Bridging and Frame Switching are
practically one and the same technology. Frame Switching is Bridging that has been speeded up.
Bridging has always been software-based and normally a bridge would just have two ports
used to connect the two LANs being bridged. Switching is hardware-based and has many ports
but all the rules that apply to bridging also apply to switching and more besides.
The MAC address is always left unchanged in bridging (barring the bit ordering change
in Translational bridging!).
LAN Frame switches can include FDDI, Token Ring or Ethernet switches. Effectively, the switch
provides single Collision Domains per switch port and each port acts as a bridge port to the rest of
the network. Forwarding tables are kept per port, different media, different speeds etc. can be
configured on a port by port basis. The speed enhancement to the network is achieved through the
'microsegmentation' of the large Collision Domain into many smaller ones.
Each port on an Ethernet switch is effectively a very fast bridge port. The switch itself has its own
MAC address e.g. 0800.a300.df00, and then each of its ports is given a MAC address, commonly in port
order; port 1 has address 0800.a300.df01, port 2 has address 0800.a300.df02 etc. If this particular
switch is the root bridge, then the MAC address 0800.a300.df00 is advertised as the root bridge, however
the BPDUs originate from whatever MAC address is assigned to the port from which the BPDU emanates.
Some switches allow you to implement a Backpressure scheme whereby, on a particular
port, jamming frames can be sent to reduce traffic coming into the switch. This stops one
port hogging the backplane on a switch thereby effecting other users. Obviously, you would not
wish to implement this on a server port, since this will affect many people and you would
wish to keep as much of the switch processing capability for the attached servers. This is
why so much play is made of the backplane capability of a particular manufacturer's switch.
A Cut-through switch first reads the Destination address of a frame and then sends the frame
straight to the destination before the rest of the frame has arrived at the switch. The
first 20 to 30 bytes of the frame need to be read to make sure that the frame is not a collision fragment.
If the destination address remains unknown, then the switch temporarily stores the frame.
Cut-through switching is fine for fixed speed networks such as all 10BaseT, and it is very fast,
however if the switch has mixed speed ports such as 10/100 autosensing ports, then there is a
bottle neck when packets are moving across the switch fabric from a 100BaseT segment to a 10BaseT
segment. Some switches, although they forward the frame as soon as they read the destination address
they still read the frame up to the CRC and if there are a certain level of errors, they can
be configured to automatically change to a Store and Forward mode.
3. Store and Forward
A Store and Forward switch, or 'buffered switch', stores each frame frame in a buffer before forwarding
it on to the appropriate port. This gets around the underflow or overflow situation that could
happen in a mixed speed environment.
4. Fragment-free Switching
This is similar to Cut Through Switching but here
the frame is checked a little further than the destination address to the
Length field in order to weed out collision fragments, before it is forwarded.
Latency of a network increases as the network gets busier. On a busy network, the backoffs (retransmits)
that could occur with Cut-through switches increase, thereby increasing latency. A Store and Forward
switch on 10Mbps LAN delays a frame by one frame time, obviously increasing latency, but there are no backoffs.