Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

LAN switching , Vlan overlapping


I have 2 questions.

1. How do i identify by cisco commands there is a vlan overlapping , which creates one big broadcast storm

is it debug commands?? can i identify from both L3 and L2

2. How do i troubleshhoot such a condition and find out where exactly the problem is.

New Member

Re: LAN switching , Vlan overlapping

What do you mean by the VLANs overlapping? Do you mean that they share IP addresses on the same subnet? I'm not sure if you can even do this. (I tried it on my 6506 with MSFC2 and it gave me an error about the addresses overlapping). However, to check the IP configuration of the VLANs on a layer 3 switch, just use the command "show running-configuration". This will show all the virtual interfaces created and their associated IP addresses.

On a layer 2 switch, the command "show vlan" will show you all the layer 2 VLAN information.

As far as a broadcast storm goes, if you aren't bridging any protocols between VLANs, the router will stop the broadcasts, so the entire network shouldn't be affected, unless you were only running one VLAN.

The command "show ip route" will also give you information about which subnets are being routed through which VLAN interfaces. This may help.

The following is copied directly from Cisco web pages:

It used to be that a broadcast storm could have the same effect on the network. Nowadays, with high-speed links and devices providing switching at hardware level, it is nearly impossible that, for instance, a single server brings down a network by broadcasting. The real way of identifying for sure a bridging loop is to capture the traffic on a saturated link and to check that similar packets are seen multiple times.

But practically, if all users in a certain bridging domain have connectivity issue at the same time, you can already suspect a bridging loop.

Check the port utilization on your devices and look for abnormal values. See the Check Port Utilization section below.

The show system command can be used to check the current backplane utilization, which is typically less than ten percent. If you believe that you are having performance related issues on a particular switch, look at the Peak field (the peak backplane utilization on the switch since it was last booted), and note the timestamp indicated by Peak-Time. Keep in mind that spikes in traffic percentage on the backplane could be a STP loop or broadcast storm.

Also, check the following link under the heading "Experiencing Poor Performance".

I hope some of this helps.