enabling the protocol on the required interfaces will allow the routers to discover directly connected label distribution peers via multicast UDP packets and, subsequently, to establish a peering relationship over TCP.

Each router will create and distribute a label binding to an FEC for each loopback interface that is defined in the router. Each physical interface that is expected to interpret and function in an LDP environment must be added to it. To enable LDP capabilities on an interface, simply add it to the LDP process.

rs(config)# ldp add interface <name|all>

To start the LDP process on the router:

rs(config)# ldp start

The loopback interface is added automatically when all interfaces are added to LDP as a group. This interface is required to establish remote LDP peering sessions, and if the all option is not used to add the interfaces to LDP, the lo0 interface must be explicitly added.^[1]

Configuring New Interfaces with Show Commands

We use Figure 2.9 to show the configuration of LDP from LER1 (far left) to LSR 1 and LSR 2. The four basic steps that were detailed above are explained along with the related show commands.

click to expand
Figure 2.9: Full Network Diagram

IGP, MPLS, and LDP are only enabled on the core-facing interfaces. This network and the associated configuration form the basis for show commands that follow.

Create and Enable the Interface

The interface is created by following the commands shown in Figures 2.10–2.12. In Figure 2.10, the interface with the IP address 192.168.1.2/30 is created.

Figure 2.10: Creating the Interface

Figure 2.11 shows the creation of the second interface on router LER1 for the 192.168.1.6/30 address. In Figure 2.12, the interface LoO with an address of 2.2.2.1 is created.

Figure 2.11: Detailed View of LER1 Interface gi 2.2

Figure 2.12: Detailed View of LER1 Local Interface Lo 0Create OSPF on Interfaces

Figures 2.13 and 2.14 show how OSPF is created in the routers. In Figure 2.13, the router ID is set; in Figure 2.14, OSPF is added to the interfaces and OSPF is started.

Figure 2.13: LER1 Global Routing

Figure 2.14: LER1 Enable OSPF

Create and Enable MPLS on New Interface

Figure 2.15 shows how MPLS is added to the interfaces LSR1 and LSR2. After MPLS is added, it must be started by a command line.

Figure 2.15: MPLS Added to Interface

Create and Enable LDP

After MPLS is added, LDP must be added to each MPLS interface. Figure 2.16 shows how LDP is added to the interfaces and then started.

Figure 2.16: LDP Started

LDP Session Information

High-level session information for each peer is shown in Table 2.6.

Table 2.6: Possible Session States
Possible Session States	Description
Nonexistent	No session exists
Connecting	TCP connection is in progress
Initialized	TCP connection established
OpenSent	Initialization or keepalive messages being transmitted
OpenRec	Keepalive message being transmitted
Operational	Session established
Closing	Closing session

RS# ldp show sessions all

Address of peer
State of the session
TCP connection state (closed, opening, open)
Time before session expires without a keepalive
Number of labels sent and received and received labels filtered

The LDP show session command is very powerful with several command extensions. Figure 2.17 is a graphic of the network represented by the show LDP session command.

Figure 2.17: Show LDP Sessions

Other Show Commands

LDP show sessions all verbose

A more detailed view of the session information is available by coding the verbose option. The following additional information is available with this option:

Session ID (comprising both LDP identifiers)
Timer values
Local and remote physical interfaces
A list of LDP-enabled interfaces on the remote peer

LDP Neighbor Information

Detailed neighbor information for each peer:

RS# ldp show neighbor verbose

Address where neighbor was discovered and interface used to reach neighbor
Label space ID indicating the LDP identifier and label space the label was issued from, :0 being from the global space
Time before session expires without a keepalive
Transport address used for establishing the session

Figure 2.18 is a graphic representation of the text command show LDP neighbor.

Figure 2.18: Show LDP Neighbor

LDP Statistical Information

Statistical information about the LDP protocol is broken into two horizontal planes, each with a cumulative and a 5-second representation. The tables are self-explanatory. One thing to note: If the statistics are cleared, all the cumulative information is lost, obviously. So when you’re reviewing the statistics following a clear, the “Event Type - Sessions Opened” field may be zero even though there are open sessions. Don’t let this field mislead you into thinking no sessions are formed. The session display command is the authority on session-related information.

RS# ldp show statistics

LDP Interface Information

A detailed view of the LDP interfaces indicates the following for each LDP enabled interface:

RS# ldp show interface all verbose

Label space indicating the LDP identifier and label space the label was issued from, :0 being from the global space
The number of neighbor sessions that exist on this interface
Timer information
Label management: retention and distribution

^[1]Provided by Riverstone.

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