The Switch-Centric Model - IBM RackSwitch G8000 Application Manual

The Switch-Centric Model

124
RackSwitch G8000: Application Guide
PVRST is switch-centric: STGs are enforced only on the switch where they are
configured. The STG ID is not transmitted in the Spanning Tree BPDU. Each
Spanning Tree decision is based entirely on the configuration of the particular
switch.
For example, in Figure
each switch is responsible for the proper configuration of its own ports, VLANs, and
STGs. Switch A identifies its own port 17 as part of VLAN 2 on STG 2, and the
Switch B identifies its own port 8 as part of VLAN 2 on STG 2.
Figure 13. Implementing Multiple Spanning Tree Groups
Chassis
Switch A
STG 2
17
VLAN 2
18
STG 2
VLAN 3
VLAN 1
8 2
1
Application
Switch C
The VLAN participation for each Spanning Tree Group in
as follows:
• VLAN 1 Participation
Assuming Switch B to be the root bridge, Switch B transmits the BPDU for VLAN
1 on ports 1 and 2. Switch C receives the BPDU on port 2, and Switch D receives
the BPDU on port 1. Because there is a network loop between the switches in
VLAN 1, either Switch D will block port 8 or Switch C will block port 1, depending
on the information provided in the BPDU.
• VLAN 2 Participation
Switch B, the root bridge, generates a BPDU for STG 2 from port 8. Switch A
receives this BPDU on port 17, which is assigned to VLAN 2, STG 2. Because
switch B has no additional ports participating in STG 2, this BPDU is not
forwarded to any additional ports and Switch B remains the designated root.
• VLAN 3 Participation
For VLAN 3, Switch A or Switch C may be the root bridge. If Switch A is the root
bridge for VLAN 3, STG 2, then Switch A transmits the BPDU from port 18.
Switch C receives this BPDU on port 8 and is identified as participating in VLAN
3, STG 2. Since Switch C has no additional ports participating in STG 2, this
BPDU is not forwarded to any additional ports and Switch A remains the
designated root.
13, though VLAN 2 is shared by the Switch A and Switch B,
Application
Switch B
8
2 1
STG 1
1
8
Application
Switch D
Figure 13 on page 124 is