Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 287

You can clear the ARP table by using the global command clear arp-cache .

Ever wondered why sometimes you try to ping a device on the local network and the first ping
fails? An ARP request goes out on the first ping and the ping ICMP echo times out as the ARP
process goes on. Then, the next four pings are successful. The following shows that when you
try the ping command for a second time, all five pings work:
router3#ping 10.1.3.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.3.2, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 1/2/4 ms
router3#ping 10.1.3.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.3.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

NOTE

Reverse Address Resolution Protocol (RARP)
RARP, as described in RFC 903, provides a method for workstations to dynamically find their
IP protocol address when they only know their hardware (MAC) address. RARP was replaced
by the Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP) as the
primary means to dynamically learn a device IP address.

Proxy ARP

Proxy ARP, as described in RFC 1027, specifies a method for which routers can respond to ARP
requests from hosts that do not have a configured gateway, by replying with its local MAC
address for destinations on other subnetworks. Cisco routers can reply to ARP requests for
destination hosts on different major networks. Also, if the IP prefix is in the routing table, Cisco
routers can send the ARP reply even if multiple segments are between the destination host and
the router. Figure 6-12 shows an example of the function of proxy-ARP.

Host A does not have a default gateway configured. It has a packet to send to Host B. As shown
in Figure 6-12, Host A sends an ARP request (Step 1) to find the MAC address of 192.168.2.20
(Host B). The router is configured for proxy-arp and has Host B's MAC address in its ARP
table. The router replies to Host A with an ARP reply (Step 2) by using its own MAC address
(00c0.a070.34a0) as the destination MAC, not Host B's MAC address. Host A then proceeds to
send the packet with Host B's IP address (192.168.2.20) but with the router's MAC address.

Proxy ARP can be disabled per interface by using the interface configuration command no ip
proxy-arp .