1 of 15

Address Resolution Protocol

Page 0

2 of 15

In order for data transmission to a network destination to be achieved it is necessary to build association between the network layer and lower layer protocols. The means by which the Address Resolution Protocol is used to build this association and prevent the unnecessary generation of additional broadcast traffic in the network should be clearly understood.

Foreword

Page 1

3 of 15

Upon completion of this section, you will be able to:

Explain how the MAC address is resolved using ARP.
Explain the function of the ARP cache table.

Objectives

Page 2

4 of 15

Data link forwarding relies on knowledge of the MAC address of the data link layer destination.

ARP

IP: 10.1.1.1/24

MAC: 00-01-02-03-04-AA

Dest IP : 10.1.1.2

Source IP : 10.1.1.1

Dest MAC : UNKNOWN

Source MAC : 00-01-02-03-04-AA

IP : 10.1.1.2/24

MAC: 00-01-02-03-04-BB

ETH_II

IP

DATA

FCS

Host A

Host B

Page 3

5 of 15

The ARP packet operates within the boundaries of the data link layer, as can be understood by the absence of an IP header.

ARP Format

FCS

ARP

Hardware Type

Hardware

Length

Protocol Type

Protocol Length

Operation Code

Source Hardware Address

Destination Hardware Address

Source Protocol Address

Destination Protocol Address

0

31

15

Ethernet_II

Page 4

The ARP packet is generated as part of the physical target address discovery process. Initial discovery will contain partial information since the destination hardware address or MAC address is to be discovered. The hardware type refers to Ethernet with the protocol type referring to IP, defining the technologies associated with the ARP discovery. The hardware and protocol length identifies the address length for both the Ethernet MAC address and the IP address, and is defined in bytes.
The operation code specifies one of two states, where the ARP discovery is set as REQUEST for which reception of the ARP transmission by the destination will identify that a response should be generated. The response will generate REPLY for which no further operation is necessary by the receiving host of this packet, and following which the ARP packet will be discarded. The source hardware address refers to the MAC address of the sender on the physical segment to which ARP is generated. The source protocol address refers to the IP address of the sender.
The destination hardware address specifies the physical (Ethernet) address to which data can be forwarded by the Ethernet protocol standards, however this information is not present in an ARP request, instead replaced by a value of 0. The destination protocol address identifies the intended IP destination for which reachability over Ethernet is to be established.

6 of 15

Host A wishes to forward data to Host C, but must identify whether it is able to reach the destination at the data link layer.

ARP Process

Host A

10.0.0.2

00-01-02-03-04-BB

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

Page 5

7 of 15

ARP Cache Lookup

Host A>arp -a

Internet Address Physical Address Type

�

Host A

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

10.0.0.2

00-01-02-03-04-BB

Page 6

The ARP cache (pronounced as [kash]) is a table for association of host destination IP addresses and associated physical (MAC) addresses. Any host that is engaged in communication with a local or remote destination will first need to learn of the destination MAC via which communication can be established.
Learned addresses will populate the ARP cache table and remain active for a fixed period of time, during which the intended destination can be discovered without the need for addition ARP discovery processes. Following a fixed period, the ARP cache table will remove ARP entries to maintain the ARP cache table’s integrity, since any change in the physical location of a destination host may result in the sending host inadvertently addressing data to a destination at which the destination host no longer resides.
The ARP cache lookup is the first operation that an end system will perform before determining whether it is necessary to generate an ARP request. For destinations beyond the boundaries of the hosts own network, an ARP cache lookup is performed to discover the physical destination address of the gateway, via which the intended destination network can be reached.

8 of 15

ARP Request Process

Host A

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

D.MAC

S.MAC

ARP

Dest IP: 10.0.0.3

Source IP: 10.0.0.1

Dest MAC: 00-00-00-00-00-00

Source MAC: 00-01-02-03-04-AA

Operation Code: Request

FF-FF-FF-FF-FF-FF

00-01-02-03-04-AA

10.0.0.2

00-01-02-03-04-BB

Page 7

Where an ARP cache entry is unable to be determined, the ARP request process is performed. This process involves generation of an ARP request packet, and population of the fields with the source and destination protocol addresses, as well as the source hardware address. The destination hardware address is unknown. As such the destination hardware address is populated with a value equivalent to 0. The ARP request is encapsulated in an Ethernet frame header and trailer as part of the forwarding process. The source MAC address of the frame header is set as the source address of the sending host.
The host is currently unaware of the location of the destination and therefore must send the ARP request as a broadcast to all destinations within the same local network boundary. This means that a broadcast address is used as the destination MAC address. Once the frame is populated, it is forwarded to the physical layer where it is propagated along the physical medium to which the host is connected. The broadcasted ARP packet will be flooded throughout the network to all destinations including any gateway that may be present, however the gateway will prevent this broadcast from being forwarded to any network beyond the current network.

9 of 15

ARP Reply Process

Host C>arp -a

Internet address Physical address Type

10.0.0.1 00-01-02-03-04-AA Dynamic

Host A

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

10.0.0.2

00-01-02-03-04-BB

Page 8

If the intended network destination exists, the frame will arrive at the physical interface of the destination at which point lower layer processing will ensue. ARP broadcasts mean that all destinations within the network boundary will receive the flooded frame, but will cease to process the ARP request, since the destination protocol address does not match to the IP address of those destinations.
Where the destination IP address does match to the receiving host, the ARP packet will be processed. The receiving host will firstly process the frame header and then process the ARP request. The destination host will use the information from the source hardware address field in the ARP header to populate its own ARP cache table, thus allowing for a unicast frame to be generated for any frame forwarding that may be required, to the source from which the ARP request was received.

10 of 15

ARP Reply Process

Page 9

Host A

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

D.MAC

S.MAC

ARP

Dest IP : 10.0.0.1

Source IP : 10.0.0.3

Dest MAC : 00-01-02-03-04-AA

Source MAC : 00-01-02-03-04-CC

Operation Code: Reply

00-01-02-03-04-CC

10.0.0.2

00-01-02-03-04-BB

Page 9

The destination will determine that the ARP packet received is an ARP request and will proceed to generate an ARP reply that will be returned to the source, based on the information found in the ARP header. A separate ARP packet is generated for the reply, for which the source and destination protocol address fields will be populated. However, the destination protocol address in the ARP request packet now represents the source protocol address in the ARP reply packet, and similarly the source protocol address of the ARP request becomes the destination protocol address in the ARP reply.
The destination hardware address field is populated with the MAC of the source, discovered as a result of receiving the ARP request. For the required destination hardware address of the ARP request, it is included as the source hardware address of the ARP reply, and the operation code is set to reply, to inform the destination of the purpose of the received ARP packet, following which the destination is able to discard the ARP packet without any further communication. The ARP reply is encapsulated in the Ethernet frame header and trailer, with the destination MAC address of the Ethernet frame containing the MAC entry in the ARP cache table, allowing the frame to be forwarded as a unicast frame back to the host that originated the ARP request.

11 of 15

ARP Cache

Host A>arp -a

Internet address Physical address Type

10.0.0.3 00-01-02-03-04-CC Dynamic

Host A

10.0.0.2

00-01-02-03-04-BB

Host C

Host B

10.0.0.3

00-01-02-03-04-CC

10.0.0.1

00-01-02-03-04-AA

Page 10

12 of 15

Proxy ARP enables data link discovery between networks.
Proxy replies with own (G0/0/0) address on behalf of Host B.

Proxy ARP

Host A

Host B

10.1.0.1/8

10.2.0.1/8

Proxy

G0/0/0

G0/0/1

10.2.0.2/16

10.1.0.2/16

ARP Reply with MAC of G0/0/0

ARP Request to 10.2.0.1

①

②

ARP Reply with MAC of Host B

③

④

Page 11

The ARP protocol is also applied to other cases such as where transparent subnet gateways are to be implemented to facilitate communication across data-link networks, where hosts are considered to be part of the same subnetwork. This is referred to as Proxy ARP since the gateway operates as a proxy for the two data-link networks. When an ARP request is generated for a destination that is considered to be part of the same subnet, the request will eventually be received by the gateway. The gateway is able to determine that the intended destination exists beyond the data-link network on which the ARP request was generated.
Since ARP requests cannot be forwarded beyond the boundaries of the broadcast domain, the gateway will proceed to generate its own ARP request to determine the reachability to the intended destination, using its own protocol and hardware addresses as the source addresses for the generated ARP request. If the intended destination exists, an ARP reply shall be received by the gateway for which the destinations source hardware address will be used to populate the ARP cache table of the gateway.
The gateway upon confirming the reachability to the intended destination will then generate an ARP reply to the original source (Host A) using the hardware address of the interface on which the ARP reply was forwarded. The gateway will as a result operate as an agent between the two data-link networks to facilitate data link layer communication, with both hosts forwarding traffic intended for destinations in different data-link networks to the relevant physical address of the “Proxy” gateway.

13 of 15

Duplicate IP addresses may be assigned in a single IP network.
ARP can be used to discover IP address conflicts.

Gratuitous ARP

Host A

IP: 10.0.0.1/24

MAC: 00-01-02-03-04-AA

Dest IP : 10.0.0.1

Source IP : 0.0.0.0

Dest MAC : 00-00-00-00-00-00

Source MAC : 00-01-02-03-04-AA

FF-FF-FF-FF-FF-FF

D.MAC

S.MAC

ARP

00-01-02-03-04-AA

Page 12

In the event that new hardware is introduced to a network, it is imperative that the host determine whether or not the protocol address to which it has been assigned is unique within the network, so as to prevent duplicate address conflicts. An ARP request is generated as a means of determining whether the protocol address is unique, by setting the destination address in the ARP request to be equal to the host’s own IP address.
The ARP request is flooded throughout the network to all link layer destinations by setting the destination MAC as broadcast, to ensure all end stations and gateways receive the flooded frame. All destinations will process the frame, and should any destination discover that the destination IP address within the ARP request match the address of a receiving end station or gateway, an ARP reply will be generated and returned to the host that generated the ARP request.
Through this method the originating host is able to identify duplication of the IP address within the network, and flag an IP address conflict so to request that a unique address be assigned. This means of generating a request based on the hosts own IP address defines the basic principles of gratuitous ARP.

14 of 15

Prior to generating an ARP request, what action must be taken by an end station?
When are gratuitous ARP messages generated and propagated on the local network?

Summary

Page 13

15 of 15

www.huawei.com

Thank You

Page 14