Understanding the network.pdf - Back to Home

also required to provide any switching between different Layer 2
network protocols (such as FDDI or Ethernet) on the same switch
fabric. This enables the switch to effectively behave like a translating
bridge.
Hybrid switches are, as the name implies, switches that use a
combination of cut-through or store-and-forward methods. Hybrid
switches are also known as modified cut-through or adaptive switches.
Hybrid switches, by default, start in a modified cut-through mode.
During the SAT lookup phase, the first 64 bytes of the frame are
examined by the switch before forwarding the frame. This allows the
switch to identify any runts or collision fragments and discard them.
In the event of numerous collisions (Ethernet) or high frame error rate
(Token Ring and FDDI), the switch will change its frame processing
method from cut-through to store-and-forward, which will perform its
own CRC checks of the frame in the buffer before sending it on. When
the frame error rates are reduced, the switch will go back to
cut-through mode and begin the monitoring process again. The frame
processing latency of hybrid switches depends on the operating mode
of the switch. In modified cut-through mode, the latency is at a fixed
rate. In store-and-forward mode, the latency is variable, again
depending on the size of the packet. The advantage of the hybrid
switch is that it provides the data integrity checking of
store-and-forward switching (if needed) and the reduced latency
(more efficient packet processing) advantage of cut-through
switching.
Source routing is also available on Token Ring switches as a
forwarding method. In terms of operation and configuration, the
same rule that applies to source route bridges applies when source
routing is used with switches. The switch must be configured with
proper LAN and port IDs and must be able to understand the RIF field
contents.
Layer 2 LAN Switching Services for Layer 3
Along with significant performance improvements, Layer 2 switching
has also provided additional functions to enhance the deployment and
performance of Layer 3 protocols. Three of these functions are Virtual
LANs (VLANs), Layer 2 traffic prioritization, and Layer 3 switching.
These functions, like port trunking and full-duplex port operation, are
made possible by adding additional logic features to the switching
logic's ASIC chip.