Allied Telesis AT-S62 - Manual (Page 491)

AT-S62 Management Software Menus Interface User’s Guide
Section IV: Spanning Tree Protocols 491
A port can be both a point-to-point and an edge port at the same time. It
operates in full-duplex and has no STP or RSTP devices connected to it.
Figure 161 illustrates a port functioning as both a point-to-point and edge
port.
Figure 161. Point-to-Point and Edge Port
Determining whether a bridge port is point-to-point, edge, or both, can be a
bit confusing. For that reason, do not change the default values for this
RSTP feature unless you have a good grasp of the concept. In most
cases, the default values work well.
Mixed STP and
RSTP Network
RSTP IEEE 802.1w is 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-8500 Series 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 are single-
instance spanning trees. 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 162. 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
Point-to-Point and Edge Port
Workstation
(Full-duplex Mode)
LINK MODE
LINK
MODE
FAULT
RPS MASTER
PWR
MODE STATUS
AT-8524M Fast Ethernet Switch