Avaya 8800 Planning And Engineering page 100

Layer 2 loop prevention
STP and BPDU forwarding
You can enable or disable STP at the port or at the spanning tree group (STG) level. If you
disable the protocol at the STG level, Bridge Protocol Data Units (BPDU) received on one port
in the STG are flooded to all ports of this STG regardless of whether the STG is disabled or
enabled on a per port basis. When you disable STP at the port level and STG is enabled
globally, the BPDUs received on this port are discarded by the CPU.
Spanning Tree and protection against isolated VLANs
Virtual Local Area Network (VLAN) isolation disrupts packet forwarding. The problem is shown
in the following figure. Four devices are connected by two VLANs (V1 and V2) and both VLANs
are in the same STG. V2 includes three of the four devices, whereas V1 includes all four
devices. When the Spanning Tree Protocol detects a loop, it blocks the link with the highest
link cost. In this case, the 100 Mbit/s link is blocked, which isolates a device in V2. To avoid
this problem, either configure V2 on all four devices or use a different STG for each VLAN.
100 Planning and Engineering — Network Design November 2010