IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

IP Addressing: ARP Configuration Guide,

Cisco IOS Release 12.2SX

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C O N T E N T S

Configuring Address Resolution Protocol Options

Finding Feature Information 1

Information About Address Resolution Protocol Options 1

Layer 2 and Layer 3 Addressing 2

Address Resolution Protocol 3

ARP Caching 4

Static and Dynamic Entries in the ARP Cache 4

Devices That Do Not Use ARP 4

Inverse ARP 5

Reverse ARP 5

Proxy ARP 6

Serial Line Address Resolution Protocol 6

Authorized ARP 6

How to Configure Address Resolution Protocol Options 7

Enabling the Interface Encapsulation 7

Defining Static ARP Entries 8

Setting an Expiration Time for Dynamic Entries in the ARP Cache 10

Globally Disabling Proxy ARP 11

Disabling Proxy ARP on an Interface 11

Clearing the ARP Cache 12

Verifying the ARP Configuration 13

Configuration Examples for Address Resolution Protocol Options 15

Static ARP Entry Configuration Example 15

Encapsulation Type Configuration Example 15

Proxy ARP Configuration Example 16

Clearing the ARP Cache Example 16

Additional References 16

Feature Information for Configuring Address Resolution Protocol Options 17

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Contents

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

iii

Configuring Address Resolution Protocol

Options

Address Resolution Protocol (ARP) performs a required function in IP routing. ARP finds the hardware

address, also known as Media Access Control (MAC) address, of a host from its known IP address. ARP

maintains a cache (table) in which MAC addresses are mapped to IP addresses. ARP is part of all Cisco

IOS systems running IP.

This document explains ARP for IP routing and the optional ARP features you can configure, such as

static ARP entries, time out for dynamic ARP entries, clearing the cache, and Proxy ARP.

• Finding Feature Information, page 1

• Information About Address Resolution Protocol Options, page 1

• How to Configure Address Resolution Protocol Options, page 7

• Configuration Examples for Address Resolution Protocol Options, page 15

• Additional References, page 16

• Feature Information for Configuring Address Resolution Protocol Options, page 17

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature

information and caveats, see the release notes for your platform and software release. To find information

about the features documented in this module, and to see a list of the releases in which each feature is

supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support.

To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Information About Address Resolution Protocol Options

• Layer 2 and Layer 3 Addressing, page 2

• Address Resolution Protocol, page 3

• ARP Caching, page 4

• Static and Dynamic Entries in the ARP Cache, page 4

• Devices That Do Not Use ARP, page 4

• Inverse ARP, page 5

• Reverse ARP, page 5

• Proxy ARP, page 6

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

• Serial Line Address Resolution Protocol, page 6

• Authorized ARP, page 6

Layer 2 and Layer 3 Addressing

IP addressing occurs at Layer 2 (data link) and Layer 3 (network) of the Open System Interconnection

(OSI) reference model. OSI is an architectural network model developed by ISO and ITU-T that consists of

seven layers, each of which specifies particular network functions such as addressing, flow control, error

control, encapsulation, and reliable message transfer.

Layer 2 addresses are used for local transmissions between devices that are directly connected. Layer 3

addresses are used for indirectly connected devices in an internetwork environment. Each network uses

addressing to identify and group devices so that transmissions can be sent and received. Ethernet (802.2,

802.3, Ethernet II, and Subnetwork Access Protocol [SNAP]), Token Ring, and Fiber Distributed Data

Interface (FDDI) use Media Access Control (MAC) addresses that are “burned in” to the Network Interface

Card (NIC). The most commonly used network types are Ethernet II and SNAP.

In order for devices to be able to communicate with each when they are not part of the same network, the

48-bit MAC address must be mapped to an IP address. Some of the Layer 3 protocols used to perform the

mapping are:

• Address Resolution Protocol (ARP)

• Reverse ARP (RARP)

• Serial Line ARP (SLARP)

• Inverse ARP

For the purposes of IP mapping, Ethernet, Token Ring, and FDDI frames contain the destination and source

addresses Frame Relay and Asynchronous Transfer Mode (ATM) networks, which are packet switched,

data packets take different routes to reach the same destination. At the receiving end, the packet is

reassembled in the correct order.

In a Frame Relay network, there is one physical link that has many logical circuits called virtual circuits

(VCs). The address field in the frame contains a data-link connection identifier (DLCI) which identifies

each VC. For example, in the figure below, the Frame Relay switch to which router Fred is connected

receives frames; the switch forwards the frames to either Barney or Betty based on the DLCI which

identifies each VC. So Fred has one physical connection but multiple logical connections.

Figure 1

Layer 2 and Layer 3 Addressing

Information About Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

ATM networks use point-to-point serial links with the High-Level Data Link Control (HDLC) protocol.

HDLC includes a meaningless address field included in five bytes of the frame header frame with the

recipient implied since there can only be one.

AppleTalk is designed for Apple computers and has a special addressing scheme that uses 24-bit addresses

and its own method for resolving addresses. Once the data reaches the internetwork, address resolution

beyond the device connecting it to the internetwork operates the same as IP address resolution. For more

information about AppleTalk networks, refer to Core Competence AppleTalk (white paper) at

www.corecom.com/html/appletalk.html.

Address Resolution Protocol

Address Resolution Protocol (ARP) was developed to enable communications on an internetwork and is

defined by RFC 826. Routers and Layer 3 switches need ARP to map IP addresses to MAC hardware

addresses so that IP packets can be sent across networks. Before a device sends a datagram to another

device, it looks in its own ARP cache to see if there is a MAC address and corresponding IP address for the

destination device. If there is no entry, the source device sends a broadcast message to every device on the

network. Each device compares the IP address to its own. Only the device with the matching IP address

replies to the sending device with a packet containing the MAC address for the device (except in the case of

"Proxy ARP"). The source device adds the destination device MAC address to its ARP table for future

reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the

data. The figure below illustrates the ARP broadcast and response process.

Figure 2

When the destination device lies on a remote network, one beyond another router, the process is the same

except that the sending device sends an ARP request for the MAC address of the default gateway. After the

address is resolved and the default gateway receives the packet, the default gateway broadcasts the

destination IP address over the networks connected to it. The router on the destination device network uses

ARP to obtain the MAC address of the destination device and delivers the packet.

Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet use

Subnetwork Access Protocol (SNAP).

The ARP request message has the following fields:

• HLN--Hardware address length. Specifies how long the hardware addresses are in the message. For

IEEE 802 MAC addresses (Ethernet) the value is 6.

• PLN--Protocol address length. Specifies how long the protocol (Layer 3) addresses are in the message.

For IPv4, the value is 4.

• OP--Opcode. Specifies the nature of the message by code:

◦ 1--ARP request.

◦ 2--ARP reply.

◦ 3 through 9--RARP and Inverse ARP requests and replies.

• SHA--Sender hardware address. Specifies the Layer 2 hardware address of the device sending the

message.

Address Resolution Protocol

Information About Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

• SPA--Sender protocol address. Specifies the IP address of the sending device.

• THA--Target hardware address. Specifies the Layer 2 hardware address of the receiving device.

• TPA--Target protocol address. Specifies the IP address of the receiving device.

ARP Caching

Because the mapping of IP addresses to MAC addresses occurs at each hop (router) on the network for

every datagram sent over an internetwork, performance of the network could be compromised. To

minimize broadcasts and limit wasteful use of network resources, ARP caching was implemented.

ARP caching is the method of storing network addresses and the associated data-link addresses in memory

for a period of time as the addresses are learned. This minimizes the use of valuable network resources to

broadcast for the same address each time a datagram is sent. The cache entries must be maintained because

the information could become outdated, so it is critical that the cache entries are set to expire periodically.

Every device on a network updates its tables as addresses are broadcast.

There are static ARP cache entries and dynamic ARP cache entries. Static entries are manually configured

and kept in the cache table on a permanent basis. They are best for devices that have to communicate with

other devices usually in the same network on a regular basis. Dynamic entries are added by the Cisco IOS

software and kept for a period of time, then removed.

Static and Dynamic Entries in the ARP Cache

Static routing requires an administrator to manually enter IP addresses, subnet masks, gateways, and

corresponding MAC addresses for each interface of each router into a table. Static routing enables more

control but requires more work to maintain the table. The table must be updated each time routes are added

or changed.

Dynamic routing uses protocols that enable the routers in a network to exchange routing table information

with each other. The table is built and changed automatically. No administrative tasks are needed unless a

time limit is added, so dynamic routing is more efficient than static routing. The default time limit is 4

hours. If the network has a great many routes that are added and deleted from the cache, the time limit

should be adjusted.

The routing protocols that dynamic routing uses to learn routes, such as distance-vector and link-state, is

beyond the scope of this document. For more information, refer to Cisco IOS IP Routing Protocols

Configuration Guide , Release 12.4.

Devices That Do Not Use ARP

When a network is divided into two segments, a bridge joins the segments and filters traffic to each

segment based on MAC addresses. The bridge builds its own address table, which uses MAC addresses

only, as opposed to a router, which has a ARP cache that contains both IP addresses and the corresponding

MAC addresses.

Passive hubs are central-connection devices that physically connect other devices in a network. They send

messages out all of their ports to the devices and operate at Layer 1, but do not maintain an address table.

Layer 2 switches determine which port is connected to a device to which the message is addressed and send

only to that port, unlike a hub, which sends the message out all its ports. However, Layer 3 switches are

routers that build an ARP cache (table).

For more information about bridges, refer to the Cisco IOS Bridging and IBM Networking Configuration

Guide , Release 12.4. For more information about switches, refer to Cisco IOS Switching Services

Configuration Guide , Release 12.4.

ARP Caching

Information About Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Inverse ARP

Inverse ARP, which is enabled by default in ATM networks, builds an ATM map entry and is necessary to

send unicast packets to a server (or relay agent) on the other end of a connection. Inverse ARP is only

supported for the aal5snap encapsulation type.

For multipoint interfaces, an IP address can be acquired using other encapsulation types because broadcast

packets are used. However, unicast packets to the other end will fail because there is no ATM map entry

and thus DHCP renewals and releases also fail.

For more information about Inverse ARP and ATM networks, refer to the “Configuring ATM” chapter of

the Cisco IOS Wide-Area Networking Configuration Guide , Release 12.4.

Reverse ARP

Reverse ARP (RARP) as defined by RFC 903 works the same way as ARP, except that the RARP request

packet requests an IP address instead of a MAC address. RARP often is used by diskless workstations

because this type of device has no way to store IP addresses to use when they boot. The only address that is

known is the MAC address because it is burned into the hardware.

Use of RARP requires an RARP server on the same network segment as the router interface. The figure

below illustrates how RARP works.

Figure 3

There are several limitations of RARP. Because of these limitations, most businesses use DHCP to assign

IP addresses dynamically. DHCP is cost effective and requires less maintenance than RARP. The most

important limitations are as follows:

• Since RARP uses hardware addresses, if the internetwork is large with many physical networks, a

RARP server must be on every segment with an additional server for redundancy. Maintaining two

servers for every segment is costly.

• Each server must be configured with a table of static mappings between the hardware addresses and IP

addresses. Maintenance of the IP addresses is difficult.

• RARP only provides IP addresses of the hosts and not subnet masks or default gateways.

The Cisco IOS software attempts to use RARP if it does not know the IP address of an interface at startup

to respond to RARP requests that they are able to answer. A feature of Cisco IOS software automates the

configuration of Cisco devices and is called AutoInstall.

AutoInstall supports RARP and enables a network manager to connect a new router to a network, turn it on,

and load a pre-existing configuration file automatically. The process begins when no valid configuration

file is found in NVRAM. For more information about AutoInstall, refer to the Cisco IOS Configuration

Fundamentals Configuration Guide , Release 12.4.

Inverse ARP

Information About Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Proxy ARP

Proxy ARP, as defined in RFC 1027, was implemented to enable devices that are separated into physical

network segments connected by a router in the same IP network or subnetwork to resolve the IP-to-MAC

addresses. When devices are not in the same data link layer network but in the same IP network, they try to

transmit data to each other as if they are on the local network. However, the router that separates the

devices will not send a broadcast message because routers do not pass hardware-layer broadcasts. The

addresses cannot be resolved.

Proxy ARP is enabled by default so the “proxy router” that resides between the local networks will respond

with its MAC address as if it is the router to which the broadcast is addressed. When the sending device

receives the MAC address of the proxy router, it sends the datagram to the proxy router that in turns sends

the datagram to the designated device.

Proxy ARP is invoked by 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 proxy ARP is disabled, a device will respond to ARP requests received on its interface only if the

target IP address is the same as its IP address, or the target IP address in the ARP request has a statically

configured ARP alias.

Serial Line Address Resolution Protocol

Serial Line ARP (SLARP) is used for serial interfaces that use High-Level Data Link Control (HDLC)

encapsulation. A SLARP server, intermediate (staging) router, and another router providing a SLARP

service may be required in addition to a TFTP server. If an interface is not directly connected to a server,

the staging router is required to forward the address resolution requests to the server, otherwise a directly

connected router with SLARP service is required. The Cisco IOS software attempts to use SLARP if it does

not know the IP address of an interface at startup to respond to SLARP requests that software is able to

answer.

A feature of Cisco IOS software automates the configuration of Cisco devices and is called AutoInstall.

AutoInstall supports SLARP and enables a network manager to connect a new router to a network, turn it

on, and load a pre-existing configuration file automatically. The process begins when no valid

configuration file is found in NVRAM. For more information about AutoInstall, refer to the Cisco IOS

Configuration Fundamentals Configuration Guide , Release 12.4.

Note

Serial interfaces that use Frame Relay encapsulation are supported by AutoInstall.

Authorized ARP

Authorized ARP addresses a requirement of explicitly knowing when a user has logged off, either

voluntarily or due to a failure of a network device. It is implemented for Public wireless LANs (WLANs)

and DHCP. For more information about authorized ARP, refer to the “Configuring DHCP Services for

Accounting and Security” chapter of the DHCP Configuration Guide , Cisco IOS Release 12.4.

Proxy ARP

Information About Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

How to Configure Address Resolution Protocol Options

ARP is enabled by default and is set to use Ethernet encapsulation by default. Perform the following tasks

to change or verify ARP functionality:

• Enabling the Interface Encapsulation, page 7

• Defining Static ARP Entries, page 8

• Setting an Expiration Time for Dynamic Entries in the ARP Cache, page 10

• Globally Disabling Proxy ARP, page 11

• Disabling Proxy ARP on an Interface, page 11

• Clearing the ARP Cache, page 12

• Verifying the ARP Configuration, page 13

Enabling the Interface Encapsulation

Perform this task to support a type of encapsulation for a specific network, such as Ethernet, Frame Relay,

FDDI, or Token Ring. When Frame Relay encapsulation is specified, the interface is configured for a

Frame Relay subnetwork in which there is one physical link that has many logical circuits called virtual

circuits (VCs). The address field in the frame contains a data-link connection identifier (DLCI) which

identifies each VC. When SNAP encapsulation is specified, the interface is configured for FDDI or Token

Ring networks.

Note

The encapsulation type specified in this task should match the encapsulation type specified in the "Defining

Static ARP Entries" task.

SUMMARY STEPS

enable

configure terminal

interface type number

arp {arpa | frame-relay | snap}

exit

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Enabling the Interface Encapsulation

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Command or Action Purpose

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Router(config)# interface ethernet0/0

Enters interface configuration mode.

Step 4

arp {arpa | frame-relay | snap}

Example:

Router(config-if)# arp arpa

Specifies the encapsulation type for an interface by type of network, such as

Ethernet, FDDI, Frame Relay, and Token Ring. The keywords are as

follows:

•

arpa --Enables encapsulation for an Ethernet 802.3 network.

•

frame-relay --Enables encapsulation for a Frame Relay network.

•

snap --Enables encapsulation for FDDI and Token Ring networks.

Step 5

exit

Example:

Router(config-if)# exit

Exits interface configuration mode.

Defining Static ARP Entries

Perform this task to define static mapping between IP addresses (32-bit address) and a MAC address (48-

bit address) for hosts that do not support dynamic ARP. Because most hosts support dynamic address

resolution, defining static ARP cache entries is usually not required. Performing this task installs a

permanent entry in the ARP cache that never times out. The entries remain in the ARP table until they are

removed using the no arp command or the clear arp interface command for each interface.

Note

The encapsulation type specified in this task should match the encapsulation type specified in the Enabling

the Interface Encapsulation" task.

SUMMARY STEPS

enable

configure terminal

arp {ip-address| vrf vrf-name} hardware-address encap-type [interface-type]

exit

Defining Static ARP Entries

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

DETAILED STEPS

Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

arp {ip-address| vrf vrf-name}

hardware-address encap-type

[interface-type]

Example:

Router(config)# arp 10.0.0.0

aabb.cc03.8200 arpa

Globally associates an IP address with a MAC address in the ARP cache. The

arguments and keyword are as follows:

• ip-address --IP address in four-part dotted decimal format corresponding to

the local data-link address.

•

vrf vrf-name --Virtual routing and forwarding instance for a Virtual Private

Network (VPN). The vrf-name argument can be any name.

• hardware-address --Local data-link address (a 48-bit address).

• encap-type --Encapsulation type for the static entry. The keywords are as

follows:

◦

arpa--For Ethernet interfaces.

◦

sap--For Hewlett Packard interfaces.

◦

smds--For Switched Multimegabit Data Service (SMDS) interfaces.

◦

snap--For FDDI and Token Ring interfaces.

◦

srp-a--Switch route processor-side A (SRP-A) interfaces.

◦

srp-b--Switch route processor-side B (SRP-B) interfaces.

• interface-type --(Optional) Interface type. The keywords are as follows:

◦

ethernet--IEEE 802.3 interface.

◦

loopback--Loopback interface.

◦

null--No interface.

◦

serial--Serial interface

◦

alias--Device responds to ARP requests as if it were the interface of

the specified address.

Step 4

exit

Example:

Router(config)# exit

Exits to privileged EXEC mode.

Configuring Address Resolution Protocol Options

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Setting an Expiration Time for Dynamic Entries in the ARP Cache

Perform this task to set a time limit for dynamic entries in the ARP cache.

SUMMARY STEPS

enable

configure terminal

interface type number

arp timeout seconds

exit

DETAILED STEPS

Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Router(config)# interface ethernet0/0

Enters interface configuration mode.

Step 4

arp timeout seconds

Example:

Router(config-if)# arp timeout 30

Sets the length of time, in seconds, an ARP cache entry will stay in the

cache. A value of zero means that entries are never cleared from the cache.

The default is 14400 seconds (4 hours).

Note

If the network has frequent changes to cache entries, the default

should be changed to a shorter time period.

Step 5

exit

Example:

Router(config-if)# exit

Exits interface configuration mode.

Setting an Expiration Time for Dynamic Entries in the ARP Cache

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Globally Disabling Proxy ARP

Proxy ARP is enabled by default; perform this task to globally disable proxy ARP on all interfaces.

The Cisco IOS software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of

routing determine the MAC addresses of hosts on other networks or subnets. For example, if hosts A and B

are on different physical networks, host B will not receive the ARP broadcast request from host A and

cannot respond to it. However, if the physical network of host A is connected by a gateway to the physical

network of host B, the gateway will see the ARP request from host A.

Assuming that subnet numbers were assigned to correspond to physical networks, the gateway can also tell

that the request is for a host that is on a different physical network. The gateway can then respond for host

B, saying that the network address for host B is that of the gateway itself. Host A will see this reply, cache

it, and send future IP packets for host B to the gateway.

The gateway will forward such packets to host B by using the configured IP routing protocols. The gateway

is also referred to as a transparent subnet gateway or ARP subnet gateway.

SUMMARY STEPS

enable

configure terminal

ip arp proxy disable

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

ip arp proxy disable

Example:

Router(config)# ip arp proxy disable

Disables proxy ARP on all interfaces.

•

The ip arp proxy disable command overrides any proxy ARP interface

configuration.

•

To reenable proxy ARP, use the no ip arp proxy disable command.

•

You can also use the default ip proxy arp command to return to the

default proxy ARP behavior, which is enabled.

Disabling Proxy ARP on an Interface

Proxy ARP is enabled by default; perform this task to disable proxy ARP on an interface.

Globally Disabling Proxy ARP

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

SUMMARY STEPS

enable

configure terminal

interface type number

no ip proxy-arp

exit

DETAILED STEPS

Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Router(config)# interface ethernet0/0

Enters interface configuration mode.

Step 4

no ip proxy-arp

Example:

Router(config-if)# ip proxy-arp

Disables proxy ARP on the interface.

•

To reenable proxy ARP, use the ip proxy-arpcommand.

•

You can also use the default ip proxy-arpcommand to return to the

default proxy ARP behavior on the interface, which is enabled.

Step 5

exit

Example:

Router(config-if)# exit

Exits to global configuration mode.

Clearing the ARP Cache

Perform the following tasks to clear the ARP cache of entries associated with an interface and to clear all

dynamic entries from the ARP cache, the fast-switching cache, and the IP route cache.

Clearing the ARP Cache

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

SUMMARY STEPS

enable

clear arp interface type number

clear arp-cache

exit

DETAILED STEPS

Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

• Enter your password if prompted.

Step 2

clear arp interface type number

Example:

Router# clear arp interface ethernet0/0

Clears the entire ARP cache on the interface. The type and number

arguments are the type of interface and the assigned number for the

interface.

Step 3

clear arp-cache

Example:

Router# clear arp-cache

Clears all dynamic entries from the ARP cache, the fast-switching cache,

and the IP route cache.

Step 4

exit

Example:

Router# exit

Exits to EXEC mode.

Verifying the ARP Configuration

To verify the ARP configuration, perform the following steps.

SUMMARY STEPS

show interfaces

show arp

show ip arp

show processes cpu | include (ARP|PID)

Verifying the ARP Configuration

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

DETAILED STEPS

Step 1

show interfaces

To display the type of ARP being used on a particular interface and also display the ARP timeout value, use the show

interfaces EXEC command.

Example:

Router# show interfaces

Ethernet 0 is up, line protocol is up

Hardware is MCI Ethernet, address is 0000.0c00.750c (bia 0000.0c00.750c)

Internet address is 10.108.28.8, subnet mask is 255.255.255.0

MTU 1500 bytes, BW 10000 Kbit, DLY 100000 usec, rely 255/255, load 1/255

Encapsulation ARPA, loopback not set, keepalive set (10 sec)

ARP type: ARPA, ARP Timeout 4:00:00

Last input 0:00:00, output 0:00:00, output hang never

Last clearing of “show interface” counters 0:00:00

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

Five minute input rate 0 bits/sec, 0 packets/sec

Five minute output rate 2000 bits/sec, 4 packets/sec

1127576 packets input, 447251251 bytes, 0 no buffer

Received 354125 broadcasts, 0 runts, 0 giants, 57186* throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

5332142 packets output, 496316039 bytes, 0 underruns

0 output errors, 432 collisions, 0 interface resets, 0 restarts

Step 2

show arp

Use the show arp EXEC command to examine the contents of the ARP cache.

Example:

Router# show arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 10.108.42.112 120 0000.a710.4baf ARPA Ethernet3

AppleTalk 4028.5 29 0000.0c01.0e56 SNAP Ethernet2

Internet 110.108.42.114 105 0000.a710.859b ARPA Ethernet3

AppleTalk 4028.9 - 0000.0c02.a03c SNAP Ethernet2

Internet 10.108.42.121 42 0000.a710.68cd ARPA Ethernet3

Internet 10.108.36.9 - 0000.3080.6fd4 SNAP TokenRing0

AppleTalk 4036.9 - 0000.3080.6fd4 SNAP TokenRing0

Internet 10.108.33.9 - 0000.0c01.7bbd SNAP Fddi0

Step 3

show ip arp

Use the show ip arp EXEC command to show IP entries. To remove all nonstatic entries from the ARP cache, use the

clear arp-cacheprivileged EXEC command.

Example:

Router# show ip arp

Protocol Address Age(min) Hardware Addr Type Interface

Internet 171.69.233.22 9 0000.0c59.f892 ARPA Ethernet0/0

Internet 171.69.233.21 8 0000.0c07.ac00 ARPA Ethernet0/0

Internet 171.69.233.19 - 0000.0c63.1300 ARPA Ethernet0/0

Internet 171.69.233.30 9 0000.0c36.6965 ARPA Ethernet0/0

Internet 172.19.168.11 - 0000.0c63.1300 ARPA Ethernet0/0

Internet 172.19.168.254 9 0000.0c36.6965 ARPA Ethernet0/0

Step 4

show processes cpu | include (ARP|PID)

Use the show processes cpu | include (ARP|PID) command to display ARP and RARP processes.

Configuring Address Resolution Protocol Options

How to Configure Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Example:

Router# show processes cpu | include (ARP|PID)

PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process

1 1736 58 29931 0% 0% 0% Check heaps

2 68 585 116 1.00% 1.00% 0% IP Input

3 0 744 0 0% 0% 0% TCP Timer

4 0 2 0 0% 0% 0% TCP Protocols

5 0 1 0 0% 0% 0% BOOTP Server

6 16 130 123 0% 0% 0% ARP Input

7 0 1 0 0% 0% 0% Probe Input

8 0 7 0 0% 0% 0% MOP Protocols

9 0 2 0 0% 0% 0% Timers

10 692 64 10812 0% 0% 0% Net Background

11 0 5 0 0% 0% 0% Logger

12 0 38 0 0% 0% 0% BGP Open

13 0 1 0 0% 0% 0% Net Input

14 540 3466 155 0% 0% 0% TTY Background

15 0 1 0 0% 0% 0% BGP I/O

16 5100 1367 3730 0% 0% 0% IGRP Router

17 88 4232 20 0.20% 1.00% 0% BGP Router

18 152 14650 10 0% 0% 0% BGP Scanner

19 224 99 2262 0% 0% 1.00% Exec

Configuration Examples for Address Resolution Protocol

Options

• Static ARP Entry Configuration Example, page 15

• Encapsulation Type Configuration Example, page 15

• Proxy ARP Configuration Example, page 16

• Clearing the ARP Cache Example, page 16

Static ARP Entry Configuration Example

The following example shows how to configure a static ARP entry in the cache and by using the alias

keyword, Cisco IOS software can respond to ARP requests as if it were the interface of the specified

address:

arp 10.0.0.0 aabb.cc03.8200 alias

interface ethernet0/0

Encapsulation Type Configuration Example

The following example shows how to configure the encapsulation on the interface. The snap keyword

indicates that interface Ethernet0/0 is connected to an FDDI or Token Ring network:

interface ethernet0/0

ip address 10.108.10.1 255.255.255.0

arp snap

Static ARP Entry Configuration Example

Configuration Examples for Address Resolution Protocol Options

IP Addressing: ARP Configuration Guide, Cisco IOS Release 12.2SX

Proxy ARP Configuration Example

The following example shows how to configure proxy ARP because it was disabled for interface

Ethernet0/0:

interface ethernet0/0

ip proxy-arp

Clearing the ARP Cache Example

The following example shows how to clear all of the entries in the ARP cache associated with an interface:

Router# clear arp interface ethernet0/0

The following example shows how to clear all of the dynamic entries in the ARP cache:

Router# clear arp-cache

Additional References