[cisco-voip] 7900 series and ARP

[Pawlowski, Adam]

Folks,

    I've seen some commentary about certain versions of Call  Manager not being suitable to sit directly on a subnet with a large number of endpoints due to a limited capacity of the ARP table, but, nothing similar from the VoIP telephones themselves.

     I have a large subnet of VoIP devices, a /19 with over 1500 phones active at this time. Within this subnet, and, specifically, within a "building", subset of network gear, there are specific telephones which are not able to complete a call to another telephone on that subnet, within the "building". Each of these devices is direct attached to a Cat 3750G placing the voice traffic on the sub mentioned before.

     What I've seen in the one time I was able to isolate the problem, was the one of the two phones in the conversation would be continually ARPing for the other device, but, the other device, despite seeing it, wouldn't reply, so the RTP stream never establishes. I toggled the GARP feature from off, to on (although we have phones elsewhere with it off and/or on) and it began working. The problem still exists with other, specific, sets, but, the feature is on and swapping it doesn't help. Often, a second try at establishing a call will work.

      I guess what I'm asking is, is there a limit to the ARP table on the device? Does it even matter, since it attempts to lookup the MAC of the target phone on each call ? Anyone else had to deal with this ?

      You'll see an error in the device's console log about 0 RTP packets received, and stream stats will show no traffic sent or received, but, of course, the phone presents as though the call is flowing and thus the end users complain.

Thanks for your insights,

Adam Pawlowski
University at Buffalo
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