STP selecting wrong BLK ports

Answered Question
Jun 3rd, 2008

Hi folks,


in my home lab environment I have 2 2924XL switchs, 1 2950, 1 2960 and 1 3550.


They are all connected to each other via redundant links.

All links are 100Mb

All the switches use the same priority.

I documented all the MAC addresses of each switch using "sh ver"


I then using my limited knowledge of STP presumed to know which ports were blocking, forwarding and root and documented this.


Bearing in mind the above setup...

I know that the root bridge is the lowest BID

I know that the root ports are closest to the root bridge,

I know that the Designated ports are chosen on the switch with lowest BID

Blocked ports are chosen on the switch with the highest BID.


Now when I verify this I conclude that a 2924 which has the highest BID of all my switches will have some blocked ports (depending on how they link together). What I find however is that the switches connected to this 2924 which have lower BID have blocked their ports, and the 2924 has all forwarding.

The 2924 is not the root bridge.


I cannot find why this is happening, is this possibly a bug with older switches? Do you need more info?


any help would be greatly received


thanks

David Hodgson

Correct Answer by royalblues about 8 years 8 months ago

Have you set the spanning-tree priority manually?


Old switches used to have a 16 bit bridge priority vlaue. The new switches use the extended system ID and the bridge priority is a 4 bit value


http://www.cisco.com/univercd/cc/td/doc/product/core/cis7600/121_8aex/swcg/spantree.htm#1037312


Since 2924 is selected as the root switch, it will have its port in the forwarding state.


HTH

Narayan

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mahmoodmkl Tue, 06/03/2008 - 23:33

Hi


As per u r post if the priorities r same then the root will be elected on lowest mac-address.


The switch with the lowest mac-address is going to become the root.


Do u have any vlan configured on this and if u r using pvst+ then each vlan will have a different root bridge.


Thanks

Mahmood

d.hodgson Wed, 06/04/2008 - 00:19

I'm not sure if vlans are configured on the 2924, all the other switches have only vlan 1, I'll have a look tonight when I get home.


thanks

Dave

Correct Answer
royalblues Tue, 06/03/2008 - 23:36

Have you set the spanning-tree priority manually?


Old switches used to have a 16 bit bridge priority vlaue. The new switches use the extended system ID and the bridge priority is a 4 bit value


http://www.cisco.com/univercd/cc/td/doc/product/core/cis7600/121_8aex/swcg/spantree.htm#1037312


Since 2924 is selected as the root switch, it will have its port in the forwarding state.


HTH

Narayan

d.hodgson Wed, 06/04/2008 - 00:13

Hi Narayan,


the 2924 is not the root bridge, there is another switch with a lower BID (in fact it's a 2924, BID 32768 00:01:24:e3:50:a3) which is the root. Doing a "sh spanning-tree" shows that the 2924 (not the root one, but the one that has it's ports forwarding) has the "root" switch as 00:01:24:e3:50:a3 which is correctt, it does not state "this is the root"


None of the switches have the priority set manually, they are all default of 32768.


I'll have a look at the link.


thanks

d.hodgson Wed, 06/04/2008 - 04:57

It's because of the old switches using the 16 bit bridge priority, so all my new switches have 32769 (32768 + vlan1), where as my old switches have just 32768.


thankyou very much

Dave

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