Cisco Systems 3750E manual Define a Static ARP Cache, Arp ip-address hardware-address type, 38-10

Chapter 38 Configuring IP Unicast Routing

Configuring IP Addressing

The switch can use these forms of address resolution:

•Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP address as input, ARP learns the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

•Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks or subnets. If the switch (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 the host through other interfaces, it generates a proxy ARP 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.

The switch also uses the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-serveraddress interface configuration command to identify the server.

For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2.

You can perform these tasks to configure address resolution:

•Define a Static ARP Cache, page 38-10

•Set ARP Encapsulation, page 38-11

•Enable Proxy ARP, page 38-12

Define a Static ARP Cache

ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the switch uses to translate IP addresses into MAC addresses. Optionally, you can also specify that the switch respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.

Beginning in privileged EXEC mode, follow these steps to provide static mapping between IP addresses and MAC addresses:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	arp ip-address hardware-address type	Globally associate an IP address with a MAC (hardware) address
		in the ARP cache, and specify encapsulation type as one of
		these:
		• arpa—ARP encapsulation for Ethernet interfaces
		• snap—Subnetwork Address Protocol encapsulation for
		Token Ring and FDDI interfaces
		• sap—HP’s ARP type

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
38-10	OL-9775-02