IVR and FSPF Cost – Don’t forget to add 1

In my studying for CCIE Storage I have read much on FSPF.  Having come from a routing and switching background, FSPF cost is very similar to spanning tree cost.  FSPF is a standard fibre channel protocol that is used by multiple vendors and it’s pretty well documented.  Cisco has extensive documentation on FSPF in its MDS cookbook and MDS configuration guides.

What I noticed, which seems to be well known, is that IVR border switches add a cost of 1.  So look at the diagram below:

IVR Diagram

So if HOST1 wanted to talk to DISK1, then the cost would seem to be 2000, for someone who has read all the documentation on cisco.com.  The reality however, is that the cost is 2001.  This is easy to see when you are viewing the FSPF cost when IVR is configured.  What is shocking is that there is no documentation on Cisco that speaks of this!  If someone finds something that mentions it, let me know, but I have submitted documentation change requests for this so that it is included.

It would seem, that there is no documentation that FSPF cost is modified by IVR in any way.  The documentation on MDS has quite a few mistakes in it.  I believe I have submitted more than 25 corrections in the last year alone.  This is just to make you aware of the pitfalls.  I may dig up my sent mail and post a list of the corrections I have found, but I have no doubt I have missed as many as I have found.  This is another reason why practical studying is important.  You literally can’t learn everything from documentation because there are always “undocumented features” or features that just aren’t well documented, and until you use them you really don’t know this.  If there had been multiple border switches in the above diagram, each would insert a cost of 1.  So anytime you leave one VSAN and hop into another, that is a cost of 1.

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