Forwarding question

Magic Johnson

PC1 has no MAC has no arp entry for PC2. This means it sends out a broadcast asking for the owner of mac blahblahblah to reply.

My question is, if there are 10 PCs on this switch, and the switch knows what PC2's MAC is, does it flood the frame out all ports bar the one it came in on? Or does it send it just out the one PC2 is attached to because it knows the MAC?

Just feel this is a little gap in my knowledge and I can't quite find absolute clarity anywhere.

late_collision

Symantics first, PC1 would send out a broadcast (ARP Request) asking for the owner of an IP address, not a mac address. In return, PC1 would expect to learn the MAC address of PC2, the owner of said IP Address, in an ARP Reply.




In your question, the switch will flood the frame out all ports, except the one it was received on. To understand why, We have to think about who the broadcast frame is addressed to. In this case, PC1 doesn't know the Mac address of PC2, so the Layer 2 destination address of this broadcast (ARP Request) is addressed to FFFF.FFFF.FFFF, which is equivalent to "All Hosts".


The switch might know PC2's Mac address, but PC2's Mac address is not in the destination field of this frame header, the broadcast address of FFFF.FFFF.FFFF is.

Magic Johnson

Excellent! That last sentence is exactly the clarity I was after! Awesome, cheers.

Magic Johnson

I think I know why I was getting confused: because if PC1 has PC2 in its arp cache, then the mac-address-table is cleared on the switch, even though PC1 has the right destination MAC, the switch still floods the frame because it doesn't know where the destination is (right?)

I thought it may work both ways, for some reason...

late_collision

Correct. If the switch doesn't know, it can't filter, so it must flood.

Glad I could help.