Address resolution is
the process of mapping network addresses to Media Access Control (MAC)
addresses. This process is accomplished using the Address Resolution Protocol
For a complete
description of the ARP commands listed in this module, refer to the
To locate documentation of other
commands that appear in this module, use the command reference master index, or
for Configuring ARP
Prerequisites for Configuring ARP
restrictions apply to configuring ARP :
Information About Configuring ARP
To configure ARP, you must understand the following concepts:
IP Addressing Overview
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.
Address Resolution on a Single LAN
The following process describes address resolution when the source and destination devices are attached to the same LAN:
End System A broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System B.
The broadcast is received and processed by all devices on the LAN, including End System B.
Only End System B replies to the ARP request. It sends an ARP reply containing its MAC address to End System A.
End System 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.)
Whenever End System 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.
Address Resolution When Interconnected by a Router
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):
End System Y broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System Z.
The broadcast is received and processed by all devices on the LAN, including Router X.
Router X checks its routing table and finds that End System Z is located on a different LAN.
Router X therefore acts as a proxy for End System Z. It replies to the ARP request from End System Y, sending an ARP reply containing its own MAC address as if it belonged to End System Z.
End System Y receives the ARP reply and saves the MAC address of Router X in its ARP cache, in the entry for End System Z.
When End System Y needs to communicate with End System Z, it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly, without using ARP requests.
Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.
ARP and Proxy
Two forms of address
resolution are supported by
Cisco IOS XR software: Address Resolution Protocol (ARP) and proxy ARP, as defined in RFC
826 and RFC 1027, respectively.
ARP is used to
associate IP addresses with media or MAC addresses. Taking an IP address as
input, ARP determines the associated media address. After a media or MAC
address is determined, the IP address or media address association is stored in
an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a
link-layer frame and sent over the network.
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:
When proxy ARP is
enabled, the networking device also responds to ARP requests that meet all the
The target IP
address is not on the same physical network (LAN) on which the request is
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.
ARP Cache Entries
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 expressly removed.
How to Configure ARP
This section contains instructions for the following tasks:
Defining a Static
ARP Cache Entry
ARP and other address resolution protocols provide a dynamic mapping between IP addresses and media addresses. Because most hosts support dynamic address resolution, generally you need not to specify static ARP cache entries. If you must define them, you can do so globally. Performing this task installs a permanent entry in the ARP cache. Cisco IOS XR software uses this entry to translate 32-bit IP addresses into 48-bit hardware addresses.
Optionally, you can
specify that the software responds to ARP requests as if it were the owner of
the specified IP address by making an alias entry in the ARP cache.
Do one of the
| ||Command or Action||Purpose|
|Step 2||Do one of the
RP/0/(config)# arp 192.168.7.19 0800.0900.1834 arpa
RP/0/(config)# arp 192.168.7.19 0800.0900.1834 arpa alias
Creates a static
ARP cache entry associating the specified 32-bit IP address with the specified
48-bit hardware address.
entry is created, then any interface to which the entry
is attached will act as if it is the owner of the specified addresses, that is,
it will respond to ARP request packets for this network layer address with the
data link layer address in the entry.
Enabling Proxy ARP
Cisco IOS XR 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 disabled by default; this task describes how to enable proxy
ARP if it has been disabled.
| ||Command or Action||Purpose|
RP/0/(config)# interface MgmtEth 0//CPU0/0
Enters interface configuration mode.
Enables proxy ARP on the interface.
Configure Learning of Local ARP Entries
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.
Enter the interface configuration mode.
RP/0/(config)# interface GigabitEthernet 0/0/0/1
Configure the IPv4/IPv6 address for the interface.
RP/0/ (config-if)# ipv4 address 184.108.40.206 255.255.255.0
Configure local ARP learning on the interface.
RP/0/(config-if)# arp learning local
Enable the interface and commit your configuration.
RP/0/(config-if)# no shut
RP/0/0/CPU0:Dec 12 13:41:16.580 : ifmgr: %PKT_INFRA-LINK-3-UPDOWN : Interface GigabitEthernet0/0/0/1, changed state to Down
RP/0/0/CPU0:Dec 12 13:41:16.683 : ifmgr: %PKT_INFRA-LINK-3-UPDOWN : Interface GigabitEthernet0/0/0/1, changed state to Up
Confirm your configuration.
RP/0/(config-if)# show running-configuration
!! IOS XR Configuration 0.0.0
!! Last configuration change at Mon Dec 12 13:41:16 2016
ipv4 address 220.127.116.11 255.255.255.0
arp learning local
Verify if local ARP learning is working as configured on the interface.
RP/0/(config-if)# do show arp idb gigabitEthernet 0/0/0/1 location 0/0/CPU0
Thu Dec 15 10:00:11.733 IST
IPv4 address 18.104.22.168, 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
(Optional) You can monitor the ARP traffic on the interface.
RP/0/(config-if)# do show arp traffic gigabitEthernet 0/0/0/1 location 0/0/CPU0
Thu Dec 15 10:13:28.964 IST
Recv: 0 requests, 0 replies
Sent: 0 requests, 1 replies (0 proxy, 0 local proxy, 1 gratuitous)
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
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