Self doubt Switches

Question

In link-layer switches ,switch performs the filtering function to discards the frame which is to be redirected to same interface from which it came.

If a frame destined for station 712B13456142 arrives at port 1, the switch consults its table to find the departing port. According to its table, frames for 7l2B13456142 leave through port 1; therefore, there is no need for forwarding, and the frame is dropped.This means that the frame has already been broadcast on
the LAN segment that contains the destination.

My doubt is

1) why does a node broadcast a frame when it has destination's IP and MAC address?

2) Is it necessary for a frame to pass through switch if both source and destination node are on same LAN and source node has destination's IP and MAC address?

 

Answer 1

Let suppose according to your question OE want to communicate with OF. both are in same LAN. OE sends the frame to Hub and as we know Hub broadcast(except source port) so in that way frame will reach OF and switch. Now as the frame reach switch , switch will first decapsulate frame received and know the source and destination MAC address and now it'll see it's MAC table , to know whether it know those MAC addresses.

If there is entry for source MAC address fine o/w switch will update it's MAC table and add this MAC to port Fa9. after this work it will check whether i have entry for  destination MAC or not. If switch don't have entry for destination MAC address then it will broadcast the frame (except source port) and when the destination device response it like , Hey i'm the one , you are seeking for , know where i 'm..and now switch will add this entry and then unicast the frame to this port.

That's a general process but in your question source and destination belongs to same LAN so in that case when switch see like ohh the destination belongs to same LAN (port Fa9)and thus i can't do anything (switch doesn't forward frame to same port from where frame came)so it discards the frame.

But through your Hub frame already reached destination OF as HUb broadcasted it.

1) Nod don't don anything it's Hub that is broadcasting ..and it broadcast because Hub is  a mindless device , don't contain any entries , don't know any MAC,IP (it's physical layer device)

2) It's absurd question , not getting your actual confusion. Switch works on the bases of physical address. they identify nodes by their physical address while router works on the bases on IP address.If source and destination belongs to same LAN , how router will take decision.. To deliver data in LAN we uses Switch as they works on the bases of MAC address... When source MAC and destination MAC are on same port in switch , it can't help then..to deliver data in LAN we can use hub but as it everytime broadcast , so security is in danger and it increases network congestion (it's quite cheap so we use it).Switches are smart devices so we prefer to use those in LANs.

Comments

Is it possible to have a configuration such as above with switch (ie replacing router in above pic with switch) ?

If it is possible then in that case there would not be any hub to broadcast the frame.Even my Second doubt  iswith respect to such a configuration.

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To transfer data in different networks , you can't use Switch , you need router..every company has their own representation for MAC so MAC can't be used as identification unit for transmitting data.
I'm not saying you can't use switch to communicate with different network , but it would not be a good approach..switch broadcast in first time so network congestion issue...switch are not broadcast domain separator .. Router are..
To connect devices in LAN use switch...
to connect different LANs use router

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Thanks

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in case you yet didn't get it , i can make an example , in hope that might clear your doubt.

Let suppose you have host A(192.168.3.1/24) and host B (192.168.7.1/24) both are let suppose directly connected to switch. Now as i told you switch make their decision bases on MAC address. When host A would send the frame to switch at the data link layer header encapsulation it have to encapsulate source MAC address and Destination MAC address. of course host A knows it's MAC address but to know Destination (host B) MAC address it have to first know whether it's destination reside in same network or not ? so as you can see host A is in network 192.168.3.0 but host B is in 192.168.7.0 so both are in different networks. Host A can reach this different network only by the help of a router(default gateway) which can help host A to get the MAC address of destination...But to communicate with default gateway it needs default gateway's MAC address , The ARP request from A reaches which is for MAC address of A's default gateway...which switch will flood , ARP in that way reaches Host B but it's asking for A's default gateway which B doesn't know so in that way A would never have MAC for it's default gateway and so won't have MAC for destination B so it would never be able to encapsulate packet with data link layer header.....If there you use Router instead of switch , things would have worked fine..

This was a general scenario , how switch fails to send data in different networks...