Chapter 14
| IP Configuration
Address Resolution Protocol
– 429 –
Address Resolution Protocol
The switch 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 switch (or any standards-based switch/router),
it first looks up the MAC address corresponding to the destination IP address in the
ARP cache. If the address is found, the switch 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 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.
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.
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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