Specifications

8-50 Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
Path selection for MSTP MSTIs
After the protocol has selected its root bridge for each MSTI, it selects which are
to be the active and blocked paths for each MSTI. The port with the best path to
the root bridge is selected as the foot port and becomes active. Other ports that
also lead to the root bridge, but via a path that is better than the path back
through the switch, are selected as alternate ports and are blocked to prevent
loops. Ports that are connected to another port on the same switch, where that
port has a better priority value, are backup ports and are blocked to prevent a
loop. All other ports that are not disabled are selected as designated ports and
are eventually made active. Figure 8-8 on page 8-50 shows which paths have
been selected.
Figure 8-8: Example MSTP MSTI Path Configuration
For MSTI 3
Between Switches 1 and 2 there are two paths available, Port 3 to Port 3, and
Port 4 to Port 4. Since no port priority has been explicitly applied, all port
configurations have their defaults. Since all ports have the same speed (100
MBPS), each port has a Port Path Cost of 200,000. Since Port 3 is numerically
lower than Port 4, the active path is the one between Switch 1 Port 3, and the
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
VLAN 12 MSTI 2
(Blocked)
VLAN 25 MSTI 3
(Blocked)
VLAN 25 MSTI 3
(Blocked)
VLAN 12 MSTI 2
(Active)
V
LA
N
25 M
STI 3
(Blocked)
V
LAN
25 M
STI 3
(A
c
tiv
e
)
VLAN
12 M
S
TI 2
(Blocked)
VLA
N
12 M
STI 2
(B
locked)
VLAN
12 M
STI 2
(Blocked)
VLAN
12 M
STI 2
(Blocked)
VLAN 12 MSTI 2
(Active)
VLAN 12 MSTI 2
(Blocked)
VLAN 25 MSTI 3
(Blocked)
VLAN 25 MSTI 3
(Active)
MSTP-MSTR CONFIG 2
Switch 1 - Port Configuration
MSTI 3
Designated Bridge
Root Port: Port 3.
Alternate Ports: Ports 4, Port 5,
Port 6, Port 7, Port 8.
MSTI 2
Designated Bridge
Designated Ports: Port 3, Port 4,
Port 7, Port 8.
Root Port: Port 5.
Alternate Port: Port 6.
Switch 2 - Port Configuration
MSTI 3
Root Bridge
Designted Ports: Port 3, Port 4,
Port 7, Port 8, Port 9, Port 10.
Port 3 (Active), Port 4 (blocked)
MSTI 2
Designated Bridge
Root Port: Port 9.
Alternate Ports: Port 3, Port 4, Port 7,
Port 8, Port 10.
Switch 3 - Port Configuration
MSTI 3
Designated Bridge
Designated Ports: Port 5, Port 6
Root Port: Port 9.
Alternate Ports: Port 10.
MSTI 2
Root Bridge
Designated Ports: Port 5, Port 6
Port 9, Port 10.