Brain Twister Part II

llllvllll

How Can a switch learn same Mac-address on two of its ports?
P.S :- There are no loops

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NetworkVeteran

llllvllll wrote: »

How Can a switch learn same Mac-address on two of its ports?
P.S :- There are no loops

"Switch" unqualified refers to a L2 device, and if no loops, the end device has only one active link.

Link bundling tends to learn MAC addresses on either the logical aggregate port or the primary physical port, not on two physical ports at once. Its hashing algorithm can use src/dst, but that isn't considered the MAC learning process.

HSRP or spoofing can be ruled out, because only one source MAC is actually learned by the switch at any time.

The most obvious case remains--two devices on different VLANs with the same MAC address.

Not sure, if you have something less obvious in mind, what it is.

llllvllll

Connect Router B on switch's fa0/1 , switch will learn its Mac, then take out its cable and plug it in on switch fa0/5 so now switch will learn same Mac address. :P

Sett

llllvllll wrote: »

Connect Router B on switch's fa0/1 , switch will learn its Mac, then take out its cable and plug it in on switch fa0/5 so now switch will learn same Mac address. :P

Nope. Try it in a lab and you'll see.

networker050184

What happens to the MAC entries when the interface goes down?

busines4u

I believe when an interface goes down the MAC Address will be flushed out of the Mac-Address-Table immediately rather than aged out after a given amount of time.

d6bmg

phoeneous wrote: »

They age out?

Who knows? I don't think there is clarification about this issue in cisco documentation.

networker050184

Yes there is clarification in documentation and you can see for yourself with a simple lab set up.

powmia

Like Network Veteran said, the answer is two different vlans.

When a link goes down.. that's a Topology Change (Spanning-Tree) and causes a CAM flush.

powmia

Also, if you've ever attempted MAC spoofing (causing the same mac to be seen on two ports), you get a mac-flap notification... as in the mac flaps between ports. It will not be retained in the mac-address table for the same vlan on multiple ports.

dmcneil330

Isn't normal switching operation to add the source mac address of the frame to the MAC table for the port the frame was received on. If a duplicate entry exists the old one is removed and the new one replaces it. This comes straight from the Cisco SWITCH OCG. The mac flap notification would be generated if this were occurring frequently. So, unplugging the router from one switch port to the other would just make it move it's already learned Mac address to port mapping to a different port. So it's not learning one mac on two ports, it's moving the mac address mapping to another port.