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Configuring the Switch
Class of Service (CoS): Managing Bandwidth More Effectively
Configuring the Switch
Using Type of Service (ToS) Criteria to Prioritize IP
Traffic
Every IP packet includes a Type of Service (ToS) field. This field carries
priority settings that are read and used, but not altered by the switch. When
CoS is configured to use ToS criteria, the switch reads the content of the
packet’s ToS field and takes actions based on any CoS configuration that
applies to the packet.
In order to use ToS to configure priority, you need to anticipate the ToS field
settings in IP packets entering the switch from upstream devices. This involves
having knowledge of how an upstream device or application will set the bits
in the ToS field of IP packets sent to the switch.
The switch can use the ToS field in either of two ways:
■ Use the Differentiated Services bits to select the packets to prioritize (ToS
Differentiated Services option)
■ Use the Precedence bits to prioritize a packet (ToS IP precedence option).
The following shows an example of the ToS field in the header for an IP packet,
and illustrates the diffserve bits and precedence bits in the ToS field. (Note
that the Differentiated Services bits and the Precedence bits are two different
interpretations of the same field.)
Figure 6-91. The ToS Field (or Byte) in an IP Header
Field: Destination
MAC Address
Source MAC
Address
IP Identifier Type &
Version
ToS Byte . . .
Packet: FF FF FF FF FF FF 08 00 09 00 00 16 08 00 45 E0 . . .
Differentiated Services Bits
Not
Used
Precedence
Bits
1 1100000
E 0