Mac – Does a switch know the mac address of other port of a bridge

bridgemac addressnetworkingswitch

Let's say we have switches A and B, they are connected by bridge C.

Does A knows about the mac of nics connected to B?

Is it true that every time when nic on A want to send a frame to nic on B, the switch A has to broadcast so B can receive the message otherwise the nic on B has to stay the same?

Does the switches know there is a bridge connect to them?


Best Answer

  • When starting up (turned off) or if looking for a MAC address that's not in its table (commonly there is space for 2000-8000 addresses in the table, at least for the switches I use), A [per ylearn's comment] "Unicast floods" the packet out all other ports.

    Per my own recollection of additional details, it also populates the table from any incoming traffic (so if a packet came in port 17 from AA:BB:CC:DD:EE:FF the switch scribbles down AA:BB:CC:DD:EE:FF => port 17 and thus knows where to send anything addressed to AA:BB:CC:DD:EE:FF

    When it gets a response from the B switch it stores the MAC address and the A port (leading to B) that the MAC address answered from.

    A knows nothing about what's connected to B - A just knows that a particular A port leads to a particular MAC address. It's has no real awareness of C (other than knowing that C's MAC address is out there on the same port that leads to things that are connected to B - but again, it doesn't know what they are connected to, just that it sends them out this particular A port.)

    The next time it needs to send something to the MAC address, it sends it out the A port that's stored (leading to B.) If the MAC is actually connected to E and gets there via D from B, A knows nothing about that.

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