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New Member

FXS and Dial-peer configuration for FAX


I am looking for configuration  for FXS to work with FAX machine,

I have E1 with DID (6993XXX) the numbers 6993001,6993002,6993003,6993004 will be used for FAXs , I will attach one 4FXS module to cisco router 2821  for FAX calls,

any body can help to find the proper configuration for this scenario.

  1. the inbound calls will come from the E1 to one of the FXS port.
  2. The outbound calls will come from FXS to the E1

the call manager I think will not be part of the fax calls

-Cisco Router


-Call Manager 4.3


Thanks in advance

Hall of Fame Super Gold

Re: FXS and Dial-peer configuration for FAX

Are you using H.323 or SIP to the CM?

In that case you should familiarize yourself first with the dial-peer configuration

before requesting configurations from a forum, that you may not understand and not even work.

Otherwise if using MGCP, the CM will be involved, and you must configure it in addition to the router.

New Member

Re: FXS and Dial-peer configuration for FAX

Thanks for the reply,,

in fact my problem is not with dial-peer , my problem with choosing the fax protocol , how to configure it with dial peer and how to configure the FXS for FAX calls , I want to use H323 protocol ,

I cannot open the link you it gives me forbidden

Thanks once again


VIP Super Bronze

Re: FXS and Dial-peer configuration for FAX

If you're doing this intra-router, it doesn't really matter whether you use relay or pass-through. It's easiest to just configure fax relay globally in my opinion. Either way, here is the documentation on both:

Be advised that SuperG3 (33.6kbps) faxing speeds are not supported until very recently in IOS 15.1(1)T.

Please remember to rate helpful responses and identify helpful or
Hall of Fame Super Gold

Re: FXS and Dial-peer configuration for FAX

I would love to know the reason for the low ratings given to 100% correct, complete and helpful posts by Steven and myself.

Hall of Fame Super Gold

Re: FXS and Dial-peer configuration for FAX

Link corrected.

No configuration is necessary for local fax, but make sure you have network-clock-select on the E1.

Please remember to rate useful posts clicking on the stars below.

Cisco Employee

Re: FXS and Dial-peer configuration for FAX

If both ports are on the same DSP backplane, and they aren't MGCP, we won't even invoke a DSP for the call, so fax over IP configuration is irrelevant.  That's the easy configuration.  You just need two POTS dial-peers, and that's it.

If the ports are on different DSP backplanes (i.e. one of them is on an NM with DSPs) then a DSP will be invoked and it will trigger some sort of fax switchover.  In this scenario, it is often beneficial to configure 'voice service pots/fax rate disable' to prevent it from going to cisco fax relay on the POTS to POTS call.

If you do need to intentionally invoke a fax protocol, I'd recommend modempassthrough when going between two Cisco decices on the same LAN since that supports high speed fax.