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

[Justin Steinberg]

What firmware are you running on these phones ?   You are running a /24
subnet for your phones, which is pretty standard.   I have never heard of
this kinda of problem happening before due to ARP issues in a /24 subnet.

Are you having this problem at more than one location?    Can you run a
packet capture on that subnet ?   I wonder if you have someone messing
around on your network (e.g. man in the middle attack)

On Mon, Oct 8, 2012 at 5:41 PM, Amanda Lalli-Cafini <alalli at iinet.net.au>wrote:

> 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****
>
> 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****
>
> _______________________________________________
> cisco-voip mailing list
> cisco-voip at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-voip****
>
> ** **
>
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