VG224 and QoS not acceptable

Unanswered Question
Jan 21st, 2008

We have a VG224 handling some analog devices (phones/faxes/paging) and notice that the only time QoS is not acceptable is when this device is at either end of a call. Auto QOS is enabled on switchports (same switch/vlan as the Call Manager) and on the VG224.

Its mainly FAX machines that are triggering the bad QoS.

Any ideas?


I have this problem too.
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Tim Smith Mon, 01/21/2008 - 15:18

Hi Charles,

Fax can be sensitive to network quality.

If the fax calls are going to the PSTN, is the PSTN gateway on the same switch also?

Are you configured for SIP, SCCP, MGCP?

Are you using fax relay?

Are you talking about the QoS report from ART / CAR? Or are you actually having fax transmission problems?



c.yeo Wed, 01/23/2008 - 06:16


The PSTN gateway is also plugged into the same switch as the VG224.

e have configured the VG224 using WMGCP, although most of the other devices in our voice system have had to use SCCP because of a very basic ISDN service we have to subscribe to for the PSTN link.

For FAXing, we are using MODEM passthrough (we use Call Manager 4.3.2, apparenlty the FAX component isn't compatible). Keep in mind, this QoS issue also shows up on plain voice calls from analog devices attached to the box.

The QoS results are from the CDR application. We currently don't have any other tools, but we are looking at some Fluke equipment (when we get budget). The FAXes through the box are working (albeit not at the speed we'd like), and we haven't have much in the line of complaint on voice quality though the box. It's mostly the reports from the CDR/CAR app that has me going down this road.

I've been able to deal with all other QoS issues up to now. This one is just 'hanging on'. Any suggestions would be welcome.


dennisbehrens Wed, 01/23/2008 - 07:07


As for the QoS configuration, I'm assuming that you have used "auto qos voip trust" on both the PSTN gateway and the VG224 ports. Is this correct?

As for the VG224 running MGCP and Fax Passthrough, I believe that Fax Passthrough is supported as of 12.3(8)T code. If you are running this code or newer on your PSTN gateway and VG224, you should be able to put a "no ccm-manager fax protocol cisco" command in the configuration. Make sure this is on both gateways.


c.yeo Wed, 01/23/2008 - 13:13

Yes, we are using 'auto qos voip trust'.

The IOS is 12.4+ on both the gateways.

The 'no ccm-manager fax procotol cisco' command does what? Force T38 or something like that?

I just Check the VG224 (with IOS 12.4(15)T1 and the PSTN gateway router (with IOS 12.4(13r)T5). The command does not allow the 'cisco' modifier on the end just: 'no ccm-manager fax protocol'. I didn't think that would be right.

The common switch is a 3750 with 12.1(14e)EA1a on it. Is there anything other than the 'auto qos voip trust' on the switchport that is required? (yes, the auto-generated QoS lines are intact).


dennisbehrens Wed, 01/23/2008 - 16:39

The 'no ccm-manager fax procotol cisco' command disables Fax Relay and enables Fax Passthrough.

I believe that you may not see the whole command if you use the ? mark, but if you type it, the router should accept the command.

There shouldn't be anything else required. I highly doubt that the issue is really a QoS issue on your LAN considering that the two devices are on the same switch.

You may want to also check your PRI for any clock slips or other errors. Do a "show controllers t1" and see if anything looks out of wack.


c.yeo Thu, 01/24/2008 - 06:21


Part of our complaint with our telco is that we can't get PRI service...that's why we have banks of BRI (ISDN) VICs in our routers. (Kind of a pain as it's old technology). However the BRIs are used for all devices, not just the VG224, and there is no problem there. FWIW, the Data and Bearer channels all look good.

So, there's no configuration needed on the CCMs to use the FAX passthrough, just on the VG and the router(s)?


dennisbehrens Thu, 01/24/2008 - 06:29


That's obnoxious. I'm glad they're running clean.

Yes, you just need it on the VG224 and ISR. I'd suggest trying it during and off time and reverting if it makes it worse.



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