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New Member

ARP Question

Hi,

I have attached a file regarding ARP. Now what i think is that the answer would be "d". but it says answer is "c". so help me out here guys and do give reason for your statement.

Regards,

query.jpg

  • LAN Switching and Routing
2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

ARP Question

Without knowing the MAC address of the default gateway, you cannot send the dataframes to the default gateway.

so to get the MAC address of the default gateway , it uses ARP.

so ARP comes first and then the dataframes is sent later.

may be a slight deviation from the topic, the action that router does here is called "Proxy ARP"

-Vijay

New Member

ARP Question

hi,

i would say the question is a bit blurry.

cause host a will not arp the mac of host b, it will actually arp the mac of the gateway and it would be more clear if the question would state that the mac of the gateway is not known yet.

host a would only arp the mac of host b if the ip of host b is still in the same subnet and then the router would perform a "proxy arp" as vijay mentioned.

so with this question there is a lot of space to conjecture.

florian

4 REPLIES
Cisco Employee

ARP Question

Hi ahmed,

I would also go with option C. Reason being..

When Host A tries to communicate with Host B, it knows the following.

L3 Source, L3 Destination, L2 Source and what is unknown is L2 Destination.

so it performs a binary "AND" Operation b/w the (Host A's IP and SNM) say X and (HostB's IP and Host A SNM.) say Y

If X and Y matching, then it will directly send an ARP request to the destination.

In this case, Host A and host B are in different Network. so the ARP request is not send to the destination directly but it is sent to the default Gateway.

Remember only after the ARP is known, the dataframes are sent. so I would choose option c instead of option d.

hope this helps

-Vijay

New Member

ARP Question

So what you are saying is that arp is first sent to the default gateway if the hosts are in different networks, that being said isnt the right answer is "d" cos it says "send the data frames t default gateway"

Cisco Employee

ARP Question

Without knowing the MAC address of the default gateway, you cannot send the dataframes to the default gateway.

so to get the MAC address of the default gateway , it uses ARP.

so ARP comes first and then the dataframes is sent later.

may be a slight deviation from the topic, the action that router does here is called "Proxy ARP"

-Vijay

New Member

ARP Question

hi,

i would say the question is a bit blurry.

cause host a will not arp the mac of host b, it will actually arp the mac of the gateway and it would be more clear if the question would state that the mac of the gateway is not known yet.

host a would only arp the mac of host b if the ip of host b is still in the same subnet and then the router would perform a "proxy arp" as vijay mentioned.

so with this question there is a lot of space to conjecture.

florian

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