Avaya 8800 Planning And Engineering, Network Design page 265

RSMLT redundant network with bridged and routed VLANs in the core
All client stations that are members of a VLAN receive every broadcast packet. Each station
analyzes each broadcast packet to decide whether the packet are destined for itself or for another
node in the VLAN. Typical broadcast packets are Address Resolution Protocol (ARP) requests, RIP
updates, NetBios broadcasts, or Dynamic Host Control Protocol (DHCP) requests. Broadcasts
increase the CPU load of devices in the VLAN.
To reduce this load, and to lower the impact of a broadcast storm (potentially introduced through a
network loop), keep the number of VLAN members below 512 in a VLAN/IP subnet (you can use
more clients per VLAN/IP subnet). Then, use Layer 3 routing to connect the VLANs/IP subnets.
You can enable IP routing at the wiring-closet access layer in networks where many users connect
to wiring-closets. Most late-model high-end access switches support Layer 3 routing in hardware.
To reduce network convergence time in case of a failure in a network with multiple IP client stations,
Avaya recommends that you distribute the ARP request/second load to multiple IP routers/switches.
Enabling routing at the access layer distributes the ARP load, which reduces the IP subnet sizes.
Figure 138: Redundant network design on page 264 shows how to enable routing at the access
layer while keeping the routing protocol design robust and simple.
June 2016 Planning and Engineering — Network Design 265
Comments on this document? infodev@avaya.com