Voice VLANs - Best practices

[ouhakheme]

Hello --

We're starting an IPT project that will involve multiple access VLANs for different organizational groups. That said, should we also create multiple voice VLANs per group/building or will one flat VLAN work for voice? The backbone is GigE, so we aren't too concerned about bandwidth.

What are the pros and cons, and best practices that the community has seen?

Please let me know.

Thanks,

OOU

[Chris Deren]

I would recommand using 1:1 ratio of data to voice VLANs, so if you have a 10 story building with seperate data VLAN for each floor I would apply the same methedology to voice VLANs. The technical reason to have more VLANs is the device limitation per VLANs as well as making your broadcast domain smaller.

Chris

[aamercado]

On a minor detail, I would add on to user lower numbers for voice vlan and higher numbers for data. As lower number vlans converges faster

For instance, VVlan 10,20,etc.. for Voip and corresponding 110, 120, etc for data.

Also note that the recommend max device per vlan was btwn 192-254. There is an exact figure I just don't remember off the top of my head but am sure it is btwn that range.

[ouhakheme]

Is the broadcast domain the only good reason to implement multiple voice vlans? I was under the impression that the phones weren't going to broadcast much data anyway.

What about troubleshooting and STP convergance time, wouldn't one VLAN make things easier?

[dgahm]

From the IP Telephony SRND:

http://www.cisco.com/en/US/customer/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a0080447513.html#wp1043378

Campus Access Layer

The access layer of the Campus LAN includes the portion of the network from the desktop port(s) to the wiring closet switch.

Proper access layer design starts with assigning a single IP subnet per virtual LAN (VLAN). Typically, a VLAN should not span multiple wiring closet switches; that is, a VLAN should have presence in one and only one access layer switch (see Figure 3-2). This practice eliminates topological loops at Layer 2, thus avoiding temporary flow interruptions due to Spanning Tree convergence. However, with the introduction of standards-based IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) and 802.1s Multiple Instance Spanning Tree Protocol (MISTP), Spanning Tree can converge at much higher rates. In situations where RSTP and/or MISTP can and have been configured on the access layer switch, there is no need for concern about topological loops. More importantly, confining a VLAN to a single access layer switch also serves to limit the size of the broadcast domain. 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. A good rule of thumb is to limit the number of devices per VLAN to about 512, which is equivalent to two Class C subnets (that is, a 23-bit subnet masked Class C address). Typical access layer switches include the stackable Cisco Catalyst 2950, 3500XL, 3550, and 3750, as well as the larger, higher-density Catalyst 4000 and 6000 switches.