D-link i2Eye between two Cisco Pix 501's -- premature timeout


My church nationally uses a Cisco Pix 501 as a firewall/router in each of its buildings that have a broadband connection. We would like to establish a video phone call between two of our buildings for the purpose of offering a supplemental gospel instruction class to a few members in a remote area that would like to participate despite their limited numbers. Locally our budget is limited, so we are experimenting with a pair of D-Link i2Eye DVC-1100 wireless broadband video phones for this project.

Each building has an ADSL line with a single dynamically assigned IP address. I have assigned the D-Link i2Eye video phones static private IP addresses and made static assignments in the Pix to map ports on the external interface to coresponding ports on the internal IP. The D-Link video phones are H323 compatible, using TCP port 1720 to setup the call and TCP/UDP ports 15328 through 15333 to do the audio/video magic. It also connects to an LDAP directory server to register itself so that you can use the speed dial feature to call the other one without having to determine the current IP address, which is helpful considering the less-technically savy instructor that will be operating this device on the other end. I had to open up all of these ports on the Pix's access-list.

In order to get this device to operate I found that it was also necessary to turn on a couple of "fixup" commands for LDAP and H323. I also found it necessary to increase the timeout setting for H323 from the default of 5 minutes to instead be a few hours, as we will need the call to last nearly 2 hours.

After doing all of this the video phones work and the quality is pretty well. However, after a random number of minutes they hang up on each other and say the connection was lost. You can immediately redial and connect back up, but this is very disruptive. So far I haven't found a pattern. Sometimes it'll last as little as 90 seconds. Other times it will last over half an hour. I am getting 0% packet loss, and don't experience any other connectivity problems besides this.

I wish the D-Link device would give a better description of why the connection was lost. Unfortunately I'm not really sure what I'm trying to fix, and am afraid I might be fixing something that's not broken. Are there any other settings on the Pix that would be causing the H323 connection to timeout after a seemingly random number of minutes, usually between :05 and :35?

D-link suggested that I might try adjusting the MTU settings being that both connections are through PPPoE over DSL. I found them to be at the default of 1500 on the Pix, so I have tried adjusting this to 1492 on one end. Doing the same on the other end will be my next step, but I will have to go back out there and havne't had a chance yet. I'm not really too familer as to sure how H323 handles itself with MTU--will two different MTU settings at each end result in different packet sizes for the same call, or will the protocal limit itself to the smaller of the two automatically?

I'll keep bumping down the MTU settings at each end and testing this thing, but I'm wondering if there are any other things I could have overlooked.


Jeff Phillips

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The J-Man
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