From CUCM SRND:
Layer 2 Access Design Recommendations
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.
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).
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.
HTH,
Chris
