Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Cisco Recomendation

Hi Guys,

Can anyone explain why cisco recomends one subnet per vlan.



Super Bronze

Re: Cisco Recomendation

Assuming we're considering host subnets, likely to avoid the same issues of a non-VLAN host subnets sharing the same wire. Such issues as broadcast and multicast containment. There's also security to consider and the issue of dynamic IP address allocation functioning.

The only time I recall having multiple host subnets on the same wire was an advantage, whether real or virtual, was during readdressing of hosts.

Re: Cisco Recomendation

Hi Mahmood,

There is the potential for large numbers of devices within a single VLAN or broadcast domain to generate large amounts of broadcast traffic periodically, which can be problematic. This is one of the reasons not have a large number of hosts in a single VLAN.

In addition to this if you are look at a voice VLAN, Cisco's recommendation is to limit the number of devices in a single Unified Communications VLAN to approximately 512, 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 a 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.


-> Sushil

CreatePlease to create content