Detailed operation of LFA protection using backup node SID

As shown in Figure: Backup ABR node SID, LFA for seamless MPLS supports environments where the boundary routers are either:

Figure: Backup ABR node SID

The following steps describe the configuration and behavior of LFA Protection using Backup Node SID:

  1. The user configures node SID 100 in ABR1 for its loopback prefix 1.1.1.1/32. This is the regular node SID. ABR1 advertises the prefix SID sub-TLV for this node SID in the IGP and installs the ILM using a unique label.

  2. Each router receiving the prefix sub-TLV for node SID 100 resolves it as described in Segment routing in shortest path forwarding. Changes to the programming of the backup NHLFE of node SID 100 based on receiving the backup node SID for prefix 1.1.1.1/32 are defined in Duplicate SID handling.

  3. The user configures a backup node SID 200 in ABR2 for the loopback 1.1.1.1/32 of ABR1. The SID value must be different from that assigned by ABR1 for the same prefix. ABR2 installs the ILM, which performs a swap operation from the label of SID 200 to that of SID 100. The ILM must point to a direct link and next hop to reach 1.1.1.1/32 of ABR1 as its primary next hop. The IGP examines all adjacencies established in the same area as that of prefix 1.1.1.1/32 and determines which ones have ABR1 as a direct neighbor and with the best cost. If more than one adjacency has the best cost, the IGP selects the one with the lowest interface index. If there is no adjacency to reach ABR2, the prefix SID for the backup node is flushed and is not resolved. This is to prevent any other non-direct path being used to reach ABR1. As a result, any received traffic on the ILM of SID 200 traffic is blackholed.

  4. If resolved, ABR2 advertises the prefix SID sub-TLV for this backup node SID 200 and indicates in the SR Algorithm field that a modified SPF algorithm, referred to as ‟Backup-constrained-SPF”, is required to resolve this node SID.

  5. Each router receiving the prefix sub-TLV for the backup node SID 200 performs the following steps:

    Note: The following resolution steps do not require a CLI command to be enabled.

    1. The router determines which router is being backed up. This is achieved by checking the router ID owner of the prefix sub-TLV that was advertised with the same prefix but without the backup flag and which is used as the best route for the prefix. In this case, it should be ABR1. Then the router runs a modified SPF by removing node ABR1 from the topology to resolve the backup node SID 200. The primary next hop should point to the path to ABR2 in the counter clockwise direction of the ring.

      The router does not compute an LFA or a remote LFA for node SID 200 because the main SPF used a modified topology.

    2. The router installs the ILM and primary NHLFE for the backup node SID.

      Only a swap label operation is configured by all routers for the backup node SID. There is no push operation, and no tunnel for the backup node SID is added into the TTM.

    3. The router programs the backup node SID as the LFA backup for the SR tunnel to node SID of 1.1.1.1/32 of ABR1. In other words, each router overrides the remote LFA backup for prefix 1.1.1.1/32, which is normally PQ node AGN5.

    4. If the router is adjacent to ABR1, for example AGN1, it also programs the backup node SID as the LFA backup for the protection of any adjacency SID to ABR1.

  6. When node AGN2 resolves a BGP label route for an inter-area prefix for which the primary ABR exit router is ABR1, it uses the backup node SID of ABR1 as the remote LFA backup instead of the SID to the PQ node (AGN5 in this example) to save on the pushed label stack.

    AGN2 continues to resolve the prefix SID for any remote PE prefix that is summarized into the local area of AGN2 as usual. AGN2 programs a primary next hop and a remote LFA next hop. Remote LFA uses AGN5 as the PQ node and pushes two labels, as it would for an intra-area prefix SID. There is no need to use the backup node SID for this prefix SID and force its backup path to go to ABR1. The backup path may exit from ABR2 if the cost from ABR2 to the destination prefix is shorter.

  7. If the user excludes a link from LFA in the IGP instance (config>router>ospf>area>interface>loopfree-alternate-exclude), a backup node SID that resolves to that interface is not used as a remote LFA backup in the same way as regular LFA or PQ remote LFA next hop behavior.

  8. If the OSPF neighbor of a router is put into overload or if the metric of an OSPF interface to that neighbor is set to LSInfinity (0xFFFF), a backup node SID that resolves to that neighbor is not used as a remote LFA backup in the same way as regular LFA or PQ remote LFA next hop behavior.

  9. LFA policy is supported with a backup node SID. See Application of LFA policy to backup node SID tunnel.

Parent topic: LFA protection using segment routing backup node SID