Configuring the ECN330-switch
4051553-KDU 137 365 Uen D 2006-06-16
6.20.5 Address Resolution Protocol
If IP routing is enabled (page 401), 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
(that is, 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 22 Address Resolution Protocol
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.
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
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