Foundation Summary 203
1.
For entire bus
2.
From device to hub/switch
3.
Numbers shown are for half/full duplex
The internal processing methods of LAN switches are listed in Table 4-23.
Table 4-24 lists several features relating to segmenting LANs with bridges, switches, and
routers.
1000BaseLx
802.3z
3000 m
Single-mode or
multimode fiber
1
1000BaseCx
802.3z
25 m
Shielded copper
2
1000BaseT
802.3ab
100 m
Category 5 UTP
2
Table 4-23
Switch Internal Processing
Switching Method
Description
Store-and-forward
The switch fully receives all bits in the frame (store) before forwarding the
frame (forward). This allows the switch to check the FCS before forwarding
the frame. (FCS is in the Ethernet trailer.)
Cut-through
The switch performs the address table lookup as soon as the destination
address field in the header is received. The first bits in the frame can be sent
out the outbound port before the final bits in the incoming frame are received.
This does not allow the switch to discard frames that fail the FCS check.
(FCS is in the Ethernet trailer.)
FragmentFree
This performs like cut-through, but the switch waits for 64 bytes to be
received before forwarding the first bytes of the outgoing frame. According
to Ethernet specifications, collisions should be detected during the first 64
bytes of the frame; frames in error due to collision are not forwarded. The
FCS still cannot be checked.
Table 4-24
Comparison of Segmentation Options
Feature
Bridging
Switching
Routing
Forwards LAN broadcasts?
Yes
Yes
No
Forwards LAN multicasts?
Yes
Yes; can be
optimized
with CGMP
No
1
Table 4-22
Ethernet Standards (Continued)
Standard
MAC Sublayer
Specification
Maximum Cable
Length
Cable Type
Pairs
Required
continues
ch04.fm Page 203 Monday, March 20, 2000 5:02 PM