Root port and designated port

kt2003
kt2003 used Ask the Experts™
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I am trying to build the following switch configuration:
At L3 (distribution layer) I have two 3500 switches (L3SW1 and L3SW2). L3SW1 is the root switch and L3SW2 the backup switch. Both switches are linked by a trunk port.
At L2 two 2900 (L2SW1 and L2W2) will be connected to the two switches on L3.
The servers and work stations will be hooked onto L2SW1 and L2SW2.
L2SW1 will have a trunk port f0/1 connected to L3SW1 and f0/2 also trunk connected to L3SW2. The other ports on the switch belong to a VLAN called Sales.
L2SW2will have port trunk port f0/1 connected to L3SW1 and f0/2 also trunk connected to L3SW2. The other ports on the switch belong to a VLAN called Sales.
After the root/backup briegde has been designated I have to select a root port for each switch and a designated port.
My question is:
1-In the above configuration how many root ports can I select per switch? Which ports on L2SW1 and L2SW2 will you select as the root port?
2-How can I configure the designated port? Which ports on L2SW1 and L2SW2 will you select as the designated port?
 


 L3SW1 is been set as the root switch and L3SW2 as the backup root switch.
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Anton NikitinNetwork Security Administrator

Commented:
1 - there is only one root port on every non-root switch, that is the shortest cost to the root bridge. That is a fa0/1 on L2SW1 and fa0/1 on L2SW2 (the port on the L3SW2 that connects to the L2SW1 is also root port).
2 - ports fa0/2 on L2SW1 and fa0/2 on L2SW2 will be blocking, their other side on the L3SW2 will be designated and all ports of the L3SW1 will be designated as well.

In that case there is nothing you have to do to configure designated ports, they will get their roles as soon as the root bridge is elected (using priority and MAC address)  and the spanning tree converges.

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Author

Commented:
Cisco-help thanks a lot. Just two more questions.
A. If STP PVST+ is used do we have to set a root port? What about Rapid-PVST+? Do we need a root port and a designated port?
B. Can a trunk port (as in this case L2SW1- fa0/1 and L2SW2-fa0/1) be a root port?
Network Security Administrator
Commented:
A. The difference for the PVST and Rapid PVST is that the topology may be different for each VLAN you have on the network (PVST stands for Per VLAN Spanning Tree, that means each VLAN has it's own Spanning Tree topology that may be different or the same as other VLANs). So the root bridge is elected for each VLAN. To make my topology right for every VLAN make sure that L3SW1 is the root bridge in every VLAN (show spanning-tree).
B. Considering what I said in the first answer, trunk may be a root port for some or all VLANs on that trunk.

Author

Commented:
Thanks

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