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NFAS Issues

d.kettler
Level 1
Level 1

We have two local T1s from AT&T using NFAS. Int 2/1 is configured as the primary and 2/0 as the backup. ATT is sending inbound calls starting at channel one of the primary. When configuring dial-peers with NFAS you can only choose the primary serial 2/1:23 as the port to send outbound calls. We would like outbound calls to be bottom up. The outbound calls are starting on channel 23 of 2/1, the primary, not 2/0 the secondary. This is causing an issue because if an outbound and inbound call try to sieze the same dsp resource it will 'lock-up' the resource making it unavailable for either. After a heavy call period most of the dsp resources are unavailable and this causes inbound calls to be dropped. How can I force outbound calls to start with the secondary line remembering that I cannot choose 2/0:23 as a port for it is the secondary of the NFAS group and the router, a 3745, will not even allow me to use it as a choice.

Thanks

Dave

1 Reply 1

jasyoung
Level 7
Level 7

First off, your router should resolve this situation gracefully. It's called "glare" and the router and telco switch should mutually figure out that they're trying to use the same channel, and abort or move calls accordingly. You should not permanently lock up DSPs. This is a bug and you should contact TAC about it.

That said, it's probably a good idea to start at the top of your trunk group when dialing out. I believe the "isdn bchan-number-order descending" command will do what you want. You apply this on your single NFAS Serial interface and it should start at the top of the group as a whole.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113aa/113aa_3/natisdn.htm#xtocid1945315

Correction: I did some more research and this seems to be the default, and I'm not 100% sure it will have the effect you're looking for on NFAS, which is to say it will try the "last" trunk first. Try it anyway, though. There is a series of "trunk-group" commands that allow close control of what channels to use, but that is only available on AS5[38]XX. Also, please do open a TAC case for the DSP lockup problem.

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