LAN Switch Types

LAN Switch Types
One last thing we need to cover before we can move on—the actual forwarding techniques used by
switches. LAN switching forwards (or filters) frames based on their hardware destination—the
MAC address. There are three methods by which frames can be forwarded or filtered. Each method
has its advantages and disadvantages, and by understanding the different LAN switch methods
available, you can configure your switches to make smarter switching decisions.
Here are the three switching modes:
Store-and-forward With the store-and-forward mode, the complete data frame is received on
the switch’s buffer, a cyclic redundancy check (CRC) is performed on the frame, and then the
destination address is looked up in the MAC filter table.
Cut-through With the cut-through mode, the switch waits for only the destination hardware
address to be received and then looks up the destination address in the MAC filter table.
FragmentFree FragmentFree is the default mode for the Catalyst 1900 switch; it is sometimes
referred to as modified cut-through. The switch checks the first 64 bytes of a frame for fragmentation
(because of possible collisions) before forwarding the frame.
Figure 15.7 shows the different points where the switching mode takes place in the frame.
The different switching modes are discussed in detail next.
100BaseT
10BaseT
Root port (F)
Designated port (F)
Designated port (F)
Root port (F)
Nondesignated port (BLK)
Port 0
Port 0
Port 1
Port 0
Port 1
Non-root bridge
Root bridge
Non-root bridge
2950C
MAC 0c00c8222222
Default priority 32768
2950B
MAC 0c00c8111111
Default priority 32768
2950A
MAC 0c00c8110000
SYST RPS Default priority 32768
STRT UTIL DUPLXSPEED
MODE
1 2 3 4 5 6 7 8 9 10 11 12
10Base-T/100Base-TX Catalyst 2950 SERIES
SYST RPS
STRT UTIL DUPLXSPEED
MODE
1 2 3 4 5 6 7 8 9 10 11 12
10Base-T/100Base-TX Catalyst 2950 SERIES
SYST RPS
STRT UTIL DUPLXSPEED
MODE
1 2 3 4 5 6 7 8 9 10 11 12
10Base-T/100Base-TX Catalyst 2950 SERIES
LAN