IS-IS, MPLS-TE, and wide metrics

I came across an interesting topic in the book Traffic Engineering with MPLS yesterday. The topic involves migration from narrow to wide metrics. As you know, wide metrics are required to support MPLS TE, so if you have a network that is currently deployed with narrow metrics (the default) you will have to migrate to the wide metric style. You may even have devices that only support narrow metrics, so you’ll have to figure out what to do with them in the meantime – perhaps they won’t be in the path of TE tunnels, so you might not need to replace/upgrade them immediately. Anyway, starting on page 465, the book details two procedures to migrate from narrow to wide metrics:

  1. migrating from narrow to wide metrics in two steps
  2. migrating from narrow to wide metrics in three steps

Why the difference in steps? Well, the first one involves enabling all routers to advertise & accept both styles of metrics, which will increase the size of your LSDB by roughly 2x. The three step program takes longer but does not require the same amount of memory.

Here’s a good link from cisco referenced in the book. Transitioning IS-IS to a New Technology

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