Configuring ARP

Address resolution is the process of mapping network addresses to Media Access Control (MAC) addresses. This process is accomplished using the Address Resolution Protocol (ARP).


Note

For a complete description of the ARP commands listed in this module, refer to the Cisco IOS XR IP Addresses and Services Command Reference for the Cisco XR 12000 Series RouterTo locate documentation of other commands that appear in this module, use the command reference master index, or search online.

Feature History for Configuring ARP

Release 3.2

This feature was introduced.

Release 3.3.0

The vrf keyword and vrf-name argument were added to arp commands. Merged the Setting ARP Encapsulation section with the Defining a Static ARP Cache Entry.

Prerequisites for Configuring ARP

  • You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.

Restrictions for Configuring ARP

The following restrictions apply to configuring ARP :

  • Reverse Address Resolution Protocol (RARP) is not supported.

  • Due to a hardware limitation in the Ethernet SPA interfaces installed on all routers, when a packet contains a wrong destination address, the corresponding SPA drops the packet even if the ingress packet count is already incremented in the output of the show interfaces command.

  • ARP throttling is not supported.


    Note

    ARP throttling is the rate limiting of ARP packets in Forwarding Information Base (FIB).

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:

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

  4. 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.)

  5. 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):

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

  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 is located on a different LAN.

  4. 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.

  5. 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.

  6. 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.

  7. Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.

ARP and Proxy ARP

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. Cisco IOS XR software also supports a form of ARP called local proxy ARP.

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:

  • 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 such as, private VLANs that are Layer 2-separated. Local proxy ARP supports all types of interfaces supported by ARP and unnumbered interfaces.

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.

SUMMARY STEPS

    1.    configure

    2.    Do one of the following:

    • arp [vrf vrf-name] ip-address hardware-address encapsulation-type
    • arp [vrf vrf-name] ip-address hardware-address encapsulation-type alias

    3.    commit


DETAILED STEPS
     Command or ActionPurpose
    Step 1 configure
          		 
    Step 2Do one of the following:
    • arp [vrf vrf-name] ip-address hardware-address encapsulation-type
    • arp [vrf vrf-name] ip-address hardware-address encapsulation-type alias


    Example: 
    RP/0/0/CPU0:router(config)# arp 192.168.7.19 0800.0900.1834 arpa

    or 

    RP/0/0/CPU0:router(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.

    Note   

    If an alias 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.

     
    Step 3 commit
          		 

    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.

    SUMMARY STEPS

      1.    configure

      2.    interface type number

      3.    proxy-arp

      4.    commit


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 configure
              		 
      Step 2 interface type number


      Example: 
      RP/0/0/CPU0:router(config)# interface MgmtEth 0/0/CPU0/0
       

      Enters interface configuration mode.

       
      Step 3 proxy-arp


      Example: 
      RP/0/0/CPU0:router(config-if)# proxy-arp
       

      Enables proxy ARP on the interface.

       
      Step 4 commit
              		 

      Enabling Local Proxy ARP

      Local proxy ARP is disabled by default; this task describes how to enable local proxy ARP.

      SUMMARY STEPS

        1.    configure

        2.    interface type number

        3.    local-proxy-arp

        4.    commit


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 configure
                  		 
        Step 2 interface type number


        Example: 
        RP/0/0/CPU0:router(config)# interface TenGigE 0/0/0/0
         

        Enters interface configuration mode.

         
        Step 3 local-proxy-arp


        Example: 
        RP/0/0/CPU0:router(config-if)# local-proxy-arp
         

        Enables local proxy ARP on the interface.

         
        Step 4 commit