[c-nsp] Migration to Rapid-PVST

[Per Carlson]

On Sat, May 13, 2006 at 09:39:47AM +0200, Robert Hass wrote:

> How procedure of migration to Rapid-PVST should looks ? Should I first
> upgrade core switches and next access-switches ? Is this migration
> bring some outages in network (eg. caused by spanning-tree
> recalcuations).

If you are going to change the STP, why not take the full step to
to MST? When using "desktop switches" in the network, you will
sooner or later run into the STP instances limitation (128
instances). By running MST, this will no longer be an issue.

The most deterministic procedure are to start with the root
switches in the network, and change the STP-mode one neighbour
after one other.  

Yes, there will be some outages in the network. How long those
outages will be are dependent of several factors: 

- how the edge ports (customer ports) are configured (are those
  running STP or not)

- the number of MAC-addresses that needs to be flooded and
  relearned

- STP topology, i.e where are the root switches

If you are going to MST, I would recommend this procedure (tested
successfully in our lab):

- Configure MST on all switches in the network, but do not change
  the STP mode.

- Configure the root switches as MST-root switches for all
  instances, including instance 0.

- Change the STP mode on the switches to MST starting with the
  root switches. Work your way through the network one neighbour
  at a time.

Note: Even though two switches are configured with exactly the
same MST configuration, they can end up in different domains. This
is caused by the first MST switch remembering receiving PVST BPDUs
from the other switch. To fix that, clear the MST migration
process by issuing "clear spanning-tree detected-protocols
[interface <interface-id>]" on one of the switches.

Be aware that there are two different MST implementations in IOS,
one pre-standard and one standard. As far as I have seen, those
will interoperate without any problems. But running the same type
in the whole network is probably a Good Thing (tm).

The MST support in IOS are:

Switch		Pre-standard	Standard
------		------------	----------
3550		12.1(9)EA1	12.2(25)SEC
3560		12.1.(19)EA1	12.2(25)SEC
3750		12.2.(18)SE	12.2(25)SEC
6500/Sup2	12.1(13)E	12.2(18)SXF2
6500/Sup720	12.2(14)SX	12.2(18)SXF

-- 
Per Carlson, Sr. Network Developer