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Port-Priority Configured for VLAN but stil not able to be root

cindylee27
Level 1
Level 1

Hi Experts,

I have one question here.Have tried to set Port-priority for one VLAN (VlAN 20) to ensure that the interface g1/1 will be root for VLAN20 instead of g2/1.

To my surprise, g2/1 still is the root for VLAN20. I have put it lower than 128.. which tried at 80.

But when I modified the root port path to 1, I managed to control the root to the other interface.

What is the reason for this? Any factors that causing this?

thanks in advanced.

2 Accepted Solutions

Accepted Solutions

Cindy,

If 2 switches are connected this way:

Switch1 (g1/1) Root FWD -----TRUNK---(DESG Fwd g1/1) Switch 2

Switch1 (g1/2) ALT BLK ------TRUNK---(DESG FWD) g1/2)Switch2.

Now in this case, if you want to make Switch1's g 1/2 as the root, you can do it in 2 ways.

1) Lower the STP cost for port g1/2 On SWITCH1

2) Lower the priority for port g1/2 on SWITCH2.

HTH

View solution in original post

Port priority wouldn't work in your scenario. It would only work both ports (g1/1 & 2/1) connect to the same upstream switch. Since they are connected to two different switches the proper way to influence root port election is to change the port cost on the 3750 switch. Lower the port cost of g1/1 to something lower than gig2/1 on the 3750 switch to make g1/1 become the root port.

HTH

Sundar

View solution in original post

9 Replies 9

It sounds like you may be trying to configure the port priority on the root port. Configure the port priority on the designated port (DP) (upstream switch) and see if that influences the root port election.

HTH

Sundar

deveshkumar
Level 1
Level 1

Lower the priority...higher the chance of getting elected as roo port.

u shld increase the prioroty of G2/1 higher than G1/1 in order to get G1/1 as root port.

Try it mate and let me know if this helps..

Hi Sundar,Devesh,

Actually I am trying to configure the root port at the BLOCKED Port interface but to no avail. I have tried to lower down the port-priority to 80 ony G1/1 but to no avail. The block Port, G1/1 still showing BLOCK. I can;t influence it until I configure the path cost to be 1.

Am trying to figure out what is the reason or how does the spanning tree being calculated on certain parameter influentials...

Thanks for your reply, guys..

Cindy,

As I stated in my previous post port priority when configured on root port doesn't affect the root port election. Can you configure the priority on the port of the other switch (upstream) to which the root port is connected to and that should influence the root port election. The reason why it doesn't work is because it's being configured on the incorrect switch.

HTH

Sundar

Sundar,

Current Connection as below:

4506 (SwC01)--Blocked Port (g1/1 of C3750)

|

4506 (SwC02)

|

|

Root port (g2/1 of C3750)

Are you referring to configure it at C4506 Switch instead of C3750 Switch where the root/block ports in action?

Thanks again.

Cindy,

If 2 switches are connected this way:

Switch1 (g1/1) Root FWD -----TRUNK---(DESG Fwd g1/1) Switch 2

Switch1 (g1/2) ALT BLK ------TRUNK---(DESG FWD) g1/2)Switch2.

Now in this case, if you want to make Switch1's g 1/2 as the root, you can do it in 2 ways.

1) Lower the STP cost for port g1/2 On SWITCH1

2) Lower the priority for port g1/2 on SWITCH2.

HTH

Pthiagas,

Thanks for that reply.I am much more clearer now..:)

but are the option 2( priority) can't set on Switch1 is a by default a behaviour of spanning tree features ?

Thanks for the great reply and I rated your post.

Nope. The priorities are the same for all ports by default.

Also, I hope this URL might be helpful for understanding STP.

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Port priority wouldn't work in your scenario. It would only work both ports (g1/1 & 2/1) connect to the same upstream switch. Since they are connected to two different switches the proper way to influence root port election is to change the port cost on the 3750 switch. Lower the port cost of g1/1 to something lower than gig2/1 on the 3750 switch to make g1/1 become the root port.

HTH

Sundar

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