Difference between unknown unicast and ARP ?

---

Dear All,

The topic is self explanatory, can anyone tell me what is the difference between unknown-unicast and ARP ? because to me they are the same.

With Regards

@Saif-Yaseen,

ARP is intentionally sent out of the source as a broadcast 255.255.255.255. A switch will do the mac address table lookup will find that 255.255.255.255 and map to ffff.ffff.ffff (ALL possible MAC addresses). This means that the switch will send ARP requests out every other port but the one it arrived on. This is not looking to populate the MAC address table.

When a frame is sent out of the with an IP ADDRESS (e.g. 192.168.1.100) and a MAC--it is a unicast, Let's say the switch doesn't find the corresponding MAC address. The switch believes, that the received frame had it right and that the unknown mac address has to be connected and it just hasn't learned about it yet. So the switch, instead will flood it out every port with the intention of discovering the MAC and populating the table.

I hope that was clear...I never know.

@Ronnie-Wong It is kinda confusing for me to understand how a layer two switch understands that a frame has an IP, because layer 2 devices are not suppose to deal with layer 3 information, hence they shouldn't care whether there is an IP or not.

@Saif-Yaseen,

Remember that what is sent to the switch is a frame that is where the MAC addresses reside. That is what the switch reads. If the switch receives a frame with ffff.ffff.ffff it will flood it out, regardless what the ip address is...so it's not really looking at Layer 3. The source is what puts the layer 3 and layer 2 headers on it. The switch only acts on the Layer 2 information.

If a source sends to 192.168.1.100 (Layer 3 header) and has it's it arp table there that it should correspond to aaaa.bbbb.cccc (Layer 2 header). The switch receives the frame and reads the layer 2 header (aaaa.bbbb.cccc). The mac address table has no corresponding entry of that MAC to port. The switch will simply say "it's gotta be here somewhere." and send it out every other port. The advantage here is that if aaaa.bbbb.cccc is out there and didn't report in to the switch. It will have received that frame first. The destination address then sends a reply. The switch will read the reply's Layer 2 frame and add it to the mac address table and forward that reply out to the original source.

@Ronnie-Wong Thank you for the clear explanation, could not of been explained better.