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Spanning-tree Port-Priority

dgragg002
Level 1
Level 1

I am trying to connect 3 switches using Port priority.

I would like to have vlans 1-98 on one leg and 99 and
up on the other. With the vlans able to switch in the
event of a failure.

Vlan 1-98 conect Sw1 gig 4/0/16 to Sw2 gig 1/0/2
Vlan 99-4094 connect Sw1 gig 3/0/50 to Sw3 gig 1/0/49
Sw2 and Sw3 are connect by Sw2 1/0/49 and Sw3 1/0/50

The Config is as follows

<<Switch 1>>

interface GigabitEthernet3/0/50
 switchport mode trunk
 spanning-tree vlan 99-1005 port-priority 32
 spanning-tree vlan 1-98 port-priority 240

interface GigabitEthernet4/0/16
 switchport mode trunk
 switchport voice vlan 2
 spanning-tree vlan 1-98 port-priority 32
 spanning-tree vlan 99-4094 port-priority 240

<<Switch 2>>

interface GigabitEthernet1/0/2
 switchport mode trunk
 switchport voice vlan 2
 spanning-tree vlan 1-98 port-priority 32
 spanning-tree vlan 99-4094 port-priority 240

interface GigabitEthernet1/0/49
 switchport mode trunk

<<Switch 3>>

interface GigabitEthernet1/0/49
 switchport mode trunk
 spanning-tree vlan 99-1005 port-priority 32
 spanning-tree vlan 1-98 port-priority 240

interface GigabitEthernet1/0/50
 switchport mode trunk


The Problem I am having is that it goes into a loop and locks
Everything up. Sounds like the Spanning -tree is not working.

Any assistance would be appriciated.

3 Replies 3

Shaunak
Cisco Employee
Cisco Employee

Hi dgragg002,

Which switch is the root for the VLANs at the moment? 

As per my understanding we are working with a triangle topology, right and I'm assuming SW1 is the root for all the VLANs if this the case then I would suggest that you alter the cost on the downstream switches so that the path via SW2 is favourable for VLANs 1-98 and the path through SW3 is favourable for the other remaining VLANs.

The port-priority is a tie breaker when we have the same cost to reach the root along with the same upstream bridge ID, that is when the upstream switch advertises it's port priorities to the downstream switch and the lesser one wins. The port-priority is only used when we have multiple connections to a single upstream switch in your scenario we have single links between three unique switches in this case alter the port cost to reach the root.

Remember cost is calculated from the perspective of the downstream switch i.e. the outgoing interface to the root and the priority is advertised by the upstream switch to influence the port used by the downstream switch for forwarding.

Thanks,

Shaunak

Thank you.

I will try "cost"

I am also taking this time to learn more about the spanning tree itself.

You're very welcome.

Do revert and let us know how it turned out.

Thanks,

Shaunak

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