Layer 2 switching technology
Three functions of the switch
address learning
Frame forwarding/filtering
loop prevention
How the switch learns the location of the host
Host A sends a data frame to host C
The switch records the MAC address of host A corresponding to port E0 by learning the source MAC address of the data frame.
The data frame is forwarded to all ports except port E0 (it is not clear that the target host's unicast is flooded)
How switches filter frames
Switch A sends data frame to host C
There is a target host in the address table, and the data frame is forwarded directly without flooding
Broadcast Frames and Multicast Frames
Host D sends a broadcast frame or a multipoint frame
Flood broadcast or multipoint frames to all ports except the source port
frame exchange
Cut-through forwarding
The switch forwards the frame after detecting the destination address
As soon as the switch determines the destination MAC address of the frame and the correct port number, it forwards the frame immediately. Normally, the forwarding starts about 14 bytes from the frame header. This allows cut-through to have a smaller and relatively constant latency than store-and-forward, but it also forwards frames smaller than 64 bytes along with some bad frames, potentially wasting bandwidth.
Store and forward
The frame is received in its entirety and checked for errors before forwarding
Store and forward
Before the switch forwards the frame to the destination port, it must receive the complete frame and perform CRC check to determine the destination address. The switch stores the entire frame in a memory buffer and does not send it to its destination until it has available resources. The benefit is being able to drop frames smaller than 64 bytes and any other corrupted frames, which saves bandwidth. The disadvantage is that the delay is large and not fixed, because it receives and processes the complete frame before forwarding.
Fragment free (modified version of cut-through)—the default mode for Cat1900
(modified cut-through)
The switch forwards the frame as soon as it detects the first 64 bytes
Redundant network topology
Redundant topology eliminates network failures due to single points of failure
broadcast storm
The solution to the loop: Spanning-Tree Protocol
Blocking certain ports prevents loops in redundant network topologies