Configuring spanning-tree VLAN priority. 2 core switches.

[Roger McCarrick]

Here's my set up:

2 3750 stacks trunked together with a port channel over 4 x 1gig links.

Hanging off each 3750 stack is a pair of 2960s.

Each 2960 is trunked to it's 3750 stack with a port channel over 2 x 1gig links.

There are SANs patched into the 3750s.

This switching environment is used for VMware running the distributed virtual switch.

There are over 100 port groups in the distributed virtual switch tagged with VLAN IDs.

There are the same VLANs configured on the cisco switches.

VTP is running. All vlans exisit on all switches.

In the diagram each ESX patched into a switch really represents multiple ESX hosts.

So for instance, in a pair of 24port 2960s, there are 7 esx hosts patched in. 6 NICs per host, 3 to each switch.

A VM could reside on any host at any give time. So all vlans need to be on all cisco switches, hence VTP.

So my question is how to configure spanning-tree vlan priority.

Can the 2 3750 stacks be equal spanning tree roots?

On the two 3750 stacks I have:

  spanning-tree mode rapid-pvst

  spanning-tree portfast default

  spanning-tree extend system-id

  spanning-tree vlan 1-999 priority 24576

( I ran the command "spanning-tree vlan 1-999 root primary" ) to get that priority number.

on each of the edge switches I have:

  spanning-tree mode rapid-pvst

  spanning-tree portfast default

  no spanning-tree optimize bpdu transmission

  spanning-tree extend system-id

  spanning-tree vlan 1-999 priority 28672

does this seem right?

I read that the root is calculated with the priority and the mac address, so I  guess ultimately, even if I try to make the two 3750 stacks the same priority, one will win out as root due to its MAC address. Correct?

I am using 1-999 at the moment. As I said I have over 100 VLANs. The max is 128 vlans and I will get there.

So I will be looking into migrating to MST from PVST where I can have 4095 vlans.

This part I dont know how to do. But I want to make sure spanning tree is good now before attempting a migration.

thanks.

[jonathanaxford]

Hi,

I would consider choosing one of the 3750 stacks to be the root, then on the other, run the spanning-tree vlan XXXX root secondary command.

That way its totally predicatable which switch will be the root and backup root. The edge switches will then sort themselves out and should never ru the risk of becoming the spanning tree root (unless both 3750 stacks disappear...)

Essentially though, your config will work just fine.

MST sounds like a better solution, especially considering the number of VLANs you have and small number of redundant links through the network.

Hope this helps

Jonathan

[Roger McCarrick]

Jonathan

thanks very much for the reply.

You said the edge switches would sort themselves out. Do you mean I dont have to specify the priority on the edge switches? Or that the priority I have is fine.

thanks again.

Roger

[cadet alain]

Hi,

The Priority is used to elect the root bridge so you only have to change it on the switch you want to be the root, you can leave the default on the others.

Regards.

Alain.

[Roger McCarrick]

Well I've moved to MSTP (multiple spanning tree protocol) .... here's a snippet of the config.

Do I have to remove the " spanning-tree vlan 1-999 priority 24576" line and if so is that simply

    "no spanning-tree vlan 1-999 priority 24576" ?

And the line "spanning-tree etherchannel guard misconfig" is new. I didn't add it. What's that doing?

port-channel load-balance src-dst-ip
!
spanning-tree mode mst
spanning-tree portfast default
spanning-tree etherchannel guard misconfig
spanning-tree extend system-id
!
spanning-tree mst configuration
name cisco
revision 1
instance 1 vlan 1-1000
!
spanning-tree mst 0-1 priority 28672
spanning-tree vlan 1-999 priority 24576
!
vlan internal allocation policy ascending

thanks