TYPES OF ARP

 

Types of ARP

 

 

Table of Contents:-

1.   Types of ARP.

1.1.      RARP( Working, Drawback )

1.2.      GARP( Packet, Working, Uses )

1.3.      PARP( Packet, Working )

2.   Questions asked from TYPES OF ARP in the Network Engineer L1 Interview.

 

·   There are 4 types of Arp:-

 

1.  RARP(Reverse Arp)

2.  GARP(Gratuitous Arp)

3.  PARP (Proxy Arp)

4.  InARP (Inverse Arp)

 

With the interview perspective only GARP and PARP are important and only these are being asked. So, I’ll explain GARP and PARP only. For the understanding perspective I’ll give a brief of RARP also but that’s just from the knowledge perspective and won’t be asked in the interview.

Moving on with the types of Arp lets discuss them one by one.

 

1. RARP (Reverse Arp):-

 

RARP as could be understood from its full form is the reverse of Arp that is Arp resolves the Mac address from the IP address whereas RARP resolves the IP address from the Mac address.

 

·     Working of RARP:-

 

 A RARP request is created and broadcasted on the local network, the device that knows all the IP address will respond with a RARP reply. The machine that is requesting will have to install a RARP client and the replying machine must install RARP server program.

 

·     Drawback:-

 

 As we can see the RARP request is a broadcast one and the broadcast request is network specific that is it could be run into the same network in which its generated. So, to make it work we would need a RARP server in every network, which makes it unscalable and this is the reason it was replaced by BOOTP and later by DHCP.

 

2. GARP (Gratuitous ARP):-

 

GARP is another type of Arp, it is responsible to detect the duplicate IP address in the particular network. Its used in the Probe test to detect if any other device is also having the same IP, GARP resolves the Mac address of that device having the duplicate IP.

 

·     GARP Packet:-

 

Let us consider that the IP being configured manually or provided by the DHCP server is 10.11.11.1 and the Source Mac to be A. So, now the packet of GARP would be ( Considering only the main fields )-

 

Gratuitous ARP

 

Source Mac

A

L2 Header

Destination Mac

FFFF.FFFF.FFFF

 

 

ARP Header

Operation Code

Request or Reply

Sender Mac Address

A

Sender IP Address

10.11.11.1

Target Mac Address

0000.0000.0000

Target IP Address

10.11.11.1

 

 

·     WORKING OF GARP:-

 

1. So, now when a IP address is assigned to a device it creates a Arp packet with      the same source and target IP as shown above and broadcasts it into the network.

2. Now this request packet is received by all the devices in the network and if there is any device with the same IP, it would reply with its Mac address into the Arp reply packet.

3. That’s how the source gets to know if there is any other device having the same IP as the source.

 

 

·     USES OF ARP:-

 

1. Probe test- When an interface goes up with a configured IP address, it sends a probe to make sure no other host is using the same IP. Hence, preventing IP conflicts.

2. A virtual Mac is used to point to the active device, when the device fails, the failover control sends a GARP to ask other systems to change their Arp table entries and point the virtual Mac of the cluster to the new active device.

3. Its also used when Ethernet interface goes up; GARP sent by host to notify all systems with its IP address, instead of waiting for them to ask and send replies to each one.

4. Whenever a machine is moved or a virtual interface is created on a load balancer; GARP is used to notify other devices of the changes.

 

 

3. PARP (Proxy ARP):-

 

Proxy Arp comes into play when the destination is in another network but after AND operation source gets the result of the destination to be in same network       ( Happens mainly in the case of subnetting ). So, now the source prepares the Arp packet for the destination and not for the next hop that is router.

In this case when the Arp request is broadcasted it won’t reach the destination and would be dropped by router ( When Proxy Arp is not enabled on the router ) as router doesn’t forward broadcast traffic.

But as Proxy Arp is enabled by default on routers, the router would reply with its Mac address on behalf of the destination.

 

 

Refer this scenario to understand the working of PARP-

 



·     WORKING OF PARP:-

 

- Lets say PC1 Pings PC2 as PC1 knows source and destination IPs and the source Mac as well but for destination Mac it needs Arp.

- So, now as the Arp request packet is prepared it runs AND operation between the destination IP and its own subnet mask to find out if the destination is in the same network or different.

- But in this case AND operation will give the result that the destination is in the same network as that of source.

- So, now the source would generate the Arp request for the destination itself (With the PC2 IP filled in the place of target IP) and not for the default gateway.

 

 

 

 

ARP REQUEST

 

Source Mac

A

L2 Header

Destination Mac

FFFF.FFFF.FFFF

 

 

ARP Header

Operation Code

Request

Sender Mac Address

A

Sender IP Address

10.1.1.1

Target Mac Address

0000.0000.0000

Target IP Address

10.1.2.2

 

 

- Now, as the Arp request reaches the router the router would reply with its Mac address on behalf of PC2 ( Only If the PARP is configured on the router ) and if PARP is not configured the request would be dropped by the router. This is why we need PARP.

 

- Refer the Arp reply packet send by the router below-

 

 

ARP REPLY

 

Source Mac

B

L2 Header

Destination Mac

A

 

 

ARP Header

Operation Code

Reply

Sender Mac Address

B

Sender IP Address

10.1.2.2

Target Mac Address

A

Target IP Address

10.1.1.1

 

- Now, the ICMP Echo will be forwarded to the router from PC1 and the router will generate Arp request for the PC2 Mac address with the source Mac- C, Source IP- 10.1.2.254  and Target IP- 10.1.2.2  from the interface F0/1.

- After that the router will receive Arp reply from PC2 and it will update its Arp table. Now, the Echo will be forwarded to PC2 and PC2 will reply for 10.1.1.1 which will reach the router and forwarded to PC1 by the router.

 

** On PC1 the IP- 10.1.2.2 is bind with the MAC-B but in reality it is not like that. So, with the help of PARP this communication becomes possible.

 

 

·   INTERVIEW QUESTIONS FOR NETWORK ENGINEER L1 FROM TYPES OF ARP:-

 

## DISCLAIMER:- The questions asked in the interview are all scenario based and indirect, So you should try to understand the concept instead of cramming. You will be fortunate if you receive a direct question from the interviewer.

 

 

1. What is GARP? Describe its working?

2. A scenario could be provided for GARP and asked what would come in Target IP address field?

3. Where is GARP used?

4. Which Arp is used in the probe test?

5. What is PARP? Describe its working?

6. You would get a scenario or a topology diagram in front of you and would be asked to explain the working of PARP? 

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