Implications for CSPF

When CSPF computes the primary paths of an LSP (or secondary paths when the primary path is not active), it ignores the fate-sharing information. You always want to find the best possible path (least IGP cost) for the primary path.

When CSPF computes a secondary path while the primary path (of the same LSP) is active, the following occurs:

1. CSPF identifies all fate-sharing groups that are associated with the primary path. CSPF does this by identifying all links and nodes that the primary path traverses and compiling group lists that contain at least one of the links or nodes. CSPF ignores the ingress and egress nodes in the search.
2. CSPF checks each link in the TED against the compiled group list. If the link is a member of a group, the cost of the link is increased by the cost of the group. If a link is a member of multiple groups, all group costs are added together.
3. CSPF performs the check for every node in the TED, except the ingress and egress node. Again, a node can belong to multiple groups, so costs are additive.
4. The router performs regular CSPF computation with the adjusted topology.

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