In today’s post I would like to look closer into one feature - proxy arp. On Cisco routers it is enabled by default and I think it’s worth of writing about possible pros and cons. To be on the same page just few words about ARP (Address Resolution Protocol). ARP is used to resolve IP addresses to MAC (physical). When we want to send packet to a host with known IP address, we need first know its MAC or MAC of a next hop. This is place where ARP start its job. Let’s look into below diagram. Assume that R1 has never contacted with R3 and I’m going to check its ARP table: R1 #sh ip arp Protocol Address Age ( min ) Hardware Addr Type Interface Internet 10.0 . 0.1 - ca00 . 18c4.0008 ARPA FastEthernet0 / 0 R1 # As you see I have only one entry with IP and MAC of the local interface. Before I ping R3 I enable debugging to see what’s happening behind the scene. R1 #debug arp ARP packet debugging is on R1 #...