ARP Interview Questions

1.What is the destination mac-address of an ARP request frame

Broadcast

2.Is ARP used on a PPP link

No

3.Does ARP contain an IP header

ARP is a layer 2 protocol. It does not use IP header.

4.What does a switch do when it receives an ARP request frame on one of its ports

It adds the source port to its mac-address-table and broadcasts the frame out to all ports.

5.Would an ARP request frame generated on a port which is member of VLAN 2, be received on a port which is a member of VLAN 3

ARP is a layer 2 broadcast. VLAN’s are used for segregating broadcast domains. So it would not be received.

6.Can an ARP packet cross IP networks

ARP is a layer 2 broadcast. It cannot traverse across IP networks.

7.If an ARP entry is available on the local cache of a computer, would an ARP request be triggered

No. The computer would first check its ARP entry before sending out an ARP request frame.

8.Explain how an ARP packet is triggered on a computer

Assume that a user on a computer pings another computer on the same network. For the ping packet to be sent to the other computer, it has to be encapsulated on an Ethernet frame. For constructing the frame, the mac-address of the destination is needed. The computer checks its ARP cache if the corresponding ARP entry for the destination computer is available. If not an ARP request is sent out.

9.Does a hub support ARP

If the hub works on mediums like Token Ring, Ethernet etc, then ARP would be supported.

10.3 switches SW1, SW2 and SW3 are interconnected and STP deployed on the switches. PC1 is on SW1 PC2 is on SW2 and PC3 is connected to SW3. All the PC’s are on the same network address of 192.168.1.0/24. PC1 triggers ARP to find the mac-address of PC3. Will the ARP packet reach the PC3

All the PC’s are on the same network address. The PC’s are on 3 switches which makes it one large Layer 2 broadcast domain. So an ARP request which is a layer 2 broadcast will be received on all the ports on all the switches and correspondingly the PC’s

How many addresses are defined in ARP?

In summary, ARP defines both hardware (MAC) and protocol (IP) addresses in its operations, and ARP packets use these addresses to map IP addresses to corresponding MAC addresses within a local network. 

Is an ARP message routable?

No, the messages do not contain an IP header

Describe the Ethernet addressing used in the standard ARP request. Are the source and destination addresses unicast, broadcast, or multicast?

The ARP request uses a unicast address for the source and a broadcast address for the destination 

Describe the Ethernet addressing used in the standard ARP reply. Are the source and destination addresses unicast, broadcast, or multicast?

The ARP reply uses a unicast address for the source and a unicast address for the destination 

What is a gratuitous ARP?

This term refers to a node sending out an ARP request for its own IP address in order to determine if another node is using the same address. 

What information is stored in an ARP table?

The ARP table contains a mapping between host MAC and IP addresses. It also shows whether each entry is static or dynamic 

Can we send standard ARP messages directly to computers that are not on our own network?

No, ARP is not routable 

Is ARP included in IPv6?

No. 

Is ARP a secure protocol?

No. False ARP messages can be created to fool ARP tables. Hosts then make incorrect forwarding decisions. ARP transmissions are also sent in the clear. 

What is the Ethertype hexadecimal value for an ARP message? 

0806