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Does a switch always get a MAC address from a PC?

up vote 14 down vote favorite

I was wondering if there is a possibility that a switch won't get the needed MAC address to it's CAM.

For example, if the used protocol is UDP, where the sender doesn't need an answer back from the receiver?

Thank you

2 Answers

up vote 4 down vote accept

A switch learns the source MAC from the sender. If the destination is not in the CAM table, the switch floods the frame out all ports. So if the receiver never responds, the switch will never learn the receiver's MAC and it will always flood the frame.

up vote 3 down vote

An ethernet switch will get the source MAC address from the frame as it enters the switch. A MAC address is a data-link address on the frame. UDP is a transport protocol, encapsulated in the network protocol, which is encapsulated in the data-link protocol. The switch has no idea what protocols are encapsulated inside the data-link frame.


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Does a switch always get a MAC address from a PC?

up vote 14 down vote

I was wondering if there is a possibility that a switch won't get the needed MAC address to it's CAM.

For example, if the used protocol is UDP, where the sender doesn't need an answer back from the receiver?

Thank you


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up vote 9 down vote

A switch learns the source MAC from the sender. If the destination is not in the CAM table, the switch floods the frame out all ports. So if the receiver never responds, the switch will never learn the receiver's MAC and it will always flood the frame.

edit

Hi, thank you for answer. So in case the frame is received by PC with the destination MAC address it always sends the answer back so switch can add this mac into CAM table ? Or how the switch knows that the frame was received ? - Lukáš Stacke Feb 5 at 19:19

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