STP Extend System-id Question

[rcrjames]

I have a network with a 3550 switch at the core and 3 x 2900 XL switches as edge switches.

The 3550 is running IOS: 12.1(8) EA1c
The 2900 XLs are running Enterprise software: IOS: 12.0(5)WC7

The 3550 therefore supports the STP extended system ID and the 2900's do not.

There are redunant links between the 2900XL switches in order to provide basic connectivity if the 3550 fails. There are two VLANS.

A note on CCO states:
If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.

The question is: Bearing in mind the above note, will using the "spanning-tree vlan root primary" command on the 3550 force it to become the Root Switch in this configuration. Or is it unlikely ever to become the Root Switch as described above.

Thanks in advance.

 

[fmonteiro]

This "problem" occurs only if you have mixed switches that both do and do not support the extended system id. Switches running old software have a default bridge priority of 8192 while the new software (supporting extended system ID) has a default of 24576.
In this environment it is better to set the bridge priority explicitly, by the "spanning-tree vlan vlan-id priority" value command.
You may, as well, set the root priority for each vlan in only one switch and, in this case, it should work.