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

Frame Relay Network

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 (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

ARP Process

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.
• 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.

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 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.

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.

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 (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

RARP Process

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.

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 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 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.

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.

1.    enable

2.    configure terminal

3.    interface type number

4.    arp {arpa | frame-relay | snap}

5.    exit

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.

1.    enable

2.    configure terminal

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

4.    exit

1.    enable

2.    configure terminal

3.    interface type number

4.    arp timeout seconds

5.    exit

1.    enable

2.    configure terminal

3.    ip arp proxy disable

1.    enable

2.    configure terminal

3.    interface type number

4.    no ip proxy-arp

5.    exit

1.    enable

2.    clear arp interface type number

3.    clear arp-cache

4.    exit

1.    show interfaces

2.    show arp

3.    show ip arp

4.    show processes cpu | include (ARP|PID)

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

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

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

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