[cisco-voip] cisco-voip Digest, Vol 173, Issue 3

[Pawlowski, Adam]

Ed,

Caveat on all this that I set this up a year or two ago so I could be wrong on some parts:

This does DNS SRV lookup I believe first, then A. As long as you're in an IOS version that supports SRV as that appeared somewhere in 15 I believe. We are running 15.(4)3 M2.

I set up a number of SRV records to point at my UCM cluster, for example, and then have the dial-peer set to:

session target dns:prod-all.cmgroup.subzone.zone.internal

I don't see that I have specified "session transport tcp" in this router but this is a _SIP._TCP SRV record and it is up and running.

I will say that you want to be careful with using this with the busyout/options ping. In my experience when it pulls the various SRV hosts, with respect to preference and weight, it will pick the one it should be using at any given point in time but it will not test all UCM nodes. It could be possible to busyout the group if multiple failures were to occur for some reason, even if other hosts are up, based on how IOS handles the responses to this. I sort of recall watching it, and test #1 may go to subA which fails, but then test #2 goes to subB so you're good. If you'd set the weighting different you could knock the peer offline.

I think.

I know that in response to this behavior I did add this configuration to my sip user agent configuration:

sip-ua
 retry invite 2
 timers trying 100
!

This was to cause the router to try again (perhaps to a different peer) if it failed to get anywhere with one. The defaults were too long for the call to fail out to a particular peer before exceeding network timers and losing the call. 

Best,

Adam Pawlowski
SUNYAB NCS