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New Member

T.38 and FAX on CISCO gateways

I'm not here to ask help but to share some info on CISCO gw fax sending and have the comment of NetPRO people.

We use cisco UGw (5300,5350,5400) as VoIP terminations in SIP. We often find interops problems with third party devices when we try to send fax in T.38.

Recently we have discovered the reason of all these problems is on CISCO IOS T.38 negotiation.

If Re-Invite is made by CISCO gw and third party ATA or IAD use different port for T.38 respect to voice, CISCO

gw ACK the change but doesn't use the new port: T.38 packets are sent to previous RTP port used for voice.

FAX works if third party device use same

port for audio codec and T.38.

I don't think is a standard behavior.

I understand that CISCO takes as standard using the same port but why acknowledging the 200 OK with a different port?

Only if third party device makes ReInvite

before CISCO does, port can be changed.

According to me, changing ports should be

permitted on CISCO IOS (we have changed a lot of versions but nothing changes).

I'd like to know your opinion about this.

Thanks in advance.


Re: T.38 and FAX on CISCO gateways

I would try this in 12.4(22)T. It doesn't sound like an issue I'm familiar with, but it is hard to say now.

If you still have problems on 12.4(22)T, I would open a TAC case and include the packet capture and SIP logs from your 5x00.



New Member

Re: T.38 and FAX on CISCO gateways

Same behavior with 12.4(22)T

The only UGW that works correctly

is a 5400HPX: same IOS!

Re: T.38 and FAX on CISCO gateways

Sounds like an new bug, I'd open a TAC case.


New Member

Re: T.38 and FAX on CISCO gateways

No, no, it's an old bug :)

I will open a case.


New Member

Re: T.38 and FAX on CISCO gateways


I have same problem, but i can't find ios without this bug. I tried use 12.422T but nothing changed.
Can you write me, if you were successful?

Thank. Richard