how to select Switch/Bridge as root bridge

hi everybody can anybody tell me what command is used to set the periority of bridge to set it as root bridge

waiting for response

thanks

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Comments

unwritt3n

Use:

Router(config)#spanning-tree vlan 1 priority [priority #]

The priority number is in increments of 4096. So set it as 4096 so its the lowest, compared to the default 32768.

BubbaJ

unwritt3n wrote:
Use:
Router(config)#spanning-tree vlan 1 priority [priority #]
The priority number is in increments of 4096. So set it as 4096 so its the lowest, compared to the default 32768.

This sometimes varies. I know there are switch/IOS combinations that will take 0 as the priority.

The reason for this question may be looking for this as the answer:
Switch(config)#spanning-tree vlan vlan-id root primary [diameter net-diameter [hello-time seconds]]

ciscos

may I ask sth relevant?!!
In a topology with
2 switches with protities 1 and 32768 which one is going to be the root?

BubbaJ

ciscos wrote:

may I ask sth relevant?!!
In a topology with
2 switches with protities 1 and 32768 which one is going to be the root?

STP uses the lowest BID as the root bridge.

mcockrell

unless they are the same, then it uses lowest MAC

BubbaJ

mcockrell wrote:

unless they are the same, then it uses lowest MAC

The BID is the priority+MAC.

ciscos

cheers guys but it wasn't that I asking...I know all these...

I was wondering about the value of "1" as a priority if is it usable.
I read above that 4096 is the lowest.

EdTheLad

ciscos wrote:

cheers guys but it wasn't that I asking...I know all these...

I was wondering about the value of "1" as a priority if is it usable.
I read above that 4096 is the lowest.

The bpdu is 64 bits, its comprised of 48 bit MAC address,12 bit vlan tag and 4 bit priority tag.The original 802.1d bpdu doesnt have the 12bit vlan tag and doesnt support multiple instances of spanning-tree using different vlans.The original version supported a priority of 1,the current version in cisco which supports multiple instances is as follows:

If you examine the 16 bits to the left of the MAC address field

xxxx xxxxxxxxxxxx

The 12 bits from right to left are the vlan id's, the extra 4 bits are the priority
Using binary 2^0,2^1 .......2^11 this is the last bit for the vlan,i.e. 2048 vlan id's.
The 4 priority bits start in location 2^12 = 4096, so you can only have increments of 4096

0001 = 4096
0010 = 8192
0011 = 12288
Etc...