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IP R
OUTING
3-273
CLI - This example sets a primary IP address for VLAN 1, and then adds
a secondary IP address for a different subnet also attached to this router
interface.
Address Resolution Protocol
If IP routing is enabled (page 3-269), 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:
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
Console(config)#interface vlan 1
Console(config-if)#ip address 10.1.0.253 255.255.255.0 4-308
Console(config-if)#ip address 10.1.9.253 255.255.255.0 secondary
Console(config-if)#
Table 3-16 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