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Can I configure Voice VLAN with /16 subnet for IP Phones ?

Unanswered Question
May 1st, 2014
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Hi Team,

I would appreciate if could help me on my following query.

Can I configure Voice VLAN with /16 subnet for IP Phones ? Will it violet ARP Cache limitation ?

***** Based on SRND ********

The recommendation to limit the number of devices in a single Unified Communications VLAN to
approximately 512 is not solely due to the need to control the amount of VLAN broadcast traffic. For
Linux-based Unified CM server platforms, the ARP cache has a hard limit of 1024 devices. Installing
Unified CM in a VLAN with an IP subnet containing more than 1024 devices can cause the Unified CM
server ARP cache to fill up quickly, which can seriously affect communications between the Unified CM
server and other Unified Communications endpoints. Even though the ARP cache size on
Windows-based Unified CM server platforms expands dynamically, Cisco strongly recommends a limit
of 512 devices in any VLAN regardless of the operating system used by the Unified CM server platform.

**********************************************************************************************************************

Regards,

Nirmal

 

 

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Gordon Ross Thu, 05/01/2014 - 22:41
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Firstly, having a /16 subnet isn't generally advised, whether it be for IP Telephoney or networking in general.

 

Secondly, the ARP Cache limit is the number of devices, not the size of the subnet. So you could have a /8 subnet with just one IP phone on it.

 

If CUCM is on a different subnet to the phones, then this whole ARP cache issue is irrelevant, as it is the router that has to maintain an ARP cache, rather than CUCM. The ARP cache limit only applies to the subnet that CUCM itself is on.

 

GTG

Ayodeji Okanlawon Fri, 05/02/2014 - 03:07
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  • Cisco Designated VIP,

    2017 IP Telephony

Excellent point Gordon (+5)

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