C
HAPTER
16
| IP Configuration
Address Resolution Protocol
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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 switch 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
switch 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.
SETTING THE ARP
TIMEOUT
Use the IP > ARP (Configure General) page to specify the timeout for ARP
cache entries.
CLI REFERENCES
â—† "arp timeout" on page 1033
PARAMETERS
These parameters are displayed:
◆ Timeout – Sets the aging time for dynamic entries in the ARP cache.
(Range: 300 - 86400 seconds; Default: 1200 seconds or 20 minutes)
The ARP aging timeout can only be set globally for all VLANs.
The aging time determines how long dynamic entries remain in the
cache. If the timeout is too short, the switch may tie up resources by
repeating ARP requests for addresses recently flushed from the table.
When a ARP entry expires, it is deleted from the cache and an ARP
request packet is sent to re-establish the MAC address.
Table 31: 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
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