From CT3

By Ivan Pepelnjak

The VPN data forwarding model in MPLS VPN technology requires an end-to-end LSP between each pair of PE-routers. The end-to-end LSP is usually established automatically with LDP (or TDP) and selected based on the VPN route’s next-hop as propagated with MP-BGP.

When you introduce MPLS TE tunnels in MPLS VPN network, the continuous end-to-end LSP must be preserved even when the traffic flows across the MPLS TE tunnel. There are several ways to ensure the continuous end-to-end LSP between PE routers:

• MPLS TE tunnel is created on a PE-router and terminates at another PE-router. The end-to-end LSP required to forward MPLS VPN data is the tunnel LSP (Figure 1).

Figure 1: MPLS TE tunnel

• MPLS TE tunnel is created on a P router, but terminates on a PE-router. The LDP-created LSP from an upstream PE-router is automatically stitched together with the MPLS TE LSP, resulting in a continuous end-to-end LSP (Figure 2).

Figure 2: P-to-PE MPLS TE tunnel

• MPLS TE tunnel is created on a P router or terminates on a P router. Directed LDP session (enabled with the mpls ip interface configuration command on the tunnel head-end) has to be run across the MPLS TE tunnel to ensure continuous LSP across the tunnel (Figure 3).

Figure 3: P-to-P MPLS TE tunnel

If the LDP session is not run across a tunnel ends on a P router, the MPLS VPN packet entering the tunnel will lose the top label when arriving at the tunnel tail end and the tunnel tail end will try to switch the packet based on an MPLS VPN label which was assigned by another router (egress PE-router), resulting in broken MPLS VPN connectivity (Figure 4). The packet might also get propagated into an unpredictable direction.

Figure 4: Data forwarding failure across a P-to-P tunnel with no LDP