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

manuelricardo
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 ,
Manuel 

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.

Joseph W. Doherty.

Thank you for your reply to the question about switching. 

I didn't understand one of the part of your post. Can you pls tell me what do you mean by this:

"If they haven't seen the destination MAC as a source MAC."

Also can you pls explain what do you mean by this:

"If the destination unicast MAC has been seen as a source MAC".  

How is it possible that destination unicast Mac can be seen as source MAC address and how is it possible that destination Mac address can be seen as source Mac address?

I hope my question is clear

Thanks


 

Lets start by reviewing some basic concepts. Switches operating at layer 2 forward frames using mac address and not IP address. The switch builds a forwarding table where it records the mac addresses that it has seen and then uses that information to make decisions about how to forward frames.

 

Let us consider separately what the switch does as it receives a frame to be forwarded and then what it does to make a decision about how to forward the frame. So think about a switch that receives a frame on interface FastEther0/1. It looks at the mac address that is the source of this frame. As an example let us assume that this source mac is 1111.2222.3333. The switch looks into its forwarding table to see if it knows about this mac. If this mac is in the table then the switch verifies that it was associated with FastEther0/1. If the mac is not in the table then the switch creates an entry in the table for that mac and associates it with the interface.

 

Now let us think about the switch receiving a frame on interface FastEther0/5. It goes through the process of verifying the source mac and then it is ready to make a decision about how to forward the frame. It looks at the destination mac address of the frame and looks into the forwarding table. Assume that the destination mac is 1111.2222.3333. The switch will find that it should forward this frame out interface FastEther0/1.

 

The key thing here is that a given mac address may be the source mac in one frame and the destination mac in another frame. I hope that this clears up the confusion.

 

HTH

 

Rick

HTH

Rick

If they haven't seen the destination MAC as a source MAC.

They send the frame to all the other switchs 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.

anuuday709
Level 1
Level 1

frame switches

Using frame switches to speed up network traffic.

 

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