Spanning Tree Setup

[Lakeram Harrypersaud]

I have two Catalyst 3750 and 6 2960 switches. The two 3750 are stacked and i have configured etherchannel between the 3960 and 3750.. I assume spanning tree is turned on.

Can someone please verify my theory of STP setup? I have five vlans,1,2,3,4,5. Is this the correct command to enter on the switches?

spanning-tree vlan 1-5 priority 1 (3750 Core switch command to make it the root bridge)

spanning-tree vlan 1-5 priority 4096 (2960, to make it next in line if the root bridge goes down)\

spanning-tree vlan 1-5 priority 8192 (Third switch and then so on)

Any help will be greatly appreciated?

Thanks,

Lake

[Reza Sharifi · Accepted solution]

Lake,

The lowest priority is 4096.  So, 1 is not .

For priority,  the range is 0 to 61440 in increments of 4096; the default is 32768.  The lower the number, the more likely the switch will be chosen as the  root switch.

Valid priority values are 4096, 8192, 12288, 16384, 20480, 24576, 28672,  32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. All other  values are rejected.

http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_9_ea1/configuration/guide/swstp.html#wp1020666

HTH

[Richard Michael · Accepted solution]

Hi Lake,

yes, your configs looks perfect!  The one with the priority 1 would be the root bridge and then its 4096, and increments in the range of 4096.

Please also note, If you dont want the switches to be a root for any vlan you can configure the below command,

SW3(config)#spanning-tree vlan 1-4094 priority 65535
SW4(config)#spanning-tree vlan 1-4094 priority 65535

If you want the switch to be the root bridge of a vlan, use below command

SW1(config)#spanning-tree vlan 3 priority 1
SW2(config)#spanning-tree vlan 3 priority 2

Remember to do this configuration for each vlan on the network,  because in per vlan spanning-tree, there is a root bridge for each  spanning-tree instance.

Thanks,

Richard

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[Lakeram Harrypersaud]

Thanks very much guys for the quick replies.

How would this work since i have two 3750 stacked switches and six 2960. How will this work since both of the 3750 work as one switch. I don't think i enter commands on each 3750 switch separately?

Thanks,

Lake

[Reza Sharifi · Accepted solution]

Lake,

The 2 3750 stacked are logically one switch.  You only configure it on the master and then the config with sync to the other switch member.

HTH

[Lakeram Harrypersaud]

Thank you Reza.

Should i set all six 3960 switches with a priority of 65535? Is this a good practice?

Thanks,

Lake

[Jon Marshall · Accepted solution]

Lake

Generally speaking you don't need to do this because you explicitly set the STP root and secondary so you don't need to set the priorities on the other switches.

If you add another switch it's default priority will be 32768 so they will not become root or secondary. But i understand what Ricky is getting at, in that if you never want the 2960s to become STP root or secondary then setting them to that value would ensure it.

Like i say though in all the production networks i have worked you only really need to set STP root/secondary but it won't hurt if you do it.

Jon

[Lakeram Harrypersaud]

Thank you Jon.

I assume that i just set the stack as root bridge which is just entering a command and it will be replicated to the slave because everything gets replicated. Now, if the master goes down the slave becomes the root bridge by default?

Is this correct?

Thanks,

Lake

[Jon Marshall · Accepted solution]

Lake

To be honest i'm not entirely sure but i as far as i know the config is simply replicated to the other switch in the stack so no i don't think it can be STP root and secondary root. I think if you want a secondary you will need to use a 2960 as the secondary.

But you do have redundancy in the stack so it is in effect like having 2 switches which would always have root priority over the other switches.

I could be wrong about but that is my understanding.

Jon

[Lakeram Harrypersaud]

Thank you very much Jon