MPLS Targeted LDP – tLDP – Label Forwarding

The past few months I’ve received a lot of feedback on my L2VPN Pseudowire blog post. Most questions where related to tLDP (Targeted LDP), PHP (Penultimate Hop Popping), Implicit Null vs. Explicit Null and pseudowire redundancy configurations.

I will address each of the topics listed above in the upcoming posts, so keep your eyes peeled on this blog and stay tuned! These posts will be explained as easy as possible so everybody with basic networking knowledge will be able to understand them.

Today will explain tLDP, also referred to as Directed LDP and how this is implemented with L2VPN pseudowires.

Targeted LDP sessions are LDP sessions between non-directly connected peers. When a pseudowire is configured, the routers will establish an LDP session and exchange the inner “service” or “application” labels, also known as VC labels.

To associate an MPLS packet with a speific pseudowire you need a way of identifying the circuit. An MPLS L2VPN pseudowire is identified with the Pseudowire ID FEC element that contains the identifier shared by the two endpoints. This service label is part of the MPLS label stack and is marked with the S bit to mark it as the bottom label.

The Pseudowire ID FEC currently supports the following transport types:

• 0×001 – Frame Relay DLCI
• 0×002 – ATM AAL5 VCC
• 0×003 – ATM Transparent Cell
• 0×004 – Ethernet VLAN
• 0×005 – Ethernet
• 0×006 – HDLC
• 0×007 – PPP

After establishing the tLDP session and exchanging the service labels the LDP session is considered synchronized. The PE routers are now able to forward the pseudowire traffic by adding the core label on top of the service label to route the traffic to the destination router.

As you know from my previous post each router will exchange core labels with their neighbor routers to create their LFIB (Label Forwarding Information Base)

Please keep in mind the a router will only advertise a pseudowire if the attached circuit (customer interface) is up and running. If the interface goes down an LDP withdraw message is sent and the LSP is considered down.

To summarize:

• A tLDP session exists between the two endpoint routers to exchange the pseudowire ID FEC labels.
• If you provision multiple pseudowires between two routers only one tLDP session will be established.
• Pseudowire circuits have at least two labels. More if MPLS-TE and QoS are configured.
• Pseudowires are only advertised if the attached interface is up and runing.