Cisco IOS XR IP Addresses and Services Configuration Guide for the Cisco CRS-1 Router
Configuring ARP on Cisco IOS XR Software
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Configuring ARP on Cisco IOS XR Software

Table Of Contents

Configuring ARP on Cisco IOS XR Software

Contents

Prerequisites for Configuring ARP

Restrictions for Configuring ARP

Information About Configuring ARP

IP Addressing Overview

Address Resolution on a Single LAN

Address Resolution When Interconnected by a Router

ARP and Proxy ARP

ARP Cache Entries

How to Configure ARP on Cisco IOS XR Software

Defining a Static ARP Cache Entry

Enabling Proxy ARP

interface-path-idConfiguration Examples for ARP Configuration on Cisco IOS XR Software

Creating a Static ARP Cache Entry: Example

Enabling Proxy ARP: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance


Configuring ARP on Cisco IOS XR Software


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 Cisco CRS-1 publication. To locate documentation of other commands that appear in this module, use the command reference master index, or search online.


Feature History for Configuring ARP for Cisco Cisco IOS XR software

Release
Modification

Release 2.0

This feature was introduced on the Cisco CRS-1.

Release 3.0

No modification.

Release 3.2

No modification.

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.

Release 3.4.0

No modification.

Release 3.5.0

No modification.

Release 3.6.0

No modification.

Release 3.7.0

No modification.

Release 3.8.0

No modification.

Release 3.9.0

No modification.


Contents

Prerequisites for Configuring ARP

Restrictions for Configuring ARP

Information About Configuring ARP

How to Configure ARP on Cisco IOS XR Software

interface-path-idConfiguration Examples for ARP Configuration on Cisco IOS XR Software

Additional References

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 on Cisco IOS XR software:

Reverse Address Resolution Protocol (RARP) is not supported.

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 on Cisco IOS XR software, you must understand the following concepts:

IP Addressing Overview

Address Resolution on a Single LAN

Address Resolution When Interconnected by a Router

ARP and Proxy ARP

ARP Cache Entries

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.

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.

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 on Cisco IOS XR Software

This section contains instructions for the following tasks:

Defining a Static ARP Cache Entry (optional)

Enabling Proxy ARP (optional)

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

3. end
or
commit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

configure

Example:

RP/0/RP0/CPU0:router# configure

Enters global configuration mode.

Step 2 

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

or

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

Example:

RP/0/RP0/CPU0:router(config)# arp 192.168.7.19 0800.0900.1834 arpa

or

RP/0/RP0/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 

end

or

commit

Example:

RP/0/RP0/CPU0:router(config)# end

or

RP/0/RP0/CPU0:router(config)# commit

Saves configuration changes.

When you issue the end command, the system prompts you to commit changes:

Uncommitted changes found, commit them before 
exiting(yes/no/cancel)? 
[cancel]:
 
        

Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

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. end
or
commit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

configure

Example:

RP/0/RP0/CPU0:router# configure

Enters global configuration mode.

Step 2 

interface type number

Example:

RP/0/RP0/CPU0:router(config)# interface MgmtEth 0/RP0/CPU0/0

Enters interface configuration mode.

Step 3 

proxy-arp

Example:

RP/RP0/CPU0:router(config-if)# proxy-arp

Enables proxy ARP on the interface.

Step 4 

end

or

commit

Example:

RP/0/RP0/CPU0:router(config-if)# end

or

RP/0/RP0/CPU0:router(config-if)# commit

Saves configuration changes.

When you issue the end command, the system prompts you to commit changes:

Uncommitted changes found, commit them before 
exiting(yes/no/cancel)? 
[cancel]:
 
        

Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.

interface-path-idConfiguration Examples for ARP Configuration on Cisco IOS XR Software

This section provides the following ARP configuration examples:

Creating a Static ARP Cache Entry: Example

Enabling Proxy ARP: Example

Creating a Static ARP Cache Entry: Example

The following is an example of a static ARP entry for a typical Ethernet host:

configure
arp 192.168.7.19 0800.0900.1834 arpa
 
   

The following is an example of a static ARP entry for a typical Ethernet host where the software responds to ARP requests as if it were the owner of both the specified IP address and hardware address, whether proxy ARP is enabled or not:

configure
arp 192.168.7.19 0800.0900.1834 arpa alias
 
   

The following is an example of configuring a static arp entry on an SRP device:

configure
arp 192.168.8.20 0800.0900.1723 srp

Enabling Proxy ARP: Example

The following is an example of enabling proxy ARP:

configure
interface MgmtEth 0/RP1/CPU0/0
proxy-arp

Additional References

The following sections provide references related to ARP.

Related Documents

Related Topic
Document Title

Cisco IOS XR QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Quality of Service Commands on Cisco IOS XR Software module in Cisco IOS XR Modular Quality of Service Command Reference

Class-based traffic shaping, traffic policing, low latency queuing, and MDDR

Configuring Modular Quality of Service Congestion Management on Cisco IOS XR Software module inCisco IOS XR Modular Quality of Service Configuration Guide

WRED, RED, and tail drop

Configuring Modular QoS Congestion Avoidance on Cisco IOS XR Software module in Cisco IOS XR Modular Quality of Service Configuration Guide

Cisco IOS XR ARP commands

ARP Commands on Cisco IOS XR Software module in Cisco IOS XR IP Addresses and Services Command Reference

Cisco IOS XR master command reference

Cisco IOS XR Master Commands List

Cisco CRS-1 router getting started material

Cisco IOS XR Getting Started Guide

Information about user groups and task IDs

Configuring AAA Services on Cisco IOS XR Software module in Cisco IOS XR System Security Configuration Guide


Standards

Standards
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.


MIBs

MIBs
MIBs Link

To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml


RFCs

RFCs
Title

RFC 826

Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware

RFC 1027

Using ARP to implement transparent subnet gateways


Technical Assistance

Description
Link

The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport