Subnet mask for more than 1.2k phones

[Juan Carlos Arias Perez]

Working in a project of IP telephony which includes more than 1,200 ip phones, and I was wondering what it's the recommended subnet mask to use for the phones, creating several VLANs of course, creating networks with 24 or 25 bits for subnet mask??  Or which subnet mask should I use for this purpose??

Regards,

Juan Carlos Arias

[Ayodeji Okanlawon · Accepted solution]

Cisco strongly recommends a limit of 512 devices  in any VLAN. which is equivalent to two Class C subnets (that is, a 23-bit subnet masked Class C)

There are several reasons for this..

1. The need to control the amount of VLAN broadcast traffic

2. The ARP cache of CUCM (linux based) has a hard limit of 1024 devices, hence 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

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"opportunity is a haughty goddess who waste no time with those who are unprepared"

[Juan Carlos Arias Perez]

Hello aokanlawon, thank you for your comments, I'll keep in mind for my design.

One last thing, do you have a link or document where I can find this??  I mean a document from Cisco, just for documentation of project, if you don't have it, no worries.

Regards,

Juan Carlos Arias

[Ayodeji Okanlawon]

Yes, its documented in the SRND here

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/8x/netstruc.html#wp1043617

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"opportunity is a haughty goddess who waste no time with those who are unprepared"

[Juan Carlos Arias Perez]

Great, thank you for your time and comments.

Regards,

Juan Carlos Arias