To expand the coverage of the LFA backup protection in a network, RSVP LSP based IGP shortcuts can be placed selectively in parts of the network and be used as an LFA backup next-hop.
When IGP shortcut is enabled in IS-IS or OSPF on a specific node, all RSVP LSP originating on this node and with a destination address matching the router-id of any other node in the network are included in the main SPF by default.
To limit the time it takes to compute the LFA SPF, the user must explicitly enable the use of an IGP shortcut as LFA backup next-hop using one of a couple of new optional argument for the existing LSP level IGP shortcut command:
config>router>mpls>lsp>igp-shortcut [lfa-protect | lfa-only]
The lfa-protect option allows an LSP to be included in both the main SPF and the LFA SPFs. For a specific prefix, the LSP can be used either as a primary next-hop or as an LFA next-hop but not both. If the main SPF computation selected a tunneled primary next-hop for a prefix, the LFA SPF does not select an LFA next-hop for this prefix and the protection of this prefix relies on the RSVP LSP FRR protection. If the main SPF computation selected a direct primary next-hop, then the LFA SPF selects an LFA next-hop for this prefix but prefers a direct LFA next-hop over a tunneled LFA next-hop.
The lfa-only option allows an LSP to be included in the LFA SPFs only such that the introduction of IGP shortcuts does not impact the main SPF decision. For a specific prefix, the main SPF always selects a direct primary next-hop. The LFA SPF selects an LFA next-hop for this prefix but prefers a direct LFA next-hop over a tunneled LFA next-hop.
Thus the selection algorithm when SPF finds multiple LFA next-hops for a specific primary next-hop is modified as follows:
The algorithm splits the LFA next-hops into two sets:
The first set consists of direct LFA next-hops.
The second set consists of tunneled LFA next-hops (after excluding the LSPs which use the same outgoing interface as the primary next-hop).
The algorithms continues with first set if not empty, otherwise it continues with second set.
If the second set is used, the algorithm selects the tunneled LFA next-hop which endpoint corresponds to the node advertising the prefix.
If more than one tunneled next-hop exists, it selects the one with the lowest LSP metric.
If still more than one tunneled next-hop exists, it selects the one with the lowest tunnel-id.
If none are available, it continues with rest of the tunneled LFAs in second set.
Within the selected set, the algorithm splits the LFA next-hops into two sets:
The first set consists of LFA next-hops which do not go over the PN used by primary next-hop.
The second set consists of LFA next-hops which go over the PN used by the primary next-hop.
If there is more than one LFA next hop in the selected set, it picks the node-protect type in favor of the link-protect type.
If there is more than one LFA next-hop within the selected type, then it picks one based on the least total cost for the prefix. For a tunneled next-hop, it means the LSP metric plus the cost of the LSP endpoint to the destination of the prefix.
If there is more than one LFA next-hop within the selected type (ecmp-case) in the first set, it selects the first direct next-hop from the remaining set. This is not a deterministic selection and varies following each SPF calculation.
If there is more than one LFA next-hop within the selected type (ecmp-case) in the second set, it picks the tunneled next-hop with the lowest cost from the endpoint of the LSP to the destination prefix. If there remains more than one, it picks the tunneled next-hop with the lowest tunnel-id.