Can someone explain this to me?
If I have a device that doesn't have any way of communicating (say a server with no IP address, and not other protocol), will the switch see a mac address connected to the port? I can understand no arp entry because of this, but why does the switch not see a mac address?
For the switch to know the host's MAC address, it needs the host to generate a frame so it can see the source MAC address in the frame.
What happens when a device on another port sends traffic to an unknown address? When it floods the switch, why does it not add it to the table then?
Perhaps you could explain a bit more about what you mean by no way of communicating. A switch learns the mac-address of a particular device when either
1) some other device tries to talk to that device and the device can respond
2) the device itself is trying to communicate with another host
Neither is possible in your scenario because this server has no way of responding to or initiating a communication so the switch will never see the mac-address.
Here's the problem:
I have about 15 ports on a switch that show connected with no mac addresses associated with them. I traced down one of them to be a KVM that's attached to a port. I disconnected the connection from the switch side, reconnected, and it still doesn't add the mac address to the table.
I called Cisco TAC yesterday, and they couldn't answer WHY I had no mac address in the table for that device.
This is not the same as your original question. If the device whatever it be has no protocol for communicating eg. TCP/IP then you will not see a mac-address.
If the device has no IP address and no way to be contacted i'm not sure how you will communicate with this device. Did it work before ?
Oh, it's not a problem. I was just wondering because I was under the assumption that the mac was learned after the port came up in a forwarding state.
Thanks for your help
Ah, okay. Good that it's not a problem :)
Generally speaking with hosts the mac-address is learnt as when the port transitions to forwarding because hosts generally use DHCP to get an IP address.
Please make a note that MAC Address table is no way related to IP address.
MAC table will have src MAC and Port number to which it was connected.
MAC table will be populated based on Source Mac Address , not for Destination MAC address.