[c-nsp] 6500 HSRP unicast flooding

Dan Brisson dbrisson at uvm.edu
Tue Feb 18 18:59:29 EST 2014

This helped me out when I had this issue:



On 2/18/14, 6:57 PM, Peter Rathlev wrote:
> On Tue, 2014-02-18 at 18:14 -0500, Randy wrote:
>> The mac entry is only present on the active HSRP member... and the
>> flood traffic is coming from standby member, heading out all the
>> backup RSTP paths (blocked on the remote end).
> This probably means that the standby member is receiving traffic from
> elsewhere that it forwards upon receiving it because it has a connected
> route. You should be able to tell from where it comes by looking at the
> source IP addresses.
> Make sure traffic from elsewhere (other VLANs, other routers) arrives at
> the HSRP primary device. Alternatively, that the traffic when forwarded
> into the VLAN flows in a way that makes both your core devices see the
> traffic.
> Regarding TCNs, which might still be relevant since not only trunk
> interface flaps can trigger them, take a look at the output from:
>     show spanning-tree detail | include last change|executing
> And see if any VLANs (or instances) have changed recently and often.
> (P.S.: I can imagine one could suggest VSS as a solution to this
> problem, which would technically be correct. OTOH VSS might introduce
> other problems and/or be precluded for other reasons and it would not
> really cast any light on the actual problem.)

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