Altn BLK in 802.1d?

[silvia.schuckert]

Hi!

I did a couple of STP tests in my lab.

The following confused me:

SW1#sh span

VLAN0001

Spanning tree enabled protocol ieee

[...]

Int Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- --------

[...}

Gi1/0/2 Altn BLK 19 128.2 P2p

I'm running 802.1d - as you can see in the line

Spanning tree enabled protocol "ieee". I was expecting the port in "blocking state,

but the state "Altn BLK" is a 802.1w state.

Would any one of you have an explaination for me?

Greetings from Austria,

silvia

[Peter Paluch · Accepted solution]

Hi Silvia,

Don't let the output confuse you. This is normal. The states may already be printed out in RSTP terminology even if your switch runs the legacy 802.1D STP.

Before the RSTP was standardized, Cisco has implemented a number of enhancements to the 802.1D (PortFast, BackboneFast, UplinkFast) that eventually inspired very similar mechanisms in RSTP. For these Cisco proprietary mechanisms to work, the switch already needs to add additional information to the 802.1D port states, resulting in an output very similar to the RSTP. I think that this is the one of reasons why even in the 802.1D mode, the output resembles very much the RSTP output - because Cisco has its proprietary mechanisms that were eventually slightly modified and taken into RSTP, so some aspects are very, very similar.

Best regards,

Peter

[Giuseppe Larosa · Accepted solution]

Hello Silvia,

in a thread of some mounths ago Cisco expert Francois Tallet had explained that Cisco had decided to provide a "uniform" output for sh spanning-tree using terms of RSTP also for 802.1D PVST+.

you are seeing IEEE istance for vlan1 but this is PVST+.

As Peter has noted Cisco had introduced several mechanisms that can help STP convergence so uplink fast can lead to an alternate blocking port even if 802.1W is not used for this STP instance.

Hope to help

Giuseppe

[Peter Paluch]

Hello Giuseppe,

Thank you very much for extending my answer. This collaborative sharing and mutual improving of knowledge is something that I value immensely. I appreciate your efforts very much.

Best regards,

Peter

[silvia.schuckert]

Dear Guiseppe and Peter!

Thank you very much for your quick and helpful answers!!

Silvia