I thought about this for a while and I don't seem to find an answer. Let's say that two routers are connected to each other, their ARP Table is empty as well as their Routing Table. Now, to be clear, hosts use the ARP table to find the MAC Address of the Router, but routers do or do not use the ARP Table to communicate with each other? Since they have a Routing Table which tells them the right interface (Eth0, Eth1...) to send the packet to. So I don't think that they will need to know the MAC Address of the other router. Is this right? Thank you.
1 Answers
Just like regular hosts, if a router wants to exchange frames with a host in the same subnet, it needs to know its MAC address. Before exchanging frames with either host, Router will need to know their MAC addresses.
Here are the steps Router needs to take before forwarding frames to Host:
- R1 wants to communicate with Host A. R1 checks its routing table. The subnet on which Host A resides is a directly connected subnet.
- R1 checks its ARP table to find out whether the Host A’s MAC address is known. If it is not, R1 will send an ARP request to the broadcast MAC address of FF:FF:FF:FF:FF:FF.
- Host A receives the frame and sends its MAC address to R1 (ARP reply). The host also updates its own ARP table with the MAC address of the Gigabit0/0 interface on R1.
- R1 receives the reply and updates the ARP table with the MAC address of Host A.
- Since both hosts now know each other MAC addresses, the communication can occur.
On Router# show arp
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