Configuring the Switch
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Mapping IP TOS Priority
The Type of Service (TOS) octet in the IPv4 header is divided into three parts;
Precedence (3 bits), TOS (4 bits), and MBZ (1 bit). The Precedence bits indicate the
importance of a packet, whereas the TOS bits indicate how the network should
make tradeoffs between throughput, delay, reliability, and cost (as defined in
RFC 1394). The MBZ bit (for “must be zero”) is currently unused and is either set to
zero or just ignored.
The four TOS bits provide 15 different priority values, however only five values have
a defined meaning. The following table lists the defined IP TOS values and the
default mapping to CoS queues on the switch. (All the TOS values not defined are
mapped to CoS queue 0.)
Command Attributes
• IP TOS Priority Status – Enables or disables the IP TOS priority.
• IP TOS Priority Table
–
Shows the IP TOS to CoS map.
• Class of Queue Service Value
–
Maps an IP TOS value to a CoS queue. Note
that queue “0” represents low priority and “3” represent high priority.
Note:
IP TOS settings apply to all interfaces.
Web – Click Priority, IP TOS Priority Status. Set the IP TOS Priority Status to
Enabled.
Figure 3-122 Globally Enabling the IP TOS Priority Status
Table 3-16 Mapping IP TOS Values to CoS Priority Queues
IP TOS Value Requested Service Default CoS Output Queue
0 Normal service 0
1 Minimize monetary cost 0
2 Maximize reliability 1
4 Maximize throughput 2
8 Minimize delay 3
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