port priority in mst

Unanswered Question
Jan 11th, 2009
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A B C D



A connected to B fa 0/1 fa 0/2 and D on fa 20/21


B is conected to C fa 0/1 fa0/2


C is conedted to D fa0/1 fa0/2


B is also conedted to D fa 20 fa21


D is conedted to A as well fa 20 and fa21



its a full redundant topolgy



i have 6 vlans swicth A is server and the rest is client


Vlan 234567


i have config mst (on all swicthes)


instance 1 234


instance 2 567



swicth A is root for instance 1


swicth D is root for instance 2


when i do sh spanning tree mst on switch on A


all ports are blocked expept fa0/1 which is connected to switch B


now i tried to config instance 1 on fa 20 and instance 2 on fa 21 (which is connedted to switch D)


i put the comand spanning tree mst 1 port-priority 200 on fa20 and same except instance 2 on fa21


after that when i do sh priority is been changed but the port is still in blocking mode why is that i even try to put cost but still port is in blocking mode



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drolemc Mon, 01/19/2009 - 09:26
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Cisco IOS uses the port priority value when the LAN port is configured as an access port and uses VLAN port priority values when the LAN port is configured as a trunk port.

Try this example.

Router(config)# interface fastEthernet 20

Router(config-if)# spanning-tree mst 1 cost 1234567

Router(config-if)# spanning-tree mst 1 port-priority 240

Router(config-if)# ^Z

Router# show spanning-tree mst 1 interface fastEthernet 20

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