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Community Member

t 38 fax routing question

Good morning,

is a dial peer, in the router,  needed to route  incoming fax  calls to a h323 g/w, 3845 router, or should calls be able to be directed just using route groups.

Thank you

Cisco Employee

Re: t 38 fax routing question

Your question is unclear.  What is the topology of this fax?  Note all devices doing VoIP, what PSTN connection and protocols are used, and where the fax endpoints are located.

Community Member

Re: t 38 fax routing question

unclear to me

Anyhow Steven, we are installing a VM fax. i am just really trying to set up a test # in to the VM. i have, through the help of this forum, made the changes to the 3845 re: enabling t-38. but i thought i would be able to route the DN by way of route-list in CM. and that dial peer's would not be needed. now i was, as we speak adding a DP to the router, and it seems, unlike the example i was following, to be looking for cut through????? i was trying to add DP and then add IP of VM Fax as session target..this would just be a DID call into the 3845 then over to the VM  Fax.


Cisco Employee

Re: t 38 fax routing question

Again, what is the topology of the call flow?

H323 gateways always need dial-peers.  You can configure t38 specifically under those dial-peers, or globally under 'voice service voip.'

If your H323 gateway's dial-peers point to CUCM, then you also need route patterns for any instance where CUCM is sending the call out to an H323 gateway.

It sounds like you may have an H323 Fax server?  In that case you can do this:

PSTN-----GW----h323-----Fax server

That bypasses CM.  You confiugre the fax server to send all calls to the GW IP in the format the GW expects (based on your GW dial-peers).  You then have a dial-peer on the GW pointing to the fax server IP for the DIDs of your fax numbers.  I recommend this over putting the fax server off of CUCM, since it's a simplier call flow and less chance of the fax switchover to have issues at CM.  The disadvantage to this is that you lose the concept of a centralized dial plan on CUCM if you have multiple remote gateways all sending faxes to this fax server.

Either way will work though.  PSTN----GW---h323---CM---h323---Fax server is certainly fine to do.  In that case, the only changes are enabling T38 on the GW, and adding the fax server as a new H323 gateway in CM.

If this is a brooktrout fax server, make sure you set the h245 stage to 1 & disable fast start in their software.  Add it as an h323 gateway, not an ICT in CUCM.  Uncheck 'wait for far end cap...' in CM, too.

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