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New Member

Fax over IP Problem

Hi all,

I have been looking into a problem that 2 remote sites are connected by cisco 1750s where one of the voice ports is connected to a fax machine at each site. Without packet loss and bandwidth consumption problems(256K line, only 1 fax + 1 voice & few data traffic), I cannot solve:

1) The faxing speed is slow.

2) Very often the faxing process stops in the middle.

3) Very often the fax output becomes distorted..

4) Poor voice quality

Is there anything I can improve the fax operation between the 2 sites with router comamnds together with fax process?

Have you guys exprienced these before and have some hints to me?

A truncated config(both sites are similar) is as follows:

nterface Serial0

bandwidth 256

ip unnumbered FastEthernet0

ip nat outside

encapsulation ppp

ip tcp header-compression iphc-format

no ip mroute-cache

load-interval 30

priority-group 1

ip rtp header-compression iphc-format

ip rsvp bandwidth 96 24


dial-peer voice 20 voip

destination-pattern 2050

session target ipv4:

req-qos controlled-load

fax rate 9600

ip precedence 5


Thanks in advance,



Re: Fax over IP Problem


During the call, if you type "show voice dsp" - what do you read in the row "codec"? (does it mention "fax", "9600" or "g.711u"?)

If you don't read fax or 9600 you don't use fax-relay. Without fax-relay I had problems with the same symptoms too.

Be sure you use an IOS which supports fax-relay.

With fax-relay I had no problems with 14'400 fax with the g.711 codec.

I also had to enter "incoming called-number xxxx" in both dial-peers.


New Member

Re: Fax over IP Problem

Many thanks Martin,

Just want to confirm to enable fax-relay, does it mean disabling ECM by command "fax-relay ecm disable" under the dial-peers? And it will consume much more BW if using codec g711?



New Member

Re: Fax over IP Problem

you need the command

fax protocol t38 ls-redundancy 0 hs-redundancy 0

on the dial-peers. The ECM, as far as I know, should be disabled, since not all fax machines support it.