Cisco Support Community
Community Member

spanning tree questions

I have 3 questions.

1) what does the command "spanning-tree extend system-id" do?

2) we do not use vlan 1 for any data/voice traffic (on other vlan's). Can I disable vlan 1 from spanning tree? and what happens if I prune vlan 1 from the trunk ports to other switches? Will this stop the CDP traffic and all the other behind-the-scenes traffic from traversing our switches?

3) what does "spanning-tree optimize bpdu transmissions" do?


Re: spanning tree questions


1) This command enables the STP feature to support extened range vlan ID's.

Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN

The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID

2). Yes, You can disable Vlan1 from STP.If you prune the vlans from the trunk the CDP or management traffic will pass over the native vlan defined on the switch. There are some IOS which supports this , this feature is called as Vlan1 minimization.It is supported mostly on newer IOS.Check also the switch model numbers that you have for the support of this feature.

3).Didnt find much info on this.


-amit singh

Re: spanning tree questions

2)I think VTP at least still uses vlan 1, even when vlan 1 is disabled on a trunk. Basically, there are two kinds of traffic on a port, control and data traffic. When vlan 1 is disabled, only data traffic is blocked for vlan 1. The port is still configured to receive control traffic. But that's a minor point, basically, VTP and CDP should still work once vlan 1 is disabled.

3) This is an engineering command. It should never have appeared in the configuration (the fact this command shows up in the config is actually a bug). The command is disabling a specific way of sending periodic BPDUs (I don't want to enter into the detail) for troubleshooting purposes only. You can ignore this. If you have a recent IOS version, you can remove this from the config.



CreatePlease to create content