Specifications
Switching 8-49
Software Release 2.7.3
C613-03098-00 REV A
Root bridge selection for MSTP MSTIs
The MSTP protocol will select its root bridges for each MSTI. It does this by
selecting, for each MSTI, the bridge that contains (numerically) the lowest
bridge identifier. This is shown in Figure 8-7 on page 8-49.
Figure 8-7: Example MSTP MSTI configuration
Notice that the root bridges are different for the two MSTIs. The root bridge for
MSTI 2 is Switch 3 and the root bridge for MSTI 3 is Switch 2. This is because
for MSTI 2 Switch 3 it has been given the lowest MSTI priority value, 4096,
compared with 8192 for Switches 1 and 2.
Similarly, the root bridge for MSTI 3 is Switch 2 because its MSTI priority value
has been set to 4096, compared with the value 8192 set for Switches 1 and 2. If
all three bridges were configured with the same priority value for a particular
MSTI, then Switch 3 would become the root bridge for that MSTI, because it
has the lowest MAC address of the three switches.
Switch 1
Switch 3
Switch 2
Root Bridge for MSTI 3
Root Bridge for MSTI 2
7
8
10
10
987654321
10
10
987654321
10
10
987654321
V
LA
N
12 M
S
TI 2
V
LA
N
25 M
S
TI 3
V
LA
N
25 M
S
TI 3
V
LA
N
12 M
S
TI 2
VLAN 25 MSTI 3
VLAN 25 MSTI 3
VLAN 12 MSTI 2
VLAN 12 MSTI 2
VLAN 12 MSTI 2
VLAN 12 MSTI 2
VLAN 12 MSTI 2
VLAN 12 MSTI 2 VLAN 25 MSTI 3
VLAN 25 MSTI 3
Drawing Ref Number
Drawing Ref Number
Switch 3 Configuration
Configuration Name = HeadOffice
Revision Level
= 0
MSTI-2
Bridge Identifier
Priority = 4096
MAC Address = 00-00-cd-04-47-24
VLAN Members = 12
MSTI-3
Bridge Identifier
Priority = 8192
MAC Address = 00-00-cd-04-47-24
VLAN Members = 25
Drawing Ref Number
Switch 1 Configuration
Configuration Name = HeadOffice
Revision Level = 0
MSTI-2
Bridge Identifier
Priority = 8192
MAC Address = 00-00-cd-07-8a-00
VLAN Members = 12
MSTI-3
Bridge Identifier
Priority = 8192
MAC Address = 00-00-cd-07-8a-00
VLAN Members = 25
Drawing Ref Number
Switch 2 Configuration
Configuration Name = HeadOffice
Revision Level = 0
MSTI-2
Bridge Identifier
Priority = 8192
MAC Address = 00-00-cd-10-00-37
VLAN Members = 12
MSTI-3
Bridge Identifier
Priority = 4096
MAC Address = 00-00-cd-10-00-37
VLAN Members = 25