Cisco Support Community
Community Member

T.38 No signal


we use more than one CISCO 5400 as

VoIP terminations, since we are VoIP


We experience some trasmitting fax problem with T.38 FAX Relay.

It seems that many customer's CPE,

after CISCO 5400 Re-Invite, wait for

a T.38 packet before sending/receiving

T.38 packets.

But CISCO too wait a T.38 packet!!!

So communication hangs a while

then call is dropped.

We have seen that all CPEs that provide

a "T.38 No Signal" packet to CISCO

gateway after Re-Invite

succeed on fax trasmitting.

We use Cisco IOS Software (C5400-IS-M).

Is there any way to let CISCO sending

"T.38 No Signal" packets after Re-Invite?

Any help will be apreciated.



Re: T.38 No signal

The issue can arise if the tcp session mtu size is set too small.Try setting the mtu size using the command ip tcp path-mtu-discovery.

Community Member

Re: T.38 No signal

Thank you. we will test.

Do you think your command works for UDP too? (T.38 packets are like RTP packets, then UDP)

Do you think using it we should experience

gateway start sending T.38 packets?

Thank you so far.


Community Member

Re: T.38 No signal

Have it work? I got the same problem like you

Community Member

Re: T.38 No signal

No but we have discovered a problem on T.30

due to third party ATA/IAD.

Basically after the T.38 re-invite, calling device doesn't send a CNG.

That's a required message to start T.30 negotiation of speed and so on.

Generally specking CISCO TAC suggest a 12.4T

IOS to have the best behavior for FAX.

Community Member

Re: T.38 No signal

I have already use 12.4(15)XY2. Is it related to 3rd party ATA? The CNG doesn't sent from where?

And how do you resolve? Are using another ATA?

Community Member

Re: T.38 No signal

We are now working with third party ATA firmware


Re-Invite in T.38 can be done by calling or called party but a T.38 packet containing CNG

must be sent by calling party in order to start

speed handshake and fax trasmission.

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