Switches ( forwarding, flooding, filtering )

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A Cisco switch will do one of three things with an incoming frame:

 -  forward it
 -  flood it
 -  filter it

To make this decision, the switch consults its MAC address table to check if there's an entry for the destination MAC address - but first, the switch will actually check to see if there's an entry for the source MAC address of the frame, because it's that source MAC that the switch will use to actually build the table in the first place!!

We have a hub where host A and B is connected, this hub is connected to switch. Two other systems are connected to the same switch. As soon as we connected the switch connected to the network just now, so it does not know the MAC address of any systems. If we have a router it has a dynamic routing protocol to discover the MAC address of the systems but there NO such dynamic switching protocol.

There is only one way switch can know the MAC address of the systems, i.e. by examining the header of the incoming packet. The MAC address can be statically configured in the switch though.

Suppose we have a packet from Source aa-aa-aa-aa-aa-aa to go to the destination cc-cc-cc-cc-cc-cc. Initial switch does not know any thing about the destination MAC address. So what it will do is, it will make an entry of the source MAC address (aa-aa-aa-aa-aa-aa) in its switch table and will flood the packets. Flooding means it will send the packet out to each of its port except the one it came in from.

This kind of frame is know as "Unknown Unicast Frame". because the information needs to be sent to only one system whose MAC address is unknown. Thus an unkown unicast frame is always flooded.

After the flooding is done host C (cc-cc-cc-cc-cc-cc-cc) sends its own frame, and the switch thus makes an entry in the MAC address table about host C. As switch already know about Host A's MAC address the switch is not going to flood this time, instead it will send (Forward) it to only A. Thus that's how the switch learns the MAC address of all the Hosts on the network dynamically.

Now what happens if the Host A sends a frame to Host B. We have a hub connected in the middle. So here's what happens :

 -  Hub receives the incoming frames
 -  hub duplicates them and sends the frame to all other ports
 -  Host B receives the frame and also the switch receives the frame
 -  When the switch receives the frame it looks into the MAC address table
 -  Switch identifies that source and destination are present on the same port
 -  Switch then fiters the frame (i.e. it kills the frame). Switch never sends a frame back to the same port it came in from.

There are always exception to rules in networking but there is no exception to the rule :
"Switches never send a frame back out the same port it came in on."

Flooding :-
Flooding is performed when the switch has no entry for the frame's destination MAC address. When a frame is flooded, it is sent out every single port on the swtich except the one it came in one. Unknown unicast frames are always flooded.

Forwarding :-
Forwarding is performed when the switch does have an entry for the frame's destination MAC address. Forwarding a frame means the frame is being sent out only one port on the switch.

Filtering :-
Filtering is performed when the switch has an entry for both the source and destination MAC address, and the MAC table indiacates that both addresses are found off the same port.

Broadcasting :- There is one other frame type that is sent out every port on the switch except the one that received it, and that's a broadcast frmae, Broadcast frames are intended for all hosts, and the MAC broadcast address is ff-ff-ff-ff-ff-ff ( or FF-FF-FF-FF-FF-FF, as a MAC address's case does not matter. )

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