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Spanning tree priority question.

Gig

A ----- B

| |

100Mbps| |100 Mbps

| |

D-------C

2x 100Mbps Etherchannel

On switch C, link C-D (Po1) is blocked while link C-B (FA0/21) is Forward.

How can I change it so that C-D is forward instead.

I had tried increase port-priority on Fa0/21 to 160 to no avail.

Root bridge is switch A with mac addr 00d0.0447.f022.

SwitchC# sh spanning-tree vlan 35

VLAN0035

Spanning tree enabled protocol ieee

Root ID Priority 32000

Address 00d0.0447.f022

Cost 23

Port 21 (FastEthernet0/21)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32803 (priority 32768 sys-id-ext 35)

Address 0017.5a48.abc0

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/21 Root FWD 19 128.21 P2p

Po1 Altn BLK 12 128.65 P2p

4 REPLIES
Silver

Re: Spanning tree priority question.

Try to increase the cost of port Fa0/21 on switch c.

Thanks.

Bronze

Re: Spanning tree priority question.

As said in earlier mail, you need to increase the port cost with the following INTERFACE conf command.

int f0/21

spanning-tree vlan 35 cost 50

The reason why changing the port priority didn't work was port priority comes into picture only when a tie happens for root path cost which is equal through 2 paths. This was not in your case as root path costs vary in the 2 paths.

Thank you,

Regards...

-Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.

Re: Spanning tree priority question.

Messing with the cost is one of the best ways to get unexpected result from your STP. Leave it at the default values and it works ;-)

Regards,

Leo

Bronze

Re: Spanning tree priority question.

Hi Leo,

BTB, the user have default values which didn't work as he wanted anoter desired path.

I don't think it's messing with the cost rather he's trying to optimize his design for some other reasons :-)

Thank you,

Regards...

-Ashok.

With best regards... Ashok ----------- Pls kindly rate if helpful or answered your question.
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