LDP over TE

Resource Reservation Protocol (RSVP) Traffic Engineering (TE) is MPLS tunneling technology that generate LSPs to transparently transmit data packets. LDP is another MPLS tunneling technology and generates LDP LSPs. LDP over TE allows an LDP LSP to span an RSVP TE area, with the TE tunnel is as a hop of the LDP LSP.

After a TE tunnel is established, the IGP (such as OSPF or IS-IS) running on the devices at both ends of the tunnel can select the TE tunnel as the route outbound interface through local routing calculation or by advertising link state advertisements (LSAs). The source and destination devices can be considered directly connected through the TE tunnel interface (logical interface). Actually, packets are transparently transmitted over the TE tunnel.

LDP over TE Tunnel Setup

As shown in Figure 2-5, P1, P2, and P3 are located in an RSVP TE area. LDP runs between PE1 and P1 and between P3 and PE2. The LDP LSP over an RSVP TE tunnel from PE1 and PE2 is used as an example to describe how an LDP LSP is set up.

Figure 2-5 LDP over TE tunnel setup

The LDP LSP setup process is as follows:

1. An RSVP TE tunnel from P1 to P3 is established. P3 allocates RSVP-Label-1 to P2, and P2 allocates RSVP-Label-2 to P1.
2. A remote LDP session is established between P1 and P3.
3. Forwarding adjacency is enabled on P1 and P3 to ensure that traffic from P1 to P3 and PE2 is forwarded through the tunnel interface.
4. PE2 triggers LDP LSP setup and sends a Label Mapping message to P3. The Label Mapping message carries the label LDP-Label-1.
5. After receiving the Label Mapping message, P3 sends a Label Mapping message to P1 through the remote LDP session. The Label Mapping message carries LDP-Label-2.
6. After receiving the Label Mapping message, P1 sends a Label Mapping message to PE1. The Label Mapping message carries LDP-Label-3.

The LDP LSP from PE1 to PE2 across the RSVP TE area is set up.

Application Scenario

As shown in Figure 2-6, MPLS L2VPN or L3VPN needs to be deployed on the PEs to connect the VPN sites. RSVP TE needs to be deployed on the entire network to ensure smooth VPN traffic forwarding; otherwise, congestion may occur on some nodes. However, network-wide RSVP TE deployment and maintenance are complex. Besides, some PEs may not support RSVP TE. LDP over TE allows an LDP LSP to span an RSVP TE area, with the TE tunnel is as a hop of the LDP LSP. You can define a core RSVP TE area and deploy RSVP TE in the area and LDP outside the area.

Figure 2-6 LDP over TE networking

Advantages of LDP over TE: Compared with RSVP TE, the LDP LSP is easier to configure and maintain. In addition, LDP consumes less system resources than RSVP TE. With LDP over TE, you only need to deploy RSVP TE in the core area, without establishing TE tunnels between every two PEs. This technology simplifies the deployment and maintenance, and reduces the burden of PEs. In the core area, the advantages of RSVP TE in terms of protection switching, path planning, and bandwidth protection can be fully used.