Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 286

TCP/IP Protocols, Services, and Applications
This section covers the different TCP/IP protocols, services, and applications that are required
knowledge for the CCIE written exam.

ARP

When an IP packet needs to be sent over an Ethernet network, the sender must find out what
48-bit Media Access Control (MAC) physical address to send the frame to. Given the destination
IP, ARP obtains the destination MAC. The destination MAC can be a local host or the gateway
router's MAC address if the destination IP is across the routed network. ARP is described in
RFC 826 (November 1982). The local host maintains an ARP table with a list of IP addresses
to MAC addresses.

ARP operates by having the sender broadcast an ARP Request, as shown in Figure 6-11. In this
figure, the workstation with IP address 192.168.1.22 has a packet to send to 192.168.1.17 but
does not have a destination MAC address in its ARP table. An ARP request is broadcast to all
hosts in a segment, as shown in Step 1 of Figure 6-11. The ARP request contains the sender's
IP and MAC address; it also contains the target IP address. All nodes in the broadcast domain
receive and process the ARP request. As shown in Step 2, the device with the target IP address
sends an ARP reply back to the sender with its MAC address information; the ARP reply is a
unicast message that is sent to 192.168.1.22. The sender now has the target MAC address in its
ARP; it sends the frame out (Step 3).

ARP Request and Reply

Figure 6-11

Step 3

Step 1

Step 2

IP: 192.168.1.2
MAC:
00c0.a070.34a0

IP: 192.168.1.17
MAC:
0010.a070.cb02

IP: 192.168.1.1
MAC:
00c0.a070.ba01

IP: 192.168.1.22
MAC:
0010.a070.0102

Display the ARP table on Cisco routers with the show ip arp command, as shown in Example 6-1.

Example 6-1

ARP Table

router5#show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.3.1 0 0050.3ee4.5859 ARPA Ethernet0
Internet 10.1.3.2 0 0010.7b1b.6f9c ARPA Ethernet0
Internet 10.1.3.5 - 0010.7b7e.ccc3 ARPA Ethernet0
Internet 10.1.3.6 0 0010.7b7e.cccd ARPA Ethernet0