Address Resolution Protocol (ARP)

OVERVIEW

The Address Resolution Protocol (ARP) was developed to enable communications on an internetwork and is defined by RFC 826. Layer 3 devices need ARP to map IP network 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 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.

When the destination device lies on a remote network, one beyond another Layer 3 device, 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 Layer 3 device 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. 1--ARP request.
2. 2--ARP reply.
3. 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.
• 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 media access control (MAC) addresses occurs at each hop (Layer 3 device) 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, Address Resolution Protocol (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. Static entries 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 Cisco software, kept for a period of time, and 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 media access control (MAC) addresses for each interface of each device 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 devices 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.

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 Media Access Control (MAC) addresses. The bridge builds its own address table, which uses MAC addresses only, as opposed to a router, which has an Address Resolution Protocol (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 ports to the devices and operate at Layer 1, but they 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 the message 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).

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 supported only 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.

Reverse ARP

Reverse ARP (RARP) as defined by RFC 903 works the same way as the Address Resolution Protocol (ARP), except that the RARP request packet requests an IP address instead of a media access control (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 in to the hardware.

RARP requires a RARP server on the same network segment as the device interface. The figure below illustrates how RARP works.

Because of the limitations with RARP, most businesses use Dynamic Host Configuration Protocol (DHCP) to assign IP addresses dynamically. DHCP is cost-effective and requires less maintenance than RARP. The most important limitations with RARP are as follows:

• Because 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 the IP addresses. Maintenance of the IP addresses is difficult.
• RARP only provides IP addresses of the hosts but not subnet masks or default gateways.
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• Cisco software attempts to use RARP if it does not know the IP address of an interface at startup to respond to RARP requests that it is able to answer. The AutoInstall feature of the software automates the configuration of Cisco devices.
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• AutoInstall supports RARP and enables a network manager to connect a new device to a network, turn it on, and automatically load a pre-existing configuration file. The process begins when no valid configuration file is found in NVRAM.

Proxy ARP

Proxy Address Resolution Protocol, 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 IP-to-MAC addresses. When devices are not in the same data link layer network but are in the same IP network, they try to transmit data to each other as if they were 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. Therefore, the addresses cannot be resolved.

Proxy ARP is enabled by default so the "proxy router" that resides between the local networks responds with its MAC address as if it were 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, which 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 responds to ARP requests received on its interface only if the target IP address is the same as its IP address or if the target IP address in the ARP request has a statically configured ARP alias.

USEFUL COMMANDS ABOUT ARP FOR CISCO DEVICES:

Defining Static ARP Entries

• enable
• configure terminal 
• arp {ip-address | vrf vrf-name} hardware-address encap-type [interface-type] 
• end

Setting an Expiration Time for Dynamic Entries in the ARP Cache

• enable 
• configure terminal 
• interface type number
• arp timeout seconds
• end

Globally Disabling Proxy ARP

• enable 
• configure terminal 
• ip arp proxy disable
• end

Globally Disabling Proxy ARP

• enable 
• configure terminal
• interface type number
• no ip proxy-arp
• end

Clearing the ARP Cache

• enable 
• clear arp interface type number
• clear arp-cache
• exit

Verifying the ARP Configuration

• show interfaces
• show arp
• show ip arp
• show processes cpu | include (ARP|PID)

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