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


Fast Convergence Techniques: IS-IS

We would be irresponsible as network designers if we did not study and appreciate IS-IS for the problems it can solve. IS-IS is a link-state protocol similar to OSPF. IS-IS uses TLVs (similar to BGP), and is thus easily extended. IS-IS and OSPF are the two choices you have when deploying TE on MPLS networks, so you should know how IS-IS compares with OSPF when your design requires fast convergence. Cisco has several resources on their site, which I’ve distilled into a few rules of thumb:

  1. Increase LSP refresh timer to a high value
  2. Increase MAX LSP lifetime to a high value
  3. Tune PRC interval
  4. Tune SPF interval
  5. Tune LSP generation interval
  6. Use BFD in lieu of fast hellos (on multiaccess networks)
  7. Tune ISIS retransmit interval
  8. Set overload-bit on startup
  9. Disable hello padding
  10. Use a single IS type where possible (note that Cisco default is L1/L2)
  11. Use metric-style wide (not necessarily FC related, but is req’d for TE)

Also related: use isis mesh-group and configure point-to-point interface type on multiaccess interfaces where they are really point-to-point (think 2-member Ethernet segments).

Cisco Technology Support Page
Cisco IOS 12.4 IS-IS Fast Convergence
Cisco Configuration Example