Is "spanning-tree vlan priority 0" valid?

[gwhuang5398]

3560 command reference says valid priority values are 4096, 8192, ... however I tried "spanning-tree vlan priority 0" and the command was accepted. When I "show spanning-tree brief", the vlan I configured did show priority 0.

Did I read the command reference right? (I looked at 12.2(44) and 12.2(52)

Is "priority 0" a valid configuration even if IOS accepts it?

Thanks

[jonatrod · Accepted solution]

Good morning gwhuang5398

My name is Johnnatan and I am part of the Small business Support community.  This is a valid value for this command, actually has more priority than 4096. The priority order starts from 0 and then increases in 4096.

     0, 4096, 8192, 12288, …

If you configure two switches one of them with priority ¨0¨ and the other one with priority  ¨4096¨, the first one with priority ¨0¨ is going to be the root.

I hope you find this answer useful, if it was satisfactory  for you, please mark the question as Answered.

Thank you.

Johnnatan Rodriguez Miranda.

Cisco Network Support Engineer.

[pagamar2004]

Hi Jonatrod,

when two switches are set with "priority 0" the first, and "root primary" the second,

which will become root-bridge? Some say the second, most say the first.

IMHO, if I'm not wrong, the second Switch generates a Syslog complaining it can't

grant to become root-bridge, because of the presence in the network of the

"priority 0" Switch (it cannot set himself 4096 below).

So I think the two switches will set both to "priority 0", and tie break on their MAC...

The lowest MAC generates a lowest BID, and this will decide the root-bridge. Isn't it?

[CiscoPurpleBelt]

All tie breakers are based on these factors:

Lowest root bridge ID

Lowest root path cost to root bridge.

Lowest sender bridge ID.

Lowest sender port ID.

[pieterh]

When you issue the "root primary" command, this switch tries to become the switch with a priority lower than the current lowest priority
as it cannot execute this command, the active priority most likely remains unchanged (not changed to "0")
and the current switch with priority "0" remains root

[pagamar2004]

I saw sometimes a diag message, when trying "root primary"
in the presence of another switch with "priority 0", stating
something like "I cannot guarantee I will become the primary"
and this is correct, because it seems to me that the priority
was set to 0, and then the role was going to depend on the
MAC values, which are random...
Or not? I don't know which IOS version gave me the message.
Marco

[mohamedismailelhedfi]

It is indeed true if two switches are set to "priority 0" the root-bridge choice depends on the MAC address value. The one having the lowest will be chosen.