Allied Telesis AT-S39 - Manual (Page 104)

Section II: Local and Telnet Management
104
Mixed STP and
RSTP Networks
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 should be able to operate together to create a single spanning
tree domain.
There is no reason not to activate RSTP on an AT-8000 Series switch even
when all other switches are running STP. The AT-8000 Series switch can
combine its RSTP with the STP of the other switches. An AT-8000 Series
switch will monitor the traffic on each port for BPDU packets. Ports that
receive RSTP BPDU packets will operate in RSTP while ports receiving
STP BPDU packets will operate in STP.
Spanning Tree
and VLANs
The spanning tree implementation on an AT-8000 Series switch is a
single-instance spanning tree. The switch supports just one spanning
tree. You cannot define multiple spanning trees.
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. What can happen is that STP will block a data link
because it detects a data loop. This can cause fragmentation of your
VLANs.
This issue is illustrated in Figure 27. Two VLANs, Sales and Production,
span two AT-8024GB switches. Two links consisting of untagged ports
interconnect the separate parts of each VLAN. If STP is activated on the
switches, one of the links would be disabled. This problem can be
avoided by not activating spanning tree or by connecting VLANs using
tagged instead of untagged ports. (For information on tagged and
untagged ports, refer to Chapter 10, Virtual LANs Overview on page
118.
Figure 27 VLAN Fragmentation
Link
Mode
Link
Mode
100
FULL
ACT
MODE
COL
FAULT
MASTER
PWR
AT-8024
10Base-T / 100Base-TX Fast Ethernet Switch RS-232 TERMINAL PORT
Link
Mode
Link
Mode
100
FULL
ACT
MODE
COL
FAULT
MASTER
PWR
AT-8024
10Base-T / 100Base-TX Fast Ethernet Switch RS-232 TERMINAL PORT
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN