Question about unknown unicast frame flooding

When a switch forwards an unknown unicast frame, it obviously will flood the frame out of all ports except the one it came from.

Does the switch replace the frames destination mac address with mac broadcast addrress and then flood it? (FF:FF:FF:FF:FF:FF) or just leave the original destination mac address in place and then flood it?

I thought it was the former, but I think I might be wrong and would like clarification.

Thanks in advance.

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CertifiedMonkey

Does the switch replace the frames destination mac address with mac broadcast addrress and then flood it? (FF:FF:FF:FF:FF:FF) or just leave the original destination mac address in place and then flood it?

It leaves the original destination MAC in there. It is an unknown unicast address. Keyword is unicast. (FF:FF:FF:FF:FF:FF) is the broadcast MAC address.

Simrid

There are 3 scenarios when a frame will be broadcast. That is when the frame entering is either a broadcast, an unknown unicast and unknown multicast.

With regards to your question, the switch itself doesn't replace the destination MAC address as that get's done by a layer 3 hop (router or MLS for example). However, the sending device will already know if it is known by using it's arp table. Should it be unknown, it will flood the frame with destination MAC of FF:FF:FF:FF:FF:FF.

ccie14023

Simrid wrote: »

There are 3 scenarios when a frame will be broadcast. That is when the frame entering is either a broadcast, an unknown unicast and unknown multicast. With regards to your question, the switch itself doesn't replace the destination MAC address as that get's done by a layer 3 hop (router or MLS for example). However, the sending device will already know if it is known by using it's arp table. Should it be unknown, it will flood the frame with destination MAC of FF:FF:FF:FF:FF:FF.

Hmmm, I think he was talking layer 2 here, why bring layer 3 into it? An unknown unicast frame, regardless of whether there is any layer 3 info at all, will use the original destination address, not the broadcast address. In other words, you could have a pure layer 2 frame (or non-IP) and so ARP doesn't even enter into it. At layer 3, if the MAC address is unknown at the sender because there is no ARP entry, it does not flood the "frame" with that MAC because there is no frame yet. Rather, it sends an ARP request. You are correct that the ARP request is sent to the broadcast MAC address, but I think the way you phrased it just confused the question.

Simrid

ccie14023 wrote: »

Hmmm, I think he was talking layer 2 here, why bring layer 3 into it? An unknown unicast frame, regardless of whether there is any layer 3 info at all, will use the original destination address, not the broadcast address. In other words, you could have a pure layer 2 frame (or non-IP) and so ARP doesn't even enter into it. At layer 3, if the MAC address is unknown at the sender because there is no ARP entry, it does not flood the "frame" with that MAC because there is no frame yet. Rather, it sends an ARP request. You are correct that the ARP request is sent to the broadcast MAC address, but I think the way you phrased it just confused the question.

I think I perhaps misunderstood what was being asked. You're quite right, after re-reading the question - it was only layer 2 which was being questioned in this particular case.