CSPF Failed: No Route Toward Event

LSP Event

CSPF failed: no route toward ip-address

Sample Output

user@R1> show mpls lsp extensive
[...Output truncated...]
Will be enqueued for recomputation in 3 second(s).
   68 Jan  5 10:02:56  CSPF failed: no route toward 10.0.0.6
[9 times]
   67 Jan  5 09:58:33  Deselected as active
   66 Jan  5 09:58:33 CSPF failed: no route toward 10.0.0.6
[...Output truncated...]

Meaning

This LSP event indicates that the CSPF calculation on the ingress router R1 failed to find a route to the destination, in this case the egress router.

Cause

The CSPF calculation to the destination can fail for many reasons, and failures occur frequently. The failures include, but are not limited to:

• A downstream node not configured for the Resource Reservation Protocol (RSVP) or Multiprotocol Label Switching (MPLS).
• The family mpls statement not configured on routers along the LSP path.
• The loopback (lo0) interface not configured at the [edit protocols isis] hierarchy level on the ingress or egress routers
• A faulty Explicit Route Object (ERO) that causes a loop or contains a bad address.

This event always includes an address it cannot reach. The listed address may be the LSP egress address, an ERO address, or an intermediate address.

Action

Determine if the node is listed in the traffic engineering database with the show ted database detail command. If necessary, compare the LSP constraints of all links that lead to the address to determine if there is a route to the destination.

Note: The CSPF algorithm prunes the database of links that do not comply with LSP constraints, then computes the shortest path from the remaining links.

A ping to an address that is unreachable by CSPF follows the interior gateway protocol (IGP) shortest path, not the CSPF constraints. Therefore, using the ping command to verify the connection does not provide information about why CSPF failed.

To verify whether the problem is a constraint issue, configure your LSP with the no-cspf statement at the [edit protocols mpls label-switched-path lsp-path-name] hierarchy level, then determine if the router signals the LSP successfully. If it does, the traffic engineering database contains links that do not comply with your constraints for the LSP.

The CSPF algorithm follows these steps to select a path:

1. Compute LSPs one at a time, beginning with the highest priority LSP (the one with the lowest setup priority value). Among LSPs of equal priority, CSPF starts with those that have the highest bandwidth requirement.
2. Prune the traffic engineering database of all links that are not full duplex and do not have sufficient reservable bandwidth.
3. If the LSP configuration includes the include statement, prune all links that do not share any included colors.
4. If the LSP configuration includes the exclude statement for the LSP, prune all links that contain excluded colors and do not contain a color.
5. Find the shortest path towards the LSP's egress router, taking into account explicit-path constraints. For example, if the path must pass through Router A, two separate SPFs are computed, one from the ingress router to Router A, the other from Router A to the egress router.
6. If several paths have equal cost, choose the path whose last-hop address is the same as the LSP's destination.
7. If several equal-cost paths remain, select the path with the fewest number of hops.
8. If several equal-cost paths remain, apply the CSPF load-balancing rule configured on the LSP (least-fill, most-fill, or random).