ARP PACKET FORMAT AND DIFFERENT TYPES OF ARP's

ARP PACKET FORMAT

Protocol type (PTYPE) :

This field specifies the internetwork protocol for which the ARP request is intended. For IPv4, this has the value 0x0800. The permitted PTYPE values share a numbering space with those for EtherType

Operation Code:

Specifies the operation that the sender is performing: 1 for request, 2 for reply

Wireshark View Of ARP

ARP

ARP is address resolution protocol. It is mapping of Layer 3 address to a Layer 2 address.
When a host wants to send data to an IP address it requires a destinationMAC address to send the frame to.To get a corresponding Layer 2 (MAC address) it looks at its ARP table. If the MAC address is not found in the ARP table the computer sends a broadcast (ARP Request) requesting the host with that IP address to reply with his MAC address.
This process is called ARP.

Inverse ARP

Frame-relay uses inverse arp.  Inverse arp is to associate a dlci with an ip address.  From the perspective of the router doing the inverse arp, the dlci is known.  So basically inverse arp allows us to ask what IP address is at the other end of a pvc associated with a given dlci.  Arp on the other hand is an Ethernet concept and works the other way.  The router performing the arp knows the IP address it needs to communicate with, but not the mac address.  So arp allows us to send out a broadcast requesting the layer 2 information.  Inverse arp sends out a request asking for layer 3 information.  

Proxy (Promiscuous ARP)

Host A wants to send data to Host B which is not on that network. But the HostA is not configured with default gateway information and therefore does not know how to reach a router(gateway).Host A sends an ARP to get a MAC address for Host B. Local Router receiving the ARP request ,replies to Host A with its own MAC address stating itself as destination  hence when the data is sent to the destination by Host A it would be sending to the gateway (as destination MAC is given as Gateway's MAC) which would in-turn send to host B. This is called proxy arp.

Advantages: 1) Used when hosts cannot be configured with default gateway 2) Proxy ARP makes the subnetted network topology transparent to the hosts.

Enabled by default in IOS and might be disabled on a per interface basis with the command no ip proxy-arp

Gratuitous ARP

Host A sends an ARP request using source and destination address as its own address.

mainly used to update other devices about its own IP address.

GARP is used to

-          Detect duplicate address by sending an ARP packet with destination as its own IP address and if it gets any reply back that means there is another device with same IP address.

-          Used when a Host wants to update other local Hosts ARP tables.

-          A router running HSRP that is just become active uses GARP to update the ARP cache of subnet hosts

Reverse ARP

Some devices, such as diskless workstations, might not know their IP address at startup. RARP might be programmed into firmware on these devices, allowing them to issue an ARP Request that has their burned-in hardware address. The reply from a RARP server will supply the appropriate IP address.