Command:

set ip precedence


Mode:

Router(config-route-map)#


Syntax:

set ip precedence [number | name]
no set ip precedence

 

Syntax Description:

number | name (Optional) A number or name that sets the precedence bits in the IP header. The values for the number argument and the corresponding name argument are listed in the table below, from least to most important.

 

Command Description:

To set the precedence value (and an optional IP number or IP name) in the IP header, use the set ip precedence route-map configuration command. To leave the precedence value unchanged, use the no form of this command.

Number and Name Values for IP Precedence

Nameroutinepriorityimmediateflashflash-overridecriticalinternetnetwork
Number
0
1
2
3
4
5
6
7

You can set the precedence using either a number or the corresponding name. Once the IP Precedence bits are set, other QoS services such as weighted fair queueing (WFQ) and Weighted Random Early Detection (WRED) then operate on the bit settings.

The way the network gives priority (or some type of expedited handling) to the marked traffic is through the application of WFQ or weighted random early detection (WRED) at points downstream in the network. Typically, you would set IP precedence at the edge of the network (or administrative domain) and have queueing act on it thereafter. WFQ can speed up handling for high precedence traffic at congestion points. WRED ensures that high precedence traffic has lower loss rates than other traffic during times of congestion.

The mapping from keywords such as routine and priority to a precedence value is useful only in some instances. That is, the use of the precedence bit is evolving. The customer can define the meaning of a precedence value by enabling other features that use the value. In the case of Cisco high-end Internet quality of service (QoS), IP precedences can be used to establish classes of service that do not necessarily correspond numerically to better or worse handling in the network. For example, IP Precedence 2 can be given 90 percent of the bandwidth on output links in the network, and IP Precedence 6 can be given 5 percent using the distributed weight fair queueing (DWFQ) implementation on the Versatile Interface Processors (VIPs).

Use the route-map (IP) global configuration command with the match and set route-map configuration commands to define the conditions for redistributing routes from one routing protocol into another, or for policy routing. Each route-map command has an associated list of match and set commands. The match commands specify the match criteria--the conditions under which redistribution or policy routing is allowed for the current route-map command. The set commands specify the set actions--the particular redistribution or policy routing actions to perform if the criteria enforced by the match commands are met. The no route-map command deletes the route map.

The set route-map configuration commands specify the redistribution set actions to be performed when all of the match criteria of a route map are met.


Example:

The following example sets the IP Precedence value to 5 (critical) for packets that pass the route map match:

Router(config)# interface serial 0
Router(config-if)# ip policy route-map texas

Router(config)# route-map texas
Router(config-route-map)# match length 68 128
Router(config-route-map)# set ip precedence 5


Misconceptions:

Setting the precedence bit affects weighted fair queueing (WFQ). It acts as a multiplier on the WFQ weighting, using a formula of 4096 divided by the IP Precedence value plus 1. For more information, see the fair-queue command.


Related Commands:

fair-queue
ip policy route-map
route-map

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