[cisco-voip] 7900 series and ARP

[Wes Sisk]

The phones also have a very limited arp cache, I believe 16 entries.  
ARP entries corresponding to next hop for CM servers are permanently 
cached.  Recommendation is to avoid deploying phones in such a large L2 
domain.  I believe this is covered in the UC SRND.  There is some 
history behind this:

CSCdt28640 7960 Intermittant Delayed Dialtone when going off-hook

/Wes

On Wednesday, June 30, 2010 11:11:18 AM, Pawlowski, Adam 
<ajp26 at buffalo.edu> wrote:
>
> 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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>
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