Single Vs. Multiple Spanning Tree - Alcatel Omni Switch/Router User Manual

Single vs. Multiple Spanning Tree

In previous releases of the Omni Switch/Router software (4.0 and earlier), spanning tree
support was done on a per port basis. In other words, a physical port could only participate
in one instance of a spanning tree on the network. If a network is passing both untagged and
IEEE tagged frames, single spanning tree support could lead to packets being lost. Lost pack-
ets could occur if a port specifically assigned to handle one type of traffic (e.g., IEEE 802.1Q)
is blocked by spanning tree, forcing traffic for that port to move to a port not assigned to
handle IEEE 802.1Q traffic.
In the above diagram, the physical connection between Switch 1 and Switch 2 is blocked by
spanning tree. No traffic can pass over the connected ports.
Release 4.1 (and later) of the Omni Switch/Router allows for multiple spanning tree instances
on a single port. Put another way, a port can be part of separate spanning trees, with no
impact on packet delivery. This is done by basing spanning tree configuration on groups
rather than physical ports.
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Switch 1
VLAN 1 (Blocked)
The above diagram shows how traffic on VLAN 1 is blocked between Switch 1 and Switch 2,
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VLAN 1
1 2 3 4 5 6 7 8
1 2 3 4 5 6
VLAN 2
Switch 1
Blocked
Physical
Port
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Switch 2
Port Based Spanning Tree
VLAN 1
VLAN 2 (Blocked)
VLAN 2
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Switch 2
Group Based Spanning Tree
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Switch 3
VLAN 2
VLAN 1
1 2 3 4 5 6 7 8
1 2 3 4 5 6
Switch 3
VLAN 2
VLAN 1