Specifications

Configuring IP Addressing
Configuring Address Resolution Methods
IPC-14
Cisco IOS IP Configuration Guide
Use the following command in interface configuration mode to set the length of time an ARP cache entry
will stay in the cache:
To display the type of ARP being used on a particular interface and also display the ARP timeout value,
use the show interfaces EXEC command. Use the show arp EXEC command to examine the contents
of the ARP cache. Use the show ip arp EXEC command to show IP entries. To remove all nonstatic
entries from the ARP cache, use the clear arp-cache privileged EXEC command.
Setting ARP Encapsulations
By default, standard Ethernet-style ARP encapsulation (represented by the arpa keyword) is enabled on
the IP interface. You can change this encapsulation method to SNAP or HP Probe, as required by your
network, to control the interface-specific handling of IP address resolution into 48-bit Ethernet hardware
addresses.
When you set HP Probe encapsulation, the Cisco IOS software uses the Probe protocol whenever it
attempts to resolve an IEEE-802.3 or Ethernet local data-link address. The subset of Probe that performs
address resolution is called Virtual Address Request and Reply. Using Probe, the router can
communicate transparently with HP IEEE-802.3 hosts that use this type of data encapsulation. You must
explicitly configure all interfaces for Probe that will use Probe.
To specify the ARP encapsulation type, use the following command in interface configuration mode:
Enabling Proxy ARP
The Cisco IOS software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of
routing determine the media addresses of hosts on other networks or subnets. For example, if the router
receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the
router has all of its routes to that host through other interfaces, then it generates a proxy ARP reply
packet giving its own local data-link address. The host that sent the ARP request then sends its packets
to the router, which forwards them to the intended host. Proxy ARP is enabled by default.
To enable proxy ARP if it has been disabled, use the following command in interface configuration mode
(as needed) for your network:
Command Purpose
Router(config-if)# arp timeout seconds
Sets the length of time an ARP cache entry will stay in the cache.
Command Purpose
Router(config-if)# arp {arpa | probe |
snap}
Specifies one of three ARP encapsulation methods for a specified interface.
Command Purpose
Router(config-if)# ip proxy-arp
Enables proxy ARP on the interface.