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New Member

How many IP-Phones in one VLAN?

Hi Community,

we have some disussions here about the maximum number of IP-Phones in one VLAN.

In our company are some statements "hanging around" that we should NOT place more than 512 Phones

in one VLAN.

I don't see any Problems with putting more Phones into one VLAN (we are talking of about 800-900 Phones currently)

Ok - of course it would make sense to split phones of different buildings to different VLANs - but for our

system Management it would be MUCH easier to maintain only ONE Voice-VLAN - one redundant DHCP-Scope

and only one IP-Range to observe for exhaustion.

Are there any cisco-documents with recommendations? I would need an "official Statement"  - because if

in 2 month something goes wrong - say bad voice-quality - then some people will point back to me and say:

"Hey - I told you not to put more than 512 Phones, because....... " - I think you all know what I mean ;-))

From a technical view I can't see any problems. Phones won't do a lot of broadcasts (except while booting - DHCP).

Signalling is Unicast,

VoiceTraffic is Unicast,

MoH is Multicast (this is a point against furhter VLANs..)

Thanks for your Help,

Greetings,

Chris

5 REPLIES

How many IP-Phones in one VLAN?

Check out your version of CUCM SRND

From the 8.6 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....


Please let me know if this helps

New Member

How many IP-Phones in one VLAN?

Hi Gregory,

thanks for your fast response.

Our Call Manager is placed in a different VLAN so arp-handling will be done by our core-device (6500 VSS).

So if the only Problem that we can run into, is the ARP-thing - this should not be an issue in our case.

Do you see any problems with broadcasts in a phone-only network? (only cisco-phones 79xx/89xx/...)

Managing more VLANs will complicate resource management in our structure a lot... so I really want to avoid

further VLANs for Phones. And as mentioned before - I don't want to run into any "political problems" here ;-)

greets,

Chris

VIP Purple

How many IP-Phones in one VLAN?

Hi,

You can also refer

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/install/8_6_1/install/cmins861.html

    •          Do not install Cisco Unified Communications Manager in a large Class A or Class B subnet that contains a large number of devices.
  • When you install Cisco Unified Communications Manager in a large subnet with a large number devices in that subnet, the Address Resolution Protocol (ARP) table can fill up quickly (maximum 1024 entries, by default). When the ARP table gets full, Cisco Unified Communications Manager can have difficulty talking to endpoints and cannot add more phones.

regds,

aman

How many IP-Phones in one VLAN?

My suggestion to you is to read through the SRND for proper design consideration.  This would be your offical documentation.

How many IP-Phones in one VLAN?

If you read the SRND I do believe it states how vlans should be configured and that one vlan is not meant to span more than one switch closet as an overall design best practice.

Hope this helps you. 

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