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Address Resolution Protocol
If IP routing is enabled (page 19-4), the router uses its routing tables to make routing
decisions, and uses Address Resolution Protocol (ARP) to forward traffic from one
hop to the next. ARP is used to map an IP address to a physical 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 corresponding to the destination 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 frame on to
the next hop. IP traffic passes along the path to its final destination in this way, with
each routing device mapping the destination IP address to the MAC address of 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 router will broadcast an
ARP request packet to all devices on the network. The ARP request contains 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 address does not match
the destination IP address in the message. However, if it does match, they write their
own hardware address into the destination MAC address field and send the
message back to the source hardware address. When the source device receives a
reply, it writes the destination IP address and corresponding MAC address into its
cache, and forwards the IP traffic on to the next hop. As long as this entry has not
timed out, the router will be able forward traffic directly to the next hop for this
destination without having to broadcast another ARP request.
19-8
IP Routing
19