rapid-pvst, uplink fast, backbone fast

[lcaruso]

Hi,

In reading other posts on this forum, I'm told if you enable rapid-pvst, uplink fast and backbone fast are both enabled automatically even though the commands to enable them are accepted at the cli but have no effect.

Can anyone confirm this is stil the case?

I checked this on a 3750 running 12.2(55) and got this result, so I don't know if that was old information or doesn't apply to the 3750.

s-1-1#sh span summary
Switch is in rapid-pvst mode
Root bridge for: VLAN0001
Extended system ID           is enabled
Portfast Default             is disabled
PortFast BPDU Guard Default  is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default            is disabled
EtherChannel misconfig guard is enabled
UplinkFast                   is disabled
BackboneFast                 is disabled
Configured Pathcost method used is short

[Peter Paluch · Accepted solution]

Hello,

In reading other posts on this forum, I'm told if you enable 
rapid-pvst, uplink fast and backbone fast are both enabled automatically
 even though the commands to enable them are accepted at the cli but 
have no effect. 
Can anyone confirm this is stil the case? 

There seems to be an ongoing misunderstanding about the relation of BackboneFast and UplinkFast to RSTP.

The RSTP alone, by itself, has built-in mechanisms that provide a similar functionality to Cisco's proprietary BackboneFast and UplinkFast STP extensions. Therefore, activating the RSTP (or the RPVST/RPVST+) immediately gives you the advantages of BackboneFast and UplinkFast. However, the BackboneFast and UplinkFast themselves are distinct extensions and they are not activated along with RSTP. In fact, even if you configured them together with running RSTP, they would not be active because RSTP already provides their functionality, although by slightly different mechanisms. That fact is confirmed by your output of show span sum

To sum it up, the RSTP already incorporates the functionality of UplinkFast and BackboneFast (although not implemented exactly in the way the UplinkFast and BackboneFast implement it), and when you activate RSTP, you get UplinkFast-like and BackboneFast-like functionality. The UplinkFast and BackboneFast alone, however, are not and will not be activated.

Please feel free to ask further.

Best regards,

Peter

[lcaruso]

I really appreciate the time and effort you have taken to clarify this.

Regarding Cisco's proprietary BackboneFast and UplinkFast STP extensions, I believe there exist standing recommendations to use BackboneFast on all switches and UplinkFast on distribution switches if I'm not mistaken.

Since the functionality of UplinkFast and BackboneFast are enabled with rapid-pvst,  is there a similar recommendation as to which switches should run or not run rapid-pvst?

Thanks again.

[Peter Paluch · Accepted solution]

Hello,

You are heartily welcome

Regarding Cisco's proprietary BackboneFast and UplinkFast STP extensions, I
 believe there exist standing recommendations to use BackboneFast on all
 switches and UplinkFast on distribution switches if I'm not mistaken.

Well, as far as I know, the BackboneFast was intended to be activated on all switches indeed, and the UplinkFast was most suitable for access and perhaps selected distribution layers switches because it modified the bridge priority and port costs as a side effect. The following two documents provide a very nice insight into both mechanisms' operations:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800c2548.shtml

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094641.shtml

Since the functionality of UplinkFast and BackboneFast are enabled with rapid-pvst,  is there a similar recommendation as to which switches should run or not run rapid-pvst?

No similar recommendations exist for RSTP. If you decide to run RSTP, you absolutely should run it in your entire switched network. The RSTP and its per-VLAN modifications implement the BF/UF similar functionality in a way that does not preclude it from running on any switch in the network. Running the RSTP in the entire network is the only way to make full use of the RSTP features and benefits.

Best regards,

Peter

[lcaruso]

very nice

Thanks again for your expertise, time, and effort !!

[Peter Paluch]

Hello,

You are welcome. Thank you very much for your generous ratings!

Best regards,

Peter