Cisco Systems 78-11741-02 manual Setting ARP Encapsulations, Enabling Proxy ARP, IPC-14

Configuring IP Addressing

Configuring Address Resolution Methods

Use the following command in interface configuration mode to set the length of time an ARP cache entry will stay in the cache:

Command	Purpose
Router(config-if)# arp timeout seconds	Sets 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-cacheprivileged 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:

Command	Purpose
Router(config-if)# arp {arpa probe	Specifies one of three ARP encapsulation methods for a specified interface.
snap}

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)# ip proxy-arp	Enables proxy ARP on the interface.

Cisco IOS IP Configuration Guide

IPC-14