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Command Reference Guide
According to RFC 791, the ToS field contains the following bits:
0
Precedence
The three-bit IP precedence values (0 through 7) are specified as:
111
110
101
100
011
010
001
000
The IP precedence values provide network routers with information about the kind of traffic contained in
the IP packet. Based on the IP precedence values, some networks (when supported) can offer special
handling to certain packets. In addition, providing IP precedence values to critical traffic (such as route
information) ensures that critical packets will always be delivered regardless of network congestion. This
traffic is often critical to network and internetwork operation. In general, the higher the IP precedence
value, the more important the traffic and the better handling it should receive in the network. It is important
to remember that not all equipment in the public IP network will be configured to recognize and handle IP
precedence values. While it is a good idea to set the values for critical traffic, it does not guarantee special
handling.
In addition to the IP precedence values, RFC 791 specifies bits for delay, throughput, and reliability to help
balance the needs of particular traffic types when traveling on the IP network infrastructure. When these
bits are set to 0, they are handled with normal operation. When set to 1, each bit specifies premium
handling for that parameter. For example, a 1 in the delay position indicates that the traffic is delay
sensitive and care should be taken to minimize delay. A 1 in the throughput position indicates that the
traffic has higher bandwidth requirements that should be met. A 1 in the reliability position indicates that
the traffic is sensitive to delivery issues and care should be taken to ensure proper delivery with all packets
of this type. These extra bits are rarely used because it is quite difficult to balance the cost and benefits of
each parameter (especially when more than one bit is set to 1).
Usage Examples
The following example instructs the route map named MyMap to match the IP precedence value of
critical:
(config)#route-map MyMap permit 100
(config-route-map)#match ip precedence critical
60000CRG0-35E
1
2
3
4
Delay
Throughput
Network Control Packets
Internetwork Control Packets
Critical Traffic
Flash Override
Flash
Immediate Servicing
Priority Traffic
Routine Data
Copyright © 2012 ADTRAN, Inc.
5
6
7
Unused
Reliability
Route Map Command Set
3312
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