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Legacy Backbonefast...

Had a weird issue this weekend during a change.

We were swapping out the old 6509 with a 6509E.

(see image below for topology)

I moved HSRP over to 6509-2, delayed my uplink from 6509-1, then EIGRP passived the uplink.

From a layer 3 standpoint, this switch was isolated.  When I shut it down, I lost 8 pings to the Internet.

Since I'd done this at five other sites without issue, I wasn't expecting to interrupt connectivity.

When I brought it back online, I got a bunch of Spantree Loopguard block messages from all my downstream switches.

This is the only site I had that still had backbonefast enabled.  Could this have caused a problem?

STP config for this site:

spanning-tree mode rapid-pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

spanning-tree backbonefast

spanning-tree vlan 1-10,24-52,130-134,150-152,160,200,302,304,310 priority 8192

spanning-tree vlan 311-312,410-414,420-422,500-502,504-505,555 priority 8192

spanning-tree vlan 600-602,800-801,803,810 priority 8192

STP config for a site that had no issues:

spanning-tree mode rapid-pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

spanning-tree vlan 1-3,104-124,132,152-154,160,200-204,300,350 priority 8192

spanning-tree vlan 500-503,505-506,541,555,600-602,700,800-801 priority 8192

spanning-tree vlan 803,810 priority 8192



Ven Taylor

Legacy Backbonefast...


Backbone fast has nothing to do with this. If we check the mode = RPVST , we already know that backbone fast does not apply to RPVST .

Do you have portfast on edge ports ? I belive that your problem could be related to the Sync process, also related to the fact that you shutdown the Root (6509-1) , and then bring it up.

Where was the source that was interupted ( vlan id) ?

What is the source's port config ( stp related ) ?

Who is the root ( in normal condition - both 6509 up)?

Who was the root ( after shutdown ) ?


Legacy Backbonefast...

Hi Ven,

As Dan said, Rapid PVST has the inbuilt feature of backfonefast, it do not require to be enabled explicitly. But not very sure what are adverse effects of enabling them explicitly. To me, your approach looked right.

You modified the STP priority (lowered down on 6509-2) & increased the HSRP priority on for those VLANs on 6509-2. but just to understand, the pc from which you saw packet drops to internet was in which VLAN?

I think Dan's questions would help better understanding the situation. Most important question is Who was the root before & after.




Re: Legacy Backbonefast...

The source of the "interruption" was just my laptop on wireless (we have wisms in the 6509s).  I was pinging as a basic test of Internet connectivity and loss.

The port config was not an issue as it was a wism.

6509-1 is the normal root.

After we shut down 6509-1, 6509-2 became the root.

Once we brought 6509-1 back online, it stood back up as root.  That's when we saw the loopguard logs from all of our downstream switches on all vlans.  (syslog basically blew up).

To clarify, I DID NOT change the STP priority.  As we were using RPVST, it was not necessary.  This is the only site out of 8 others that freaked out.  Every other site was smooth as silk.  The only difference between these sites (templated) was the backbonefast legacy command.  Somebody missed this site when they moved to RPVST and I didn't notice it before I did the work.

Also, I typo'ed the config a little.  the 6509-2 STP priority is normally 16384 for all vlans, making it secondary.

mea culpa.


Ven Taylor
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