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MPLS Label Distribution Protocol, LDP – Part 1

Category: Alcatel-Lucent, LDP, MPLS
Author: gokhankosem, on 12 Jul 15 - 0 Comments
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MPLS Label Distribution Protocol, LDP

LDP (Label Distribution Protocol) is one of the two label exchange protocols of MPLS. The other is RSVP (Research Reservation Protocol). And there is also one more, Cisco specific protocol, TDP(Tag Distribution protocol). In this article, we will talk abbout the LDP. And the examples will be also for Alcatel-Lucent Service Routers(7750, 7450 etc..).

There are 3 types LDPs. These are:

– Link LDP
– Targeted LDP (TLDP)
– Contraint-based Routing LDP (CR-LDP)

transport tunnel and service tunnel, ldp and tldp

Transport Tunnel and Service Tunnel, LDP and TLDP

Link LDP is used to establish MPLS Transport Tunnels. But, TLDP is used for MPLS Service Tunnels. Lets explain it more detailly.

Think about MPLS VPNs. For MPLS we need to establish an MPLS Transport Tunnel through the core network (P routers) between PE routers. This is established via LDP. Inside this MPLS Transport Tunnel, we carry specific customers traffic. In Alcatel-Lucent terminalogy, this is called different customer services. So, this different customers’ traffic is carried in different MPLS Service Tunnels in Transport Tunnels. And these MPLS Service Tunnels are established via TLDP, between PE routers, for VPLS and VPWS services(L2 VPN Services). For VPRN (L3 VPN Service), MP-BGP is used instead of TLDP. For this lesson also check Tunnel Structure article.

Contraint-based Routing LDP (CR-LDP) is the extension of LDP that will provide traffic engineering mechanism to LDP. As you know pure LDP has no traffic engineering mechanism.

Link LDP

For Link LDP, all the directly connected neighbours need to establish LDP session each other. This must be a full mesh topology. Firstly LDP Adjacency must be done, then the LSP Session must be established. By doing this, the label distribution can be done for all the routers.

To create and maintain LDP there are 4 processes. These are:

1) Peer Discovery,
2) Session Establishment,
3) Label Management (Distribute label management)
4) Notification ( Alerting LDP peering routers about errors)

LDP session establishment, LDP messages

LDP Session Establishment and LDP Messages

Peer Discovery

The Peer Discovery is the first process and it is done via Hello Messages. We can call this process also LDP Adjacency. Hello messages are sent each other with the peer parameters.

Hello Messages use UDP port 646. Session establishment, Label Management and Notification use TCP port 646.Hello messaged send every 5 seconds by default. This can be changed. Even the configured Hello time values are different, LDP neighbour discovery is done. The lower Hello value is used.
After the Hellos are sent each other, the LDP adjacency is established.

LDP Session Establishment

After LSP adjacency, “Init messages” are sent each other. This messages use TCP 646 port. In this messages, the Session Parameters are sent. Then Keepalive Messages are sent each other and the LSP Session is established. Keepalive Messages are still sent after the LDP Session Establishment.

There are 4 categories of LDP messages :

– Discovery messages (periodically announced and maintain the precence of LDP router)
– Session messages (establish, maintain and terminate sessions)
– Advertisement messages (create,change and delete label mappings for FECs)
– Notification messages (sent for errors)

For LDP, firstly a routing protocol configuration must be done between the peers or a static configuration must be used. Basically the points must be reach eachother. This is generally done by a routing protocol(IGP).

LDP relies on routing protocol (IGP). This means that it is dependant to routing protocol (IGP). It sends the label according to choose of routing protocol. If a problem occurs because of routing protocol, the network needs to be wait for the convergence of IGP. LDP is easy to configure bu IGP depandancy is the weakness of this protocol.

MPLS Label Distribution Protocol, LDP – Part 1
MPLS Label Distribution Protocol, LDP – Part 2
MPLS Label Distribution Protocol, LDP – Part 3 (Configuration on ALU)
MPLS Label Distribution Protocol, LDP – Part 4 (ECMP)
MPLS Label Distribution Protocol, LDP – Part 5 (Export and Import Policy, Prefix Aggregation)
MPLS Label Distribution Protocol, LDP – Part 6 (T-LDP)
MPLS Label Distribution Protocol, LDP – Part 7 (CR-LDP)

For more information about LDP, T-LDP and CR-LDP, check the related following RFCs…

RFC 5036: LDP Specification
RFC 3815: Definitions of Managed Objects for the MPLS,LDP
RFC 3478: Graceful Restart Mechanism for Label Distribution Protocol
RFC 5443: LDP IGP Synchronization
RFC 7307: LDP Extensions for Multi-Topology
RFC 7361: LDP Extensions for Optimized MAC Address Withdrawal in a H-VPLS
RFC 3212: Constraint-Based LSP Setup using LDP
RFC 3213: Applicability Statement for CR-LDP 
RFC 3214: LSP Modification Using CR-LDP


…YOU CAN REACH ALL THE “MPLS” ARTICLES AND CONFIGURATIONS BELOW…

MPLS Basics
What is MPLS?
Enabling MPLS on Cisco Router

MPLS Label Distribution Protocol, LDP
MPLS Label Distribution Protocol, LDP – Part 1
MPLS Label Distribution Protocol, LDP – Part 2
MPLS Label Distribution Protocol, LDP – Part 3 (Configuration on ALU)
MPLS Label Distribution Protocol, LDP – Part 4 (ECMP)
MPLS Label Distribution Protocol, LDP – Part 5 (Export and Import Policy, Prefix Aggregation)
MPLS Label Distribution Protocol, LDP – Part 6 (T-LDP)
MPLS Label Distribution Protocol, LDP – Part 7 (CR-LDP)

MPLS, RSVP-TE
MPLS, RSVP-TE – Part 1
MPLS, RSVP-TE – Part 2(Alcatel-Lucent Configuration)

MPLS Protection And Restoration
MPLS Protection and Restoration – Part 1
MPLS Protection and Restoration – Part 2 (End-to-End Protection, Secondary Paths)
MPLS Protection and Restoration – Part 3 (Path Diversity in Secondary Paths)
MPLS Protection and Restoration – Part 4 (Local Protection, Fast Reroute)
MPLS Protection and Restoration – Part 5 (Fast Reroute Protection Types)
MPLS Protection and Restoration – Part 6 (RSVP Objects Used for MPLS Fast Reroute)
MPLS Protection and Restoration – Part 7 (Verification Commands on Alcatel-Lucent Routers)
MPLS Protection and Restoration – Part 8 (Actions After the Failure)
MPLS Protection and Restoration – Part 9 (Convergence Factors)
MPLS Protection and Restoration – Part 10 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 11 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 12 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 13 (Juniper Configuration Samples)


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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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