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New Member

ARP

Hello,

Just a quick clarification

Suppose I have a topology as thus: just two hosts connected by a switch (layer 2 switch). In the event that a ping is sent from Host A to Host B and the CAM table of the switch does not contain the mac address of host B. I know the source IP would be that of Host A and the destination would be that of Host B but my confusion lies in the MAC address. The source MAc address would be that of Host A but would the destination be the all Fs address? Would the switch strip this address and replace it with its own Mac address or would this happen only when the switch is set as the default gateway or ip routing (assuming it can support this) is turned on on the switch.

BR,

DJ.

  • Getting Started with LANs
1 ACCEPTED SOLUTION

Accepted Solutions
Purple

ARP

Hi,

if host A and host B are on the same subnet then if host A hasn't got the mac address of host B in its arp cache it will send an arp request which is broadcasted so the switch will flood the arp request out all ports except the port where host A is connected.Host B will send a unicast arp reply and so both mac addresses will be in the switch cam table and both hosts will have the mac address in their arp cache.Once the dynamic entries on the switch are aged out and if the arp cache is still populated then host B will send unicast ping to host B and the switch will learn the mac of host B and then the switch will flood the unknown unicast frame and when host B replies with unicast echo reply then switch learns also mac address of host B.

Regards

Alain

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3 REPLIES
Purple

ARP

Hi,

if host A and host B are on the same subnet then if host A hasn't got the mac address of host B in its arp cache it will send an arp request which is broadcasted so the switch will flood the arp request out all ports except the port where host A is connected.Host B will send a unicast arp reply and so both mac addresses will be in the switch cam table and both hosts will have the mac address in their arp cache.Once the dynamic entries on the switch are aged out and if the arp cache is still populated then host B will send unicast ping to host B and the switch will learn the mac of host B and then the switch will flood the unknown unicast frame and when host B replies with unicast echo reply then switch learns also mac address of host B.

Regards

Alain

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Don't forget to rate helpful posts.
New Member

ARP

hi Alain,

Thanks for the response am assuming you meant Host A in the line where you mentioned the age time out in the CAM table.

DJ

Purple

ARP

Hi

I meant both mac as they  age out at the  same time if there is no other communication happening between them

Regards

Alain

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Don't forget to rate helpful posts.
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