How to know the Bridge ID of the connected switches ?

[chandra_rc16]

Hi All,

How can i know the Bridge ID of the connected switches ?

Scenario:

Let us assume that my network is having 10 swtiches and all are running STP. Assume that none of the values are default (Bridge priority, Port priority) SW1 is acting as ROOT switch with some XX Bridge priority. So assume that there is secondary ROOT configured on one of the swiches and i want to make switch5 as a ROOT switch.

So inorder to make Switch5 as a root switch, if i just change the bridge priortiy to the lowest value, then it may or may not make that switch to be a ROOT because there may be a another switch configured with the same priority with a lower MAC ...

So can't we get the Bridge ID values of all the connected switches ? When i use "show spanning-tree" on any switch it just shows the current ROOT and its own bridge ID values ....

Regards,

Chandu

[paul driver]

Hello

by default the bridge id of switces will default to 32768 plus the vlan id value of any vlan eg vlan 2 = 32770

so if these values are equal among swithes by default the switch with the lower base mac address with take precedence

unless your manually change this by either using the root macro command or applying a root priority value if the vlans

res
paul

Sent from Cisco Technical Support Android App

[chandra_rc16]

Hi Paul,

I know that theory. But in my scenario above none of the switches are having default values.

Please take a look at it completely.

Regards,

Chandu

[devils_advocate]

Logon to each switch and check to see what the priority has been changed to?

You only have 10 switches, it won't take long.

Also, this is a poor network design so it would be worthwhile sorting it out.

[JohnTylerPearce]

Chandra,

Remember, most likely you are running, PVST+, or RPVST+, and or MST. With the exception of MST, each and every vlan will be running it's own version of STP, with it's on root switch, root ports, designated ports, and blocked ports. (It's possible to do the same with MST, but that's a story for a different time).

If you run the command 'show span vlan X' where X is the numerical value of the vlan, you will see the current root switch BID and your the current BID for the switch, from which you ran the command.

An easy way to find the root switch, if to just follow the root ports.

By default all switches have a priority of 32768, and since System ID Extension is most likely enabled, the priority will be 32768+(Vlan Number), so for VLAN 10 the priority will be 32868.

Also, if you have 'span vlan x root primary' it will have a priority of 24576 I believe + vlan number. Also, someone could have manually configured each switch prioity.

Hope this helps.

[chandra_rc16]

Hi John,

Thanks for your info. But i believe that is not resolution for my scenario above because none of the switches are having  their default values.

Regards,

Chandu

[chandra_rc16]

Hi Devil,

This is the case of 10 switches so logging on each switch is fine.

But what if, if i have 50 switches.

I think there should be a Table like MAC table that should show all the switches and their priorities and MAC's.

And how it could be poor design... may the person who changed the default values might have started with a single switch and as his network grows he went on modifying priorities as per his needs. But after a long time when he looks back to make any changes in the switched network he may confused and it will be irksome for him to logon to each switch and grep the priority to make any switch as a ROOT as i asked in my scenario .

Regards,

Chandu

[devils_advocate]

Is this a genuine issue you need help with or an exam question you don't know the answer to?

If its the latter then I am not prepared to help but if its the former then my advice above stands and John has also given some good advice.

Having random priorities on your switches can cause problems so I would read up on Spanning tree and look at the Cisco design documents in terms of where to place your root bridge. I am not familiar with your network but standard practice is to have your Distribution switch being your root bridge and your access switches having the default priority to prevent them becoming the root.

[chandra_rc16]

Hi Devil,

This is not a genuine or real time issue. I'm practicing STP for my CCNA on that way i'm creating diffferent scenarios and doing "Trial and Error" with STP.

In that view i created this scenario and was expecting a perfect resolution for it.

Regards,

Chandu

[Peter Paluch · Accepted solution]

Chandu,

One fact to consider: in STP, BPDUs are sent only from Designated ports. They are not sent from Root or from Non-Designated (and hence Blocking) ports. This means that a root switch is unable to even see Bridge IDs of its directly connected neighbors because the do not send any BPDUs back. This goes for every switch that has only Designated ports to its downstream neighbors. As these neighbors either have Root or Non-Designated ports towards this switch, they do not send any BPDUs towards it. As a result, you do not have any clue about their Bridge IDs.

Best regards,

Peter

[chandra_rc16]

Do we have any docs for Planning a perfect switched network ?

[JohnTylerPearce · Accepted solution]

This would require a network design, and each one is different. You have various technologies, and new switches coming out, so it's really hard to explain all that into one paragraph. You have to consider, how you want QoS, NHRPs, redundant links, etc.

I have personally had an interview question, where I was given about 10 switches, with lines between them for links, and told how would you improve on this design. I literally told the guy, I can't do this, because I have no idea how it's currently setup, and what the goals of thie design are. Now, if you really don't know what your doing, or are not confident I would suggest every saying that in an interview.