ARP establishes correspondences between network addresses (an IP address, for example) and Ethernet hardware addresses. A record of each correspondence is kept in a cache for a predetermined amount of time and then discarded.

You can also add a static (permanent) entry to the ARP cache that persists until explicitly removed.

When proxy ARP is disabled, the networking device responds to ARP requests received on an interface only if one of the following conditions is met:

• The target IP address in the ARP request is the same as the interface IP address on which the request is received.

• The target IP address in the ARP request has a statically configured ARP alias.

When proxy ARP is enabled, the networking device also responds to ARP requests that meet all the following conditions:

• The target IP address is not on the same physical network (LAN) on which the request is received.

• The networking device has one or more routes to the target IP address.

• All of the routes to the target IP address go through interfaces other than the one on which the request is received.

When local proxy ARP is enabled, the networking device responds to ARP requests that meet all the following conditions:

• The target IP address in the ARP request, the IP address of the ARP source, and the IP address of the interface on which the ARP request is received are on the same Layer 3 network.

• The next hop for the target IP address is through the same interface as the request is received.

Typically, local proxy ARP is used to resolve MAC addresses to IP addresses in the same Layer 3 network. Local proxy ARP supports all types of interfaces supported by ARP and unnumbered interfaces.

You can configure an interface or a sub-interface to learn only the ARP entries from its local subnet.

Use the following procedure to configure local ARP learning on an interface.

1. Enter the interface configuration mode.

   Router(config)# interface GigabitEthernet 0/0/0/1

2. Configure the IPv4/IPv6 address for the interface.

   Router(config-if)# ipv4 address 12.1.3.4 255.255.255.0

3. Configure local ARP learning on the interface.

   Router(config-if)# arp learning local

4. Enable the interface and commit your configuration.

   Router(config-if)# no shut
   Router(config-if)# commit
   RP/0/0/CPU0:Dec 12 13:41:16.580 : ifmgr[397]: %PKT_INFRA-LINK-3-UPDOWN : interface GigabitEthernet 0/0/0/1, changed state to Down 
   RP/0/0/CPU0:Dec 12 13:41:16.683 : ifmgr[397]: %PKT_INFRA-LINK-3-UPDOWN : interface GigabitEthernet 0/0/0/1 changed state to Up 

5. Confirm your configuration.

   Router(config-if)# show running-configuration 
   ..
   Building configuration...
   !! IOS XR Configuration 0.0.0
   !! Last configuration change at Mon Dec 12 13:41:16 2016
   !interface GigabitEthernet 0/0/0/1
    ipv4 address 12.1.3.4 255.255.255.0
    arp learning local
   !

6. Verify if local ARP learning is working as configured on the interface.

   Router(config-if)# do show arp idb gigabitEthernet 0/0/0/1 location 0/0/CPU0
   Thu Dec 15 10:00:11.733 IST
   
   GigabitEthernet 0/0/0/1 (0x00000040):
     IPv4 address 12.1.3.4, Vrf ID 0x60000000
     VRF Name default
     Dynamic learning: Local
     Dynamic entry timeout: 14400 secs
     Purge delay: off
     IPv4 caps added (state up)
     MPLS caps not added
     Interface not virtual, not client fwd ref,
     Proxy arp not configured, not enabled
     Local Proxy arp not configured
     Packet IO layer is NetIO
     Srg Role : DEFAULT
     Idb Flag : 2146444
     IDB is Complete

7. (Optional) You can monitor the ARP traffic on the interface.

   Router(config-if)# do show arp idb gigabitEthernet 0/0/0/1 location 0/0/CPU0 
   Thu Dec 15 10:13:28.964 IST
   
   ARP statistics:
     Recv: 0 requests, 0 replies
     Sent: 0 requests, 1 replies (0 proxy, 0 local proxy, 1 gratuitous)
     Subscriber Interface: 
            0 requests recv, 0 replies sent, 0 gratuitous replies sent
     Resolve requests rcvd: 0
     Resolve requests dropped: 0
     Errors: 0 out of memory, 0 no buffers, 0 out of sunbet
   
   ARP cache:
     Total ARP entries in cache: 1
     Dynamic: 0, Interface: 1, Standby: 0
     Alias: 0,   Static: 0,    DHCP: 0
   
     IP Packet drop count for GigabitEthernet0_0_0_1: 0
   

A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address, because it is contained in the data link layer (Layer 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). The more technically inclined person will refer to local addresses as MAC addresses, because the MAC sublayer within the data link layer processes addresses for the layer.

To communicate with a device on Ethernet, for example, Cisco IOS XR software first must determine the 48-bit MAC or local data-link address of that device. The process of determining the local data-link address from an IP address is called address resolution.

The following process describes address resolution when the source and destination devices are attached to the same LAN:

1. End System A (Node A) broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System B (Node B).

2. The broadcast is received and processed by all devices on the LAN, including End System B.

3. Only End System B replies to the ARP request. It sends an ARP reply containing its MAC address to End System A (Node A).

4. End System A (Node A) receives the reply and saves the MAC address of End System B in its ARP cache. (The ARP cache is where network addresses are associated with MAC addresses.)

5. Whenever End System A (Node A) needs to communicate with End System B, it checks the ARP cache, finds the MAC address of System B, and sends the frame directly, without needing to first use an ARP request.

The following process describes address resolution when the source and destination devices are attached to different LANs that are interconnected by a router (only if proxy-arp is turned on):

1. End System Y (Node A) broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System Z (Node B).

2. The broadcast is received and processed by all devices on the LAN, including Router X.

3. Router X checks its routing table and finds that End System Z (Node B) is located on a different LAN.

4. Router X therefore acts as a proxy for End System Z (Node B). It replies to the ARP request from End System Y (Node A), sending an ARP reply containing its own MAC address as if it belonged to End System Z (Node B).

5. End System Y (Node A) receives the ARP reply and saves the MAC address of Router X in its ARP cache, in the entry for End System Z (Node B).

6. When End System Y (Node A) needs to communicate with End System Z (Node B), it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly, without using ARP requests.

7. Router X receives the traffic from End System Y (Node A) and forwards it to End System Z (Node B) on the other LAN.