C
HAPTER
17
| General IP Routing
Address Resolution Protocol
– 442 –
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
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.
Also, if the switch receives a request for its own IP address, it will send
back a response, and also cache the MAC of the source device's IP address.
BASIC ARP
CONFIGURATION
Use the IP > ARP (Configure General) page to specify the timeout for ARP
cache entries, or to enable Proxy ARP for specific VLAN interfaces.
CLI REFERENCES
â—† "arp timeout" on page 1012
â—† "ip proxy-arp" on page 1013
COMMAND USAGE
Proxy ARP
When a node in the attached subnetwork does not have routing or a
default gateway
configured, Proxy ARP can be used to forward ARP requests
to a remote subnetwork.
When the router receives an ARP request for a
remote network and Proxy ARP is enabled, it determines if it has the best
route to the remote network, and then answers the ARP request by
sending its own MAC address to the requesting node. That node then sends
traffic to the router, which in turn uses its own routing table to forward the
traffic to the remote destination.
Figure 264: Proxy ARP
Table 23: 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
no routing,
no default
gateway
Remote
ARP Server
Proxy ARP
ARP
request
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