Avaya 8800 Planning And Engineering, Network Design page 97

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.
Figure 29: VLAN isolation
Multiple STG interoperability with single STG devices
Avaya provides multiple spanning tree group (STG) interoperability with single STG devices. When
you connect the Avaya Ethernet Routing Switch 8800/8600 with Layer 2 switches, be aware of the
differences in STG support between the two types of devices. Some switches support only one
STG, whereas the Avaya Ethernet Routing Switch 8800/8600 supports 25 STGs.
June 2016 Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
Spanning tree
97