CISCO Switching Basics : Switching Modes

3- Switching Modes :

   Cisco switches has offered 3 ways to switch the traffic, this depends on how the frame is checked before it is forwarded, so the more checking performed, the more latency will be introduced by the switch.
the three switching modes to choose from are :

• Cut-through
• Store-and-forward 
• Fragment-Free

however the default mode en modern switches is the Store-and-forward mode.

3-1- Cut-through :

   this methode is fast, so it introduces the lowest latency. the idea here is the the incoming frame is read up until the destination MAC address. when it reaches the destination MAC address, the switch then will check its CAM table (Content Addressable Memory) to retrieve the correct port to forward the frame out of and sends it. there is no error checking, so this methode has the lowest latency as advantage, but on the other hand the biggest drawback is that the switch will forward any frames containing errors.

3-2- Store-and-forward :

   in this mode here, the switch read the entire frame and then makes a copy of it into its buffers, however the frame is checked for any errors by the Cyclic redundancy check (CRC), if ever any errors are found th frame is dropped, as a result of this error checking the store-and-forward methode is considered as the one with the highest latency of all the switching modes. on the other hand, the switching table is examined and then the frame is transmitted.
there is important thing must be mentioned, the store-and-forward methode checks the length of the frame, if it's smaller than 64 bytes or larger than 1518 bytes, the switch will discard the frame.

3-3- Fragment-free :

   as we have seen earlier in this corse, the cut-through switching doesn't check for errors and the store--and-forward methode takes too long, we need a methode that is quick and reliable at the same time
we can say that fragment-free switching is a modified version of cut-through switching. only the first 64 bytes are examined for any errors, if there is no errors detected, the frame will be passed on.

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so this is it Guys, hope i covered it up as clear as possible.
Next we will talk about VLANs (virtual local area networks)

please comment for any notes or questions, propositions...

Thank you All!

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CISCO Switching Basics : Broadcaste & Collision Domains

2- Broadcaste & Collision Domains : 

2-1- Collision domains: 

   to begin with, it's vital to know that one of the advantages of modern switches is that each port on the switch is considered as a collision domain.
so when a collision is occured, the damaged frame does not pass the interface(not the Hubs' case the damaged frame will be sent to all connected devices).

figure.1 each switch port is collision domain.

   as the figure.1 shows that the switch breaks the network to 4 collision domains, so we can say the switches increase the number of collision domains which is a good thing.
on the other hand, all the ports of a Hub are considered to be 1 collision domain, so hubs do not separate collision domains.

2-2- broadcast domains :

   talking about broadcast domains, you should know that switches do not separate broadcast domains
you may ask WHO does, well Router do.
however if a switch has received a frame addressed with a broadcast destination, then it will forward it out of all interfaces, except the one where the frame was received on.

figure.2 Broadcast domains.








2-3 Sum-up :

to finish with, the following topology may be the best practice.

figure.3 collision & broadcast domains.

   so this is it Guys, hope i covered it up as clear as possible.
Next we will talk about switching Modes

please comment for any notes or questions, propositions...

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CISCO Switching Basics

1- CSMA/CD :

CSMA/CD stands for Carrier Sense Multiple Access Collision Detection, we can identifier this concept very easily with the following :
on a LAN (local area network) with a half Duplex mode collision may occur, so with CSMA/CD access methode, if there is any a collision on the link the switch will inform the other network devices to stop forwarding data on that wire until the link is free to receive incoming data.
the CSMA/CD uses an algorithm to manage data flow :

Flowchart 1 : the CSMA/CD algorithm.

 as the flowchart above illustrate where the collision is detected a jam signal is sent and it took place according to backoff strategy, then we resend data when the transmision support is free and so on until there will be no collision, data will be sent until the end of transmission.
so this is it Guys, hope i covered it up as clear as possible.
Next we will see the Broadcast & Collision Domains see you Next
please comment for any notes or questions...

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