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Community Member

Switch A is learning unnecessary MAC-addresses from other switches

Hi all,

The following is our setup with default Vlan 1 and 20 users connected to each Switch (B & C). Whenever users want to communicate first time switch flood to all ports and once the connection established it will pick it from its MAC table to communicate directly from next time.

Now my question is why the Switch A is learning all those MAC addresses again and again as it suppose to be aged out those entries after connection established between them.

20users--SwitchB--SwitchC--20users (also switch B is connected to Switch A and no users is connected to switch A) SwitchA

Your input will be highly appreciated.

Regards,

Khan.

6 REPLIES
Community Member

Re: Switch A is learning unnecessary MAC-addresses from other sw

Is anybody out there to help me on this issue........

Bronze

Re: Switch A is learning unnecessary MAC-addresses from other sw

Azmath,

Unless I'm mistaken about how switches work, every switch that is "in the same VLAN(s)" is going to be learning mac addresses of individual devices in those VLAN(s), regardless of whether those devices are directly connected or indirectly.

You could play with your mac-address-table timeout if you want to prolong how long the mac addresses stay in the CAM.

Or am I misunderstanding the question?

John

Community Member

Re: Switch A is learning unnecessary MAC-addresses from other sw

John,

Thank you very much for your feedback.

Once the connection established from there they can directly communicate with each other, because their MAC addresses are mapped in MAC table.Right!!

But In my scenario Switch A is also learning all MAC entries which are connected to other switches even after connection is established.

I hope that I have explained clearly this time.

Regards,

Khan

Community Member

Re: Switch A is learning unnecessary MAC-addresses from other sw

Please your input will be highly appreciated.

Bronze

Re: Switch A is learning unnecessary MAC-addresses from other sw

Yes, switch A will still learn MACs, even after the connection is established.

This is because, in addition to the on-going unicast traffic (which switches B and C forward directly from port-to-port once the source and destinations are learned), there is also layer 2 broadcast traffic. For example, ARPs, DHCP, certain kinds of Windows traffic like the Windows network Browser service, certain kinds of viruses, etc. will cause switch A to relearn any mac addresses that might otherwise age out of the mac-address-table.

Network management software can cause this, too, if you're inventorying workstations for hardware and software, or monitoring their activity.

Knowing this, I can say that one of the best things you can do for your networks' performance is to identify any unnecessary broadcast-based services on hosts and disable them to keep that traffic off the network. Using a program like "ethereal" and using SPAN to look at typical host traffic will help you start identifying potential culprits here. This will be an on-going project, because as new software comes available on hosts, new "junk" traffic will appear.

If you do have to have a particular broadcast-based service (e.g. DHCP), you might look at its parameters. So instead of granting a lease for 24 hours, you might consider a five or ten days, depending on the circumstances of your network.

John

Green

Re: Switch A is learning unnecessary MAC-addresses from other sw

Switch A is learning the MACs for the same reason the other two switches are: when a frame comes in from another port, Switch A has to know where to send it ... (out the port connecting Switch B & C).

The addresses are learned when the frame comes through, the switch looks at the source address of the frame, looks to see if it's been seen before (within the timeout values of the CAM / forwarding table) .... adds it if it hasn't, then looks at the destination address to see if it knows where that is (then flood or forward as appropriate).

If you were using hubs in positions B&C, then Switch A would learn the addresses as they passed through the hub, because the hub repeats the frame to all other ports on a bit-by-bit basis.

What makes a switch a switch is the multiple virtual connections from the ingress port to the egress port; a "private" connection for everything except broadcast/multicasat (and the flood for unknown MACs).

FWIW

Scott

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