Detour Merge Point

Detour Merging

 With FRR one-to-one protection, each router creates a separate

detour tunnel for each protected LSP.

 This can lead to a large number detour tunnels in the network.

 Detour Merging was introduced in RFC 4090 to help the issue.

 If multiple detour tunnels protecting the same LSP-Path use the

same outgoing link on a router, they are merged.

 The router that performs the merging operation is called a Detour

Merge Point (DMP).

 A single PATH message, consisting of a list of the detour objects of

the merged detour tunnels, is sent beyond the DMP.

 Detour Merging is a default response and cannot be disabled.

The detour calculated by router R1 is already established, as described on the previous page.

Assuming the PLR role, router R2 also calculates a detour and sends a PATH message to signal it. The Tunnel_ID, LSP_ID,

and LSP_Name fields are identical to those of the detour originated by router R1. This is a clear indication that both

detours are protecting the same LSP-Path. The detour originated by router R2 will also egress on the interface R6-R7.

Realizing this fact, router R6 terminates the detour from router R2 on itself and sends a single PATH message to router

R7.

The Detour Object included in the PATH message sent to router R7 contains the PLR_ID and Avoid_Node fields from both

merged detours.

Router R6 is named as a Detour Merge Point in the RFC terminology, after the merge operation.

Traffic is forwarded on the original primary LSP-Path.

 The detour tunnels are also established on the routers and are

ready to be activated, in case of a failure.

Router R2 detects that the connection to router R3 is lost (it could

be the result of a link or total nodal failure on router R3).

 Router R2 performs a label swap from the original primary LSP-Path

to the detour tunnel (from 200 to 60).

When a failure occurs on the link between R2-R3, or if router R3 fails, router R2 will detect the failure along the

primary LSP-Path.

Router R2 has already established a detour tunnel to protect against such failures and is responsible for locally

recovering the traffic. Therefore, router R2 switches the traffic coming in on the primary LSP-Path to the detour

protection tunnel.

The label switching operation is depicted above with simplified label values. 60, 70, 80 and 40 are the label values

previously signaled by RSVP-TE on the detour tunnel.

After the failure, router R2 swaps the incoming (outer) RSVP-TE label of 200 on the primary path with 60 on the detour

path.

Routers R6, R7 and R8 also perform label swapping along the detour path.

Router R4 is the Merge Point for the detour tunnel. Router R4 is also the terminating point for the original protected

LSP-Path.

Therefore, the detour tunnel label 40 is popped, the correct service is identified by the VPN label and the original Data

packet is delivered to the customer end.

Note that the depth of the label stack does not change at any point along the forwarding path.