Remote LFA Node-Protect Operation

SR OS supports the node-protect extensions to the Remote LFA algorithm as described in RFC 8102.

Remote LFA follows a similar algorithm as TI-LFA but does not limit the scope of the calculation of the extended P-Space and of the Q-Space to the post-convergence paths.

Remote LFA adds an extra forward SPF on behalf of the PQ node to make sure that for each destination the selected PQ node does not use a path via the protected node.

Figure 1 shows a slightly modified topology from that in TI-LFA Feature Interaction and Limitations. A new node R7 is added to the top ring and the metric for link R3-R6 is modified to 100.

Figure 1. Application of the Remote LFA Algorithm for Node Protection

Applying the node protect remote LFA algorithm to this topology yields the following steps:

Compute extended P-Space of R1 with respect to protected node R2.

This is the set of nodes Yi which are reachable from R1 neighbors, other than protected node R2, without any path transiting the protected node R2.

R1 computes a LFA SPF rooted at each of its neighbors, in this case, R7, using the following equation:

Distance_opt(R7, Yi) < Distance_opt(R7, R2) + Distance_opt(R2, Yi)

Where Distance_opt(A,B) is the shortest distance between A and B.

Nodes R7, R3 and R6 satisfy this inequality.
Compute Q-space of R1 with respect to protected link R1-R2.

This is the set of nodes Zi from which node R2 can be reached without any path transiting the protected link R1-R2.

Distance_opt(Zi, R2) < Distance_opt(Zi, R1) + Distance_opt(R1, R2)

The reverse SPF for the Q-space calculation is the same as in the remote LFA link-protect algorithm and uses the protected node R2 as the proxy for all destination prefixes.

This step yields nodes R3, R4, R5, and R6.

Therefore, the candidate PQ nodes after this step are nodes R3 and R6.
For each PQ node found, run a forward SPF to each destination D.

This step is required to select only the subset of PQ nodes which does not traverse protected node R2.

Distance_opt(PQi, D) < Distance_opt(PQi, R2) + Distance_opt(R2, D)

Of the candidates PQ nodes R3 and R6, only PQ node R6 satisfies this inequality.

Note this step of the algorithm is applied to the subset of candidate PQ nodes out of steps 1 and 2 and to which the parameter max-pq-cost was already applied. This subset is further reduced in this step by retaining the candidate PQ nodes which provide the highest coverage among all protected nodes in the topology and which number does not exceed the value of parameter max-pq-nodes.

In case of multiple candidate PQ nodes out of this step, the detailed selection rules of a single PQ node from the candidate list is provided in Step4.
Select a PQ Node.
If multiple PQ nodes satisfy the criteria in all the above steps, then R1 further selects the PQ node as follows:
1. selects the lowest IGP cost from R1
2. If more than one remains, R1 selects the PQ node reachable via the neighbor with the lowest router-id (OSPF) or system-id (ISIS);
3. If more than one remains, R1 selects the PQ node with the lowest router-id (OSPF) or system-id (ISIS).

For each destination prefix D, R1 programs the remote LFA backup path:

For prefixes of R5, R4 or downstream of R4, R1 programs a node-protect remote LFA repair tunnel to the PQ node R6 by pushing the SID of node R6 on top of the SID for destination D and programming a next-hop of R7.
For prefixes owned by node R2, R1 runs the link-protect remote LFA algorithm and programs a simple link-protect repair tunnel which consists of a backup next-hop of R7 and pushing the SID of PQ node R3 on top of the SID for the destination prefix D.
Prefixes owned by nodes R7, R3, and R6 are not impacted by the failure of R2 since their primary next-hop is R7.