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New Member

ccna vlan troubleshooting

I ran into a problem on the ccna that required me to identify out which ports an unknown unicast frame would be sent. I had forgoten the show commands that I needed to determine which ports were trunking and so forth. I have searched the cisco website and I am still not sure which commands I would need to identify the following:

1. Which ports are trunking.

2. Which vlans are broadcast thru the trunk

3. What ports will foward an unknown unicast frame

I would like to know what cmds are most helpful in discovering this kind of info.

Could anyone please help me find information on this kind of troubleshooting?


New Member

Re: ccna vlan troubleshooting

1-2: Take a look at your switch config (sh run) to find out what ports are configured as trunk ports and vlans (show vlan). Try this command " show interface fa0/x trunk " and it should tell you what port is acting as a trunk port.

3. all ports in "fowarding" status will forward an unicast frame (sh spanning-tree sum)



Re: ccna vlan troubleshooting


Try the command "show interface int f0/0 switchport"

This should answer all your queries.



New Member

Re: ccna vlan troubleshooting

There are 3 types of flooded traffic on Layer 2 switchports:

1. Broadcasts

2. Multicasts

3. Unknown Unicasts

When one of these frames arrive on a switchport they are forwarded out all ports in that vlan, including trunks that permit that vlan, EXCEPT the port the frame actually arrived on.

So your task was to determine which ports belonged to the same vlan as the unknown unicast or were trunk ports that permitted that vlan. The easiest commands to show this information are:

sh int trunk

sh int status

sh vlan

Hope this helps


New Member

Re: ccna vlan troubleshooting

Thank you all for your reply. I will try the show cmds.