Examples of SR-TE LSP Path Label Stack Reduction

Figure 1 illustrates a metro aggregation network with three levels of rings for aggregating regional traffic from edge ring routers into a PE router.

Figure 1. Label Stack Reduction in a 3-Tier Ring Topology

The path of the highlighted LSP uses admin groups to force the traffic eastwards or westwards over the 3-ring topologies such that it uses the longest path possible. Assume all links in a bottom-most ring1 have admin-group=east1 for the eastward direction and admin-group=west1 for the westward direction.

Similarly, links in middle ring2 have admin-group=east2 and admin-group=west2 and links in top-most ring3 have admin-group=east3 and admin-group=west3. To achieve the longest path shown, the LSP or path should have an include statement: include east1 west2 east3. The fully explicit path computed in Phase 1 of CSPF results in label stack of size 18.

The label stack reduction algorithm searches for the farthest node SID in that path which can replace a segment of the strict path while maintaining the stated admin-group constraints. The reduced label stack contains the SID adjacency MSR1-MSR2, the found node SIDs plus the node SID of the destination for a total of four labels to be pushed on the packet (the label for the adjacency MSR1-MSR2 is not pushed):

{N-SID MNR2, N-SID of MNR3, N-SID of MJR8, N-SID of PE1}

Figure 2 illustrates an example topology which creates two TE planes by applying a common admin group to all links of a given plane. There are a total of four ECMP paths to reach PE2 from PE1, two within the red plane and two within the blue plane.

Figure 2. Label Stack Reduction in the Presence of ECMP Paths

For a SR-TE LSP from PE1 to PE2 which includes the red admin-group as a constraint, Phase 1 of CSPF results in two fully explicit paths using adjacency SID of the red TE links:

path 1 = {PE1-P1, P1-P2, P2-P3, P3-PE2}

path 2 = {PE1-P1, P1-P4, P4-P3, P3-PE2}

Phase 2 of CSPF finds node SID of P3 as the farthest hop it can reach directly from PE1 while still satisfying the ‛include red’ admin-group constraint. If the node SID of PE2 is used as the only SID, then traffic would also be sent over the blue links.

Then, the reduced label stack is: {P3 Node-SID=300, PE2 Node-SID=20}.

The resulting SR-TE LSP path combines the two explicit paths out of Phase 1 into a single path with ECMP support.