Rapid spanning tree and vlans, Packets, Vlan – Allied Telesis AT-S84 User Manual
Page 152
Chapter 11: Rapid Spanning Tree Protocol (RSTP)
152
Section II: Menus Interface
packets.
Rapid Spanning
Tree and VLANs
The spanning tree implementation in the AT-S84 management software 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. In this situation, STP blocks a data link because it detects a data
loop. This can cause fragmentation of your VLANs.
This issue is illustrated in Figure 41. Two VLANs, Sales and Production,
span two AT-9000/24 Gigabit Ethernet switches. Two links consisting of
untagged ports connect the separate parts of each VLAN. If 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 41. VLAN Fragmentation
You can avoid this problem by not activating rapid spanning tree or by
connecting VLANs using tagged port members instead of untagged ports.
(For information on tagged and untagged ports, refer to Chapter 8, “Virtual
LANs and GVRP” on page 103.)
Sales
VLAN
Production
VLAN
Production
VLAN
Sales
VLAN
Blocked Port
Blocked Data Link
793
AT-9000/24
24 Port Gigabit Ethernet Switch
1
3
5
7
9
11
13
15
17
19 21R 23R
2
4
6
8
10
12
14
16
18
20 22R 24R
POWER
21
22
23
24
SFP
LINK
ACT
1000
10/100
PORT ACTIVITY
793
AT-9000/24
24 Port Gigabit Ethernet Switch
1
3
5
7
9
11
13
15
17
19 21R 23R
2
4
6
8
10
12
14
16
18
20 22R 24R
POWER
21
22
23
24
SFP
LINK
ACT
1000
10/100
PORT ACTIVITY