I believe this is a very simple question for you, but a interesting one for me, so I hope I don't mind you with my first question here, noob question.

We have two switches. SW1 and SW2.

First of all: Switches share the nearby Switches mac address table? I mean, if SW1 have FFFF.FFFF.FF11 F0/1 and FFFF.FFFF.FF22 F0/2 and SW2 have FFFF.FFFF.FF33 F0/1 and FFFF.FFFF.FF44 F0.2, SW1 have knowledge about FF33 and FF44 to? Or just FF11 and FF22?

And second: If I am sending a packet from PC1 through SW1 which is connected to multiple Switches, that have multiple PC's connected to them. How does the SW1 know what path to choose? Judging by the fact that the MAC destination is known only by the Switch that is connected to that PC?

I hope you guys understood the question.

Thank you.

  • All switches MAC addresses tables are independent.
    – Ron Trunk
    Dec 25, 2017 at 20:10
  • Thank you very much. A simple and complex in the same time answer. Thank you guys and I am glad I found you.
    – Andrewy
    Dec 25, 2017 at 20:54

1 Answer 1

  1. Switches don't share anything, each switch learns MAC addresses and port associations on its own. They do this by looking at the sender's address of each incoming frame and then put it in their MAC table, together with the port they received the frame on.

  2. When a frame comes in, the switch searches for the destination MAC in its MAC table and forwards the frame out the associated port from the table.

When a switch doesn't find the destination MAC in its database it floods the frame out of every port but the one it was received on - mimicking a repeater hub. Normally, the switch knows where to find each active MAC because of some earlier frames - and ARP request, ARP response or any other request that's been made by the destination.

In a larger network, not all switches know all MAC addresses. Connections not running through a given switch don't update its MAC table and a remote node might eventually age out.

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