resolving arp question

Unanswered Question
Jul 30th, 2007

I see a question related to this in CCNA where host on network A has to know the hadware address of host B , so it sends a ARP request and the question is who will answer this question ?

the give answer is the default gateway of network A will send a resposne to thsi ARP ..

But I am not sure about this please clarify .

1 )Host must be configured with a default gateway .

2) let us presume that host has already resolved its arp address

3) looking at dest ip address host will know that this machine is not on local network A , so it has to send this packet to default gateway of network A

4) as it already has hardware address of defau;t gateway .it will simply build the packet and send it across to it .

5) it will never raise a ARP query for this

please suggest ? is somthing wroing in question or my understanding

I have this problem too.
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srue Mon, 07/30/2007 - 04:37

the host on network A will not send an arp request for host B unless host A thinks host B is on network A. If that is what host A thinks (even though host B is not on the network A), then the local router might answer using proxy arp (unless it's disabled).

if host A knows host B is on a different network, it will send an arp request for its default route/gateway device.

scottmac Mon, 07/30/2007 - 18:28

You have it pretty much correct, with a few minor (but important) adjustments:

1) - OK

2) - OK

3) - By looking at the IP address AND MASK the host will determine the request is off-LAN

4)- (part 1) - OK, except the packet will be put into a FRAME (i.e., "it will simply build a FRAME and send it ..."... Layer 2 is FRAMES, Layer 3 (IP) is Packets.

4) -(part 2) You are basically correct, if the ARP cache has the IP->MAC in its table, it will not re-ARP. For as long as the address does not age out of the table (can vary depending on the host (i.e., PC, router, switch, other thing)

5) Correct, for as long as the ARP entry exists in the cache/table. Many / most devices will permit static ARP entries, but they're usually not necessary.

If the Gateway device (Router / L3 Switch) is also running "Proxy ARP", it will answer as the MAC for any off-LAN IP address.

IF the questions/answers were written as you presented them, then the "trick" to the question is the syntax (Packets versus Frames, for example).

Another wrinkle, as mentioned above I believe, is that unless the destination hosts is on the local LAN, then no ARP for that host will occur, and the source host doesn't care. MACs are Layer 2, as such, they are only locally significant and do not (generally) make it across a broadcast domain boundry.

Once the source host figure out that the destination host is off-LAN, all he needs is the address/MAC of the Gateway.

It becomes the job of the last router / L3 device to figure out that it is directly connected to the destination host's LAN and to perform the ARP to get its MAC.

Any routers along the way only concern themselves with the network portion of the destination IP address (determined by the Mask) to figure out if it is the last hop.

I hope this helps to clarify this stuff for you.

Good Luck!

Scott

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