Allied Telesis AT-S62 - Manual (Page 338)

Chapter 18: STP and RSTP
Section IV: Spanning Tree Protocols 338
Mixed STP and
RSTP Network
RSTP IEEE 802.1w is fully compliant with STP IEEE 802.1d. Your network
can consist of bridges running both protocols. STP and RSTP in the same
network can operate together to create a single spanning tree domain.
There is no reason not to activate RSTP on an AT-8524M switch even
when all other switches are running STP. The switch can combine its
RSTP with the STP of the other switches. The switch monitors the traffic
on each port for BPDU packets. Ports that receive RSTP BPDU packets
operates in RSTP mode while ports receiving STP BPDU packets operate
in STP mode.
Spanning Tree
and VLANs
The STP and RSTP implementations in the AT-S62 software is a single-
instance spanning tree. The protocols support just one spanning tree.
The single spanning tree encompasses all ports on the switch. If the
ports are divided into different VLANs, the spanning tree crosses the
VLAN boundaries. This point can pose a problem in networks containing
multiple VLANs that span different switches and are connected with
untagged ports. In this situation, STP or RSTP might block a data link
because it detects a data loop. This can cause fragmentation of your
VLANs.
This issue is illustrated in Figure 100. Two VLANs, Sales and Production,
span two AT-8524M switches. Two links consisting of untagged ports
connect the separate parts of each VLAN. If STP or RSTP is activated on
the switches, one of the links is disabled. In the example, the port on the
top switch that links the two parts of the Production VLAN is changed to
the block state. This leaves the two parts of the Production VLAN unable
to communicate with each other.
Figure 100 VLAN Fragmentation
LINK
MODE
LINK
MODE
FAULT
RPS
MASTER
PWR
MODE STATUS
AT-8524M Fast Ethernet Switch
LINK
MODE
LINK
MODE
FAULT
RPS
MASTER
PWR
MODE STATUS
AT-8524M Fast Ethernet Switch
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link