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The Concept of Switching Frames in Cisco swicthes & Switch Hardware components ?

Level 1
Level 1

Hi folks ,

I would like to know 2 things :

1. can you please explain and easy words the overall concept behing switching logic ? by that i mean how are actually the frames from switches forwarded or learned from one switch to another , as it ingress in one port and egress on another port that is on the same VLAN or on separate or different VLAN ? in shot just explain me how frames are switched in Cisco environment ?

2. What are the different harware components of a Cisco switch , what is made of , for example on PC we have CPU, Disk drives, serial Bus, etc , what about on Cisco switch what do we have and who is responsible for what please explain , and please also enlighten me what the most important logical or physical components of a switch , as a good engineer I need to know my switching gear in and out ..

Looking forward to your answers ..

Regards ,

5 Replies 5

Joseph W. Doherty
Hall of Fame
Hall of Fame


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#1 Switches don't learn from other switches.  When they receive a frame they look to see if the destination unicast MAC has been seen as a source MAC.  If so, they send the frame out the port that the source MAC was seen to arrive on.  If they haven't seen the destination MAC as a source MAC, they send the frame to all the other switch's ports in the same broadcast domain.  (The latter effectively, much like a hub.)

If the frame's destination MAC is a broadcast MAC, it too is sent to all other ports in the same broadcast domain.

If the frame's destination MAC is multicast, it's treated like broadcast unless IGMP snooping is active.  If the latter, the frame is only sent to the ports that have noted they want to receive that multicast MAC.

#2 In many ways, switch hardware is similar to PC hardware.  There's a CPU and NICs although often versions more oriented to how they will be used within a switch.

Switches likely don't have disk drives, although some have "flash" drives.  They might have a serial bus, but whether they do or don't, they generally have a "fabric" for moving frames between ports.  (Fabrics provide connectivity between any two ports without being blocked by other port-to-port traffic.)

Switches also have ASICs, which is the network equivalent of a PC having graphics hardware.  ASICs off-load much to most of the network work from the main CPU.

More information might be found on Cisco's main web site, Cisco's learning forums, and/or searching the Internet.