Allied Telesis AT-GS950/16PS, User Manual

Page: 364 / 386

Appendix A: MSTP Overview
364
Figure 143. CIST and VLAN Guideline - Example 2
When port 3 on switch B receives a BPDU, the switch notes the port sending
the packet belongs only to CIST 0. Therefore, switch B uses
CIST 0 in determining whether a loop exists. The result would be that the
switch detects a loop because the other port is also receiving BPDU packets
from CIST 0. Switch B would block port 3 to cancel the loop.
To avoid this issue, always assign all VLANs on a switch, including the Default
VLAN, to an MSTI. This guarantees that all ports on the switch have an MSTI
ID and helps to ensure that loop detection is based on MSTI, not CIST.
1 3 5 7 9 11 13 15
AT-G S 950/16P S
Gigabit Ethernet PoE+ Switch
plus
1 3 5 7 9 11 13 15R
2 4 6 8 10 12 14 16R 15 16
1 3 5 7
2 4 6 8
9 11 13 15R
10 12 14 16R
15 16
100 LINK ACT
1000 LINK
S FP
ACT
CLASS 1
LASER PRODUCT
S FP
PD ERR PD ON
ACT 1000 LINK
ACT 10/100 LINK
PORT ACTIVITY
POE MAX
S Y S TEM
POE
1 3 5 7 9 11 13 15
AT-G S 950/16P S
Gigabit Ethernet PoE+ Switch
plus
1 3 5 7 9 11 13 15R
2 4 6 8 10 12 14 16R 15 16
1 3 5 7
2 4 6 8
9 11 13 15R
10 12 14 16R
15 16
100 LINK ACT
1000 LINK
S FP
ACT
CLASS 1
LASER PRODUCT
S FP
PD ERR PD ON
ACT 1000 LINK
ACT 10/100 LINK
PORT ACTIVITY
POE MAX
S Y S TEM
POE
2719
Port 1 (Default VLAN)
S witch A
S witch B
Instances: CIST 0 and MSTI 15
Port 6 Port 8
(VLAN 3)
BPDU Packet
Instances: CIST 0
Port 3 (Blocked)
BPDU Packet