Address Resolution Protocol
If IP routing is enabled (page 19-4), the router uses its rout ing ta bles to m a ke rout ing
decisions, and uses Address Resolution Protocol (A RP) to forward t raffic from one
hop to the next. ARP is used to map an IP address to a ph ysical layer (i.e., MAC)
address. When an IP frame is received by this router (or any standards-based
router), it first looks up the MAC address co rrespon ding to the dest ination IP
address in the ARP cache. If the address is found, the router writes the MAC
address into the appropriate field in the frame header, and forwards the f rame on to
the next hop. IP traffic passes along the path to its final destination in this way, with
each routing device mapping the desti nation IP ad dress to the MAC address o f the
next hop toward the recipient, until the packet is delivered to the final destination .
If there is no entry for an IP address in the ARP cache, the rou ter wi ll broadcast an
ARP request packet to all devices on the network. The ARP request contain s the
following fields similar to that shown in this example:
Table 19-1 Address Resolution Protocol
destination IP address 10.1.0.19
destination MAC address ?
source IP address 10.1.0.253
source MAC address 00-00-ab-cd-00-00
When devices receive this request, they discard it if their ad dress doe s not m atch
the destination IP address in the message. However, if it does match, they write the ir
own hardware address into the destination MAC addres s field and send th e
message back to the source hardware address. When the sou rce devi ce re ceives a
reply, it writes the destination IP address and corresponding M AC addres s into its
cache, and forwards the IP traffic on to the next hop. As long as this entry h as not
timed out, the router will be able forward traffic directly to th e next hop for this
destination without having to broadcast another ARP request.

19-8

IP Routing

19