# root bridge in stp

Jul 2nd, 2008
• Bronze, 100 points or more

hi every body!

I need some help to understand the following.

sw1(config)# spanning-tree vlan 4 root primary.

According to David hucaby(ccnpbcmsn guide)" this command modifies the switch'bridge priority value to become less than the brige priority of the current root bridge. If the current priority is more than 24,576, the local switch sets it priorty to 24,576.If the current root priorty is less than 24,576, then local switch sets it priorty to 4096 less than the current root"

My question what happens if:

(1) The current bridge ' priorty is 24,576, what will local switch(sw1) do in this case?

(2) if the current bridge' priorty is 4096, what will local switch(sw1) do?

will it assigs itself zero priorty because 4096(current root'prioty)minus 4096 yeilds =0?

(3) how about if current' root priorty is 4000 ? will local switch assigs itself -96 priorty because 4000-4096=-96?

thanks alot and have a nice day !

Hello Sarah,

here we meet the mac address reduction (system-id concept) used in modern IOS switches.

To be able to track the root bridge id in each vlan in PVST (per vlan Spanning Tree)

STP priority in vlan X = base_value + X

the step is 4096 because 4096 is the range of Vlans in 802.1Q standard.

The command should accept only multiple of 4096 for this reason

so if the current root priority is 4010 = 4096+4 the priority for vlan 4 will be set to 0+4 = 4

(1) will set to 24,576 - 4,096

(2) 0

(3) 0

priority cannot be negative and should accept only 4096 or multiple of it if you don't disable the system-id

Be aware that this command is actually a macro if a new switch is connected with a lower priority it will take the role of root bridge

hope to help

Giuseppe

Overall Rating: 5 (1 ratings)

## Replies

Giuseppe Larosa Wed, 07/02/2008 - 12:49
• Super Silver, 17500 points or more
• Hall of Fame,

Founding Member

Hello Sarah,

here we meet the mac address reduction (system-id concept) used in modern IOS switches.

To be able to track the root bridge id in each vlan in PVST (per vlan Spanning Tree)

STP priority in vlan X = base_value + X

the step is 4096 because 4096 is the range of Vlans in 802.1Q standard.

The command should accept only multiple of 4096 for this reason

so if the current root priority is 4010 = 4096+4 the priority for vlan 4 will be set to 0+4 = 4

(1) will set to 24,576 - 4,096

(2) 0

(3) 0

priority cannot be negative and should accept only 4096 or multiple of it if you don't disable the system-id

Be aware that this command is actually a macro if a new switch is connected with a lower priority it will take the role of root bridge

hope to help

Giuseppe

sarahr202 Wed, 07/02/2008 - 14:03