[cisco-voip] setting the ARP cache timeout for phones using DHCP option

[Amanda Lalli-Cafini]

Hi Ryan,

 

Thanks for that.

We do not have any 9971 phones in this setup.

Most are 7941 and 7942 models.  There are a few 7961 phones and a few 9951
phones.

 

The problem I am trying to solve is that some of the 7942 phones just appear
to drop off of the network from time to time.

When this occurs, I can not ping the phone even from its own default
gateway, yet it is still registered to CUCM and can make/receive calls.

I however, get one way audio from that phone when the problem occurs.

 

We are thinking that their arp cache (something small like 16) is getting
full and wanted to be able to flush that out every five minutes with the
dhcp option.

Is there a way to set the ARP cache timeout from within Call Manager itself
by any chance?

 

Thanks for any help.

 

Regards

 

Amanda

 

From: Ryan Ratliff [mailto:rratliff at cisco.com] 
Sent: Monday, 8 October 2012 10:56 PM
To: alalli at iinet.net.au
Cc: cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] setting the ARP cache timeout for phones using
DHCP option

 

CSCtf62463 seems to indicate the 9971 phones don't support DHCP option 35.
This will vary by model so YMMV.

 

-Ryan 

 

On Oct 8, 2012, at 4:10 AM, alalli at iinet.net.au wrote:

 

Currently Being Moderated


Configuring the ARP cache timeout for Cisco phones
<x-msg://184/message/3751318#3751318> 


Hi All,

 

I am hoping to configure the ARP cache timeout for Cisco phones using the
dhcp option.

I was wondering if anyone here has done it or can verify that this would be
correct.

 

Say for example I have a DHCP pool named VOICE.

In this network, the CUCM is located at IP address 10.2.200.110 

 

I did some google searching and found a Wiki site with the various dhcp
options defined in it.

 

http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol

According to the site, dhcp option 35 is used as the ARP Cache Timeout value
and it is 4 bytes long.

 

Lets say that I would like to set my arp cache timeout to be 300 seconds.

I convert 300 to hex and get 12C.

Since my field will need to be four bytes long this becomes 0000.012C.

 

I think that the following dhcp pool will provide 300 seconds for the phone
ARP cache timeout.

 

ip dhcp pool VOICE

network 10.240.1.0 255.255.255.0

default-router 10.240.1.1

option 150 ip 10.2.200.110

option 35 hex 0000.0c12           

 

Can anyone help me out with a verification or point me in the direction of
the appropiate documentation?

 

Many thanks for any assistance.

 

Regards

 

Amanda Lalli-Cafini

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