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CCENT™ (Cisco Certified Entry Network Technician) Certification Exam Notes

H. Switching

1. Switches are data link layer devices that enable multiple physical LAN segments to be interconnected into a single larger network. Two widely used switching methods are store-and-forward switching and cut-through switching.

2. Switches forward packets based on the physical address (such as MAC address) whereas, routers forward packets based on logical address (such as IP address). A frame’s MAC address doesn’t change when being forwarded through a switch.

3. Important types of switching used in Cisco devices:
· Store-and-Forward switching: Here the LAN switch copies the entire frame into its buffers and computes the CRC. The frame is discarded if there are any CRC errors. Giant (more than 1518 bytes0 and Runt (less than 64 bytes) frames are also dropped, if found.
· Cut-Through (Real-Time) switching: Here, the LAN switch copies only the destination address into its buffers. It immediately looks up the switching table and starts forwarding the frame. The latency is very less because, the frame is forwarded as soon as the destination address is resolved.
· Fragment-Free switching: Here, the switch waits for the collision window before forwarding the entire frame. The collision window is 64 bytes long.

4. The Catalyst 1900 and 2820 series switches support three types of switching methods:
· FastForward (Cut-through): In this type of switching, the packet is forwarded as soon as the destination address is read. This has least latency.
· FragmentFree (Modified cut-through): This type of switching is useful when your network is experiencing large number of collisions. FragmentFree switching has a latency in between FastForward and the Store-and Forward.
· Store-and-Forward: This method stores the entire frame and checks for errors before forwarding it on to another port. Store-and-forward has the highest latency compared with both FastForward and FragmentFree.
The default switching method used by Catalyst 1900 series switches is FastForward.

5. In store-and-forward switching, an entire frame must be received before it is forwarded. This means that the latency through the switch is relative to the frame size—the larger the frame size, the longer the delay through the switch. Cut-through switching allows the switch to begin forwarding the frame when enough of the frame is received to make a forwarding decision. This reduces the latency through the switch. Store-and-forward switching gives the switch the opportunity to evaluate the frame for errors before forwarding it. This capability to not forward frames containing errors is one of the advantages of switches over hubs. Cut-through switching does not offer this advantage, so the switch might forward frames containing errors.

6. The store and forward switch works by first receives the whole packet before retransmission. Cut-through switch mode retransmits a frame as soon as it reads the destination address. Fragment-free switch mode reads the first 64 bytes before retransmission. Therefore, the latency of cut-through and fragment-free switch modes is constant, whereas that of store-and-forward varies depending on the length of the packet.

7. Cisco 2950 Switch System LED:
The system LED shows whether the system is receiving power and functioning properly.
LED Off: System is not powered up.
LED color is Green: System is operating normally.
LED color is Amber: System is receiving power but is not functioning properly. The amber color on a Cisco2950 series switch indicates that the system is malfunctioning.

8. The purpose of port security is to prevent access to the LAN from un-authorized hosts.

9. If no matching MAC entry is found in the MAC table of a switch, the frame will be broadcast to all other ports except the port through which it had arrived.

10. The MAC address table of a switch would be empty to begin with. However, it builds the MAC table learning from the frames that arrive at its ports by adding the MAC address and the corresponding port that it had arrived to the MAC table. 
11. Spanning Tree Protocol (STP) 802.1d is used to prevent routing loops. In Cisco Catalyst 5000 series switches, use BDPUs (Bridge Protocol Data Units) to determine the spanning tree topology. STP uses a Tree Algorithm (STA) to prevent loops, resulting in a stable network topology.

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