Junos CSPF for multiple dynamic paths

A quick one today, but a few people have recently asked me how the Junos CSPF process manages to always ensure the primary and secondary paths are kept diverse *automagically*. RSVP path calculation with CSPF is an interesting topic, and if you look through the archives, you’ll find I’ve touched on this as I’ve discussed SRLGs, RSVP auto-bandwidth, and other such topics.

Lets assume for the purposes of this post that we have a LSP that looks like this:

[edit protocols mpls]
[email protected]# show 
label-switched-path test-LSP-to-PE5 {
    to 10.0.0.5;
    primary primary-path-to-5;
    secondary secondary-path-to-5;
}
path primary-path-to-5;
path secondary-path-to-5;

This LSP is pretty simple. There are no constraints implemented for either of the primary or secondary paths, and no constraints on the LSP as a whole.

Unpon configuration of a LSP such as this, Junos will attempt to stand up the primary path. This will follow the best IGP metric subject to constraints (available bandwidth, admin-groups, SRLGs, etc). Only once this has stood up will the calculation for the secondary begin.

As Junos calculates the secondary path, it will first take a copy of the topology of the RSVP network and inflate the IGP metric of every link that has been used by the primary path by a factor of 8 million. This ensures that if at all possible, a diverse path is used – while still allowing the use of a shared path between the primary and secondary if this is our only option.

When the primary path re-optimizes to a new path, this process is repeated, thus ensuring that we at all times try to keep the primary and secondary as diverse as possible.

It’s worth noting that if you turn off CSPF for the LSP, the above functionality will not be implemented, and both paths will simply follow the best IGP path.

Hope this helps!

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