Showing results for 
Search instead for 
Did you mean: 

Spanning-tree loop -> broadcast storm -> network congestion


Hi there,

I'm facing an issue most probably with the spanning-tree loops causing broadcast storm.

Please see the attached picture for more details.

If I connect switch Stratix5700#1 (Rockwell version of Cisco IE2000) with only one trunk, there is ot issue. But if I connect both trunks the broadcast storm appears (only for VLANs 301,302 and 334). Unfortuntely this stupid switch with the lite version of IOS does not support port-channel neither flex links. So the spanning-tree is the only one option how to have redundant trunks. The problem is that the network is operating 24/7 so I cannot do deep investigation as the issue has an impact to entire network. My plan is to wait for the next maintenance window to enable BPDU skew detection to debug STP events. 


The network is in the middle of tech refrest (thats the reason why you see so many distribution blocks). I know about old IOS version and obsolete hardware, but It's a long story


Any idea about the root cause of this issue would be appreciated.


Thank you very much for your help in advance




MHM Cisco World

Friend If the SW not support the PortChannel then the best solution here is 
using PVST, this make each link block all VLAN and allow only one VLAN.
in this way you can use all link but make each link not forward traffic for all VLAN but only for specific VLAN.

Hi. the network is configured to use RAPID PVST. As you can see in the picture only VLANS 13,301,302,334 & 666 are allowed on trunks.

VIP Guru VIP Guru
VIP Guru

As you mentioned you are in a tech refresh project to remediate all Switches to modern hardware.


Question here? - do you really need Layer 2 external that switch (which causing the issue ?)

Layer 3 or p2p Link can be considered here, rather than the whole network outage?

what end device connected to that switch, what is the requirement for the devices ? to reach the servers ?





***** Rate All Helpful Responses *****

How to Ask The Cisco Community for Help

paul driver
VIP Expert VIP Expert
VIP Expert

I would say the STP mode you are running is fine no need to change this.
Make sure the vlan STP priorities for this switch is less preferred than its upstream switch
Changing the physical ports/sfps, cabling
Changing the from UDLD aggressive to normal?
Disable storm control ( temporary)

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: