Re: PVST+ & Rapid-PVST

Hi Muhammad,

First of all, you are a little bit confused about RSTP and R-PVST+.
RSTP is a single spanning-tree IEEE protocol with rapid convergence
built-in. Rapid-PVST+ is an adaptation of RSTP to PVST+, Cisco's
proprietary multiple spanning-tree protocol, where every VLAN runs a
separate copy of RSTP (not STP, like PVST+ does). There are some
discrepancies with this implementation, but in most cases it works
just fine.

Now for the ALTN port role. This is a logical role that an STP
algorithm may assign to a port, irrespective of the fact the role is
being used for rapid transition or not. You may see this role being
assigned to ports in PVST+ or R-PVST+ modes; however, PVST+ will use
this information for rapid transition only if UplinkFast feautre has
been enabled. In fact, this feature made the ALTN port role usable by
STP. By default, a direct failure of a root port will take
2xForwardTime to recover without UplinkFast. You may read more about
STP convergence here:

http://blog.internetworkexpert.com/2009/03/07/understanding-stp-convergence-part-i/
http://blog.internetworkexpert.com/2009/03/14/understanding-stp-conv-2/

HTH,

-- 
Petr Lapukhov, petr_at_INE.com
CCIE #16379 (R&S/Security/SP/Voice)
Internetwork Expert, Inc.
http://www.INE.com
Toll Free: 877-224-8987
Outside US: 775-826-4344
2009/10/31 Muhammad Anser Khan <manserkhan_at_gmail.com>:
> Hello,
>
> I am little bit confused between the theory and the output results.
>
> As per my knowledge, PVST+ is the extension of 802.1d(STP) and
> Rapid-PVST+ is RSTP(802.1w).
>
> Then Alternate port role belongs to Rapid-PVST+ not  PVST+, But in the
> output of PVST+, It shows  "ALTR  BLK". It should be in the blocking
> state.
>
> and same output shows in the Rapid-PVST+  "Altr  BLK".
>
> Whereas debug results are fine:
>
> Debug PVST+
>
>  "blocking" 01:09:27: STP: VLAN0017 Fa0/24 -> blocking
>
> Debug Rapid-PVST+
>
> 01:08:51: RSTP(17): updt roles, received superior bpdu on Fa0/24
> 01:08:51: RSTP(17): Fa0/24 is now alternate
>
> PVST+ = blocking
> Rapid-PVST+ = alternate
>
> Is there any 'show' command where I can see the accurate result.
>
> One more thing to ask that How much time PVST+ will take to come in
> the Forwarding state from the Blocking state.( in default settings)
>
> Regards,
> Anser
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net