Understanding STP configurations

Unanswered Question
Jun 8th, 2009

Hi all,

I have been studying this material here:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

Can somebody please explain me in simple language what these commands actually do? I appreciate the material from Cisco, but I need better clarifications.

1. Issue the set spantree root vlan_id command in order to set the priority of the switch to 8192 for the VLAN or VLANs that the vlan_id specifies.

Question: What is the relevance of "8192"?

2. There is also this command: "set spantree priority 8192 1". I suppose this is also setting the bridge priority.

Question: What is the relevance of "8192" and then "1"?

3. I suppose the "set spantree root 1,200-204" would set the switch as the root bridge for VLAN 1, 200-204.

Question: Right?

Thanks.

Kind regards

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Giuseppe Larosa Mon, 06/08/2009 - 04:53

Hello Neb,

1) set spantree root

this is a macro that is executed only once: the system looks at the current root bridge priority and uses a value multiple of 4096 to assign priority to local device in order to make it the new root.

The choice of a value multiple of 4096 derives from the extended-id concept that uses the following formula:

priority (vlan X) = base_priority + X

being 4094 the max number of vlans per 802.1Q standard the base_priority has to be given as multiple of 4096.

8192 is used if current root bridge priority is between 16,000 and 20,000

2)

the macro set spantree root is actually expanded in some commands.

One of this commands is

set spantree pri 8192

in your example the current switch uses priority 8192 for vlan 1.

3)

yes your understanding is correct the vlan-list parameter can be given as a list of single vlans or ranges separated with ',' without any space in the middle.

Notice that the macro can have to assign different priorities to the different vlans depending on the findings of root bridge for each.

Notice also that if tomorrow someone on another switch issues

set spantree pri 4096 1,200-204 it becomes the new root.

The set spantree macro is executed only once and cannot react to later changes on other switches

Hope to help

Giuseppe

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