BCMSN Spanning tree Scenario

My topology has four switches, A, B, C, D. and C becomes the elected root for all VLANS (Lowest MAC), default priority on all.

I issued the following command on switch D
spanning-tree vlan 150 priority 20480. Which immediately made it the root

I went back to SW C (root switch) and issued a command
spanning-tree vlan 150 root primary.

It, however made itself the root for VLAN 150 but changed the priority to 20480, same as switch D, Now theoretically shouldn’t it change its priority to 4096 less than the current root?.

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Comments

Mrock4

I tested this, and it appears that the only bridge that will do this is the ORIGINAL root bridge, because it has the lowest MAC, therefore setting it equal will still cause it to become the root bridge.

Verified this by going to all bridges and setting the "root primary" command over and over, and every bridge (except the original root, aka the one with the lowest MAC) will set the priority to 4096 less than the current root. The switch with the lowest MAC will set it's priority to be equal of the current root, but will become root itself due to lowest MAC..

Good catch. This was pretty interesting, I've never noticed that.

ConstantlyLearning

Strange one.

In my switching lab I went through the same steps as yourself and it worked as normal. i.e. Issuing the 'spanning-tree VLAN X root primary' command reduced the priority to 4096 less than the current root switch's priority.

What IOS is running on switch C?

I wonder is it possible that the root macro on switch C uses differant methods? Maybe it takes into account the mac address as well. Says: "Well let's just match the current root switches priority because I'll become the root switch for VLAN X anyway on account of my lower mac address".

EDIT: Good work Mrock4! I obviously didn't go through the EXACT same steps as the OP.

singh8281

That's right Mrock, i just tried on one of the other non root switches and it did reduce it to 16534 as expected.

I appreciate your inputs fellas.

Mrock4

To twist it even further, I noticed that if you use the root primary command on say, Switch B..it becomes root..do it on Switch A...Switch A becomes the root...if you do it AGAIN on Switch B....then switch B *should* repeat this odd behavior, and make itself have equal priority...only under the condition that it's MAC is lower though.

ConstantlyLearning

Something else of note in relation to this.

In the authorized self-study guide (4th ed.) it doesn't say that the priority will be change to 4096 less than the current root bridges priority. It says that it will change to 1 less than the current root bridges priority...

ConstantlyLearning

Mrock4 wrote: »

To twist it even further, I noticed that if you use the root primary command on say, Switch B..it becomes root..do it on Switch A...Switch A becomes the root...if you do it AGAIN on Switch B....then switch B *should* repeat this odd behavior, and make itself have equal priority...only under the condition that it's MAC is lower though.

Cool. Here's my interpretation then.

1. Some switch is the root
2. Make some other switch the root (Current root priority minus 4096)
3. Make the previous root switch the root switch again (Is mac address lower? Yes - match priority / No - current root priority minus 4096)

Mrock4

From cisco.com

"If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value as shown in Table 10-1.)"

Catalyst 2950 Desktop Switch Software Configuration Guide, 12.1(9)EA1 - Configuring STP [Cisco Catalyst 2950 Series Switches] - Cisco Systems

Should be noted that guide is for a 2950, but the behavior is the same in this case.

Mrock4

ConstantlyLearning wrote: »

Cool. Here's my interpretation then.

1. Some switch is the root
2. Make some other switch the root (Current root priority minus 4096)
3. Make the previous root switch the root switch again (Is mac address lower? Yes - match priority / No - current root priority minus 4096)

Couldn't have worded #3 better.

singh8281

Constantly, It will reduce by 1 on Cat OS based switches. On IOS based it will reduce by 4096

ConstantlyLearning

singh8281 wrote: »

Constantly, It will reduce by 1 on Cat OS based switches. On IOS based it will reduce by 4096

Ah, cheers.

I guess the guys writing that part of the self-study guide had Cat OS on their mind.