Does the router or switch help a host find the mac address of a host when using arp ?

[dolanduck.]

to continue the question Does the switch or router broadcast the ff,ff,ff,ff,ff,ff request or does it look in its cam table ?

[Marlon AJ]

Hi @dolanduck.  ,

Lets say that PC1 is connected to SW1 and lets say that PC1 sends an ARP request with a broadcast destination MAC address FFFF.FFFF.FFFF . When the SW receives the frame, it checks the destination MAC address. For a broadcast address FFFF.FFFF.FFFF it will forward the frame to every port except the port in which the switch received the frame. That would be the action taken by the switch.

If the destination is in the same LAN, it will reply the broadcast frame with a unicast frame with PC1 Mac and IP address as the destination.

If the destination is in a different LAN, a router will construct an ARP request with a destination MAC address FFFF.FFFF.FFFF. Then, it will forward the request out of its directly connected ports. That would be the action taken by the router.

If the destination is reachable by the router (directly connected or through another switch), it will reply the broadcast frame with a unicast frame with R1 Mac address and PC1 IP address as the destination.

Answering your question, the switch and the router will forward the ARP requests and replies.

Hope this helps!

[dolanduck.]

 ok, i understand now that it forwards the broadcast but does the router or switch checks cam table if it has an entree ?.

[ngkin2010]

Concretely, switch's CAM table (MAC address, Port) not like ARP table (MAC address, IP Address). Switch cannot help to answer the ARP request using CAM table.

[Cristian Matei · Accepted solution]

Hi,

     For broadcast frames (destination MAC of FFFF.FFFF.FFFF), the switch does not do a CAM table lookup, by design. The MAC address of FFFF.FFFF.FFFF can never be added to the CAM table, as the switch does MAC learning by looking at the source MAC of Ethernet frames, and FFFF.FFFF.FFFF can never be a source MAC.

Regards,

Cristian Matei.