CCNA Cisco Certified Network Associate Study Guide - FTP Server

FIGURE 2.6 Different switching modes within a frame
Store and Forward
6 bytes 1 byte 6 bytes 6 bytes 2 bytes
Preamble SFD
LAN Switch Types 85
Store-and-forward switching is one of three primary types of LAN switching.
With the store-and-forward switching method, the LAN switch copies
the entire frame onto its onboard buffers and computes the cyclic redundancy
check (CRC). Because it copies the entire frame, latency through the
switch varies with frame length.
The frame is discarded if it contains a CRC error, if it’s too short (less than
64 bytes including the CRC), or if it’s too long (more than 1518 bytes including
the CRC). If the frame doesn’t contain any errors, the LAN switch looks
up the destination hardware address in its forwarding or switching table and
determines the outgoing interface. It then forwards the frame toward its destination.
This is the mode used by the Catalyst 5000 series switches and cannot
be modified on the switch.
Cut-Through (Real Time)
Destination
hardware
addresses
Source
hardware
addresses
Default switching
cut-through:
no error checking
Up to
1500 bytes 4 bytes
Length DATA FCS
FragmentFree:
checks for
collisions
Store-and-forward:
all errors filtered;
has highest latency
Cut-through switching is the other main type of LAN switching. With this
method, the LAN switch copies only the destination address (the first six
bytes following the preamble) onto its onboard buffers. It then looks up the
hardware destination address in the MAC switching table, determines the
outgoing interface, and forwards the frame toward its destination. A cutthrough
switch provides reduced latency because it begins to forward the
frame as soon as it reads the destination address and determines the outgoing
interface.