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200 Chapter 4: Bridges/Switches and LAN Design
Foundation Summary
The Foundation Summary is a collection of tables and figures that provide a convenient review
of many key concepts in this chapter. For those of you already comfortable with the topics in
this chapter, this summary could help you recall a few details. For those of you who just read
this chapter, this review should help solidify some key facts. For any of you doing your final
prep before the exam, these tables and figures will be a convenient way to review the day before
the exam.
Figure 4-32 summarizes the various LAN header formats.
Table 4-20 summarizes the fields used for identifying the types of data contained in a frame.
Table 4-20
Protocol Type Fields in LAN Headers
Field Name
Length
LAN Type
Comments
Ethernet Type
2 bytes
Ethernet
RFC 1700 (Assigned Numbers RFC)
lists the values. Xerox owns the
assignment process.
802.2 DSAP and
SSAP
1 byte each
IEEE Ethernet,
IEEE Token Ring,
ANSI FDDI
The IEEE Registration Authority
controls the assignment of valid values.
The source SAP (SSAP) and
destination SAP (DSAP) do not have to
be equal, so 802.2 calls for the sender's
protocol type (SSAP) and the
destination's type (DSAP).
SNAP Protocol
2 bytes
IEEE Ethernet,
IEEE Token Ring,
ANSI FDDI
Uses EtherType values. Used only when
DSAP is hex AA. It is needed because
the DSAP and SSAP fields are only 1
byte in length.
ch04.fm Page 200 Monday, March 20, 2000 5:02 PM