Spanning Tree Root Bridge Priority

siksika55 · ‎08-17-2007

I am working towards my CCNA and i have a question about root bridge priorities. Several references have conflicting guidance on setting the root bridge priority. Everyone agrees that the default is 32768 and if all are at default, the lowest MAC wins. When you lower the priority, can you use any number between 1 and 32768, or doess it have to be in blocks of 4096 ( 0, 4096, 8092, 12288, etc)? All help is appreciated.

sundar.palaniappan · ‎08-17-2007

I believe you would have set the bridge priority in blocks of 0, 4096, 8092 etc.. If you try to enter a different # the switch you generate an error and ask you to enter a number from that block.

HTH

Sundar

msdesai · ‎08-17-2007

Hi

Yoi can use number between 1-61440 in increment of 4096.

you will get below error message if you enter the value other than the increment of 4096:

% Bridge Priority must be in increments of 4096.

switch(config)#spanning-tree vlan 1 priority ?

<0-61440> bridge priority in increments of 4096

switch(config)#spanning-tree vlan 1 priority 4000

% Bridge Priority must be in increments of 4096.

% Allowed values are:

0 4096 8192 12288 16384 20480 24576 28672

32768 36864 40960 45056 49152 53248 57344 61440

Regards

nambi_gct · ‎08-20-2007

As per 802.1t specification 12 bits from the priority were taken off and are being used for Per VLAN SPT.(12 bits for 4096 vlans) Hence you have the priority value in multiples of 4096.

Francois Tallet · ‎08-20-2007

Yes, that's what Cisco called "extended system id". You can only control the 4 upper bits of the priority fields, the others are the instance ID. When MST is used or when this extended system id feature is enabled, you can only configure the priority by increments of 4096. (some cat6k chassis have the feature enabled by default, with no option to disable it).

F.