Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
OL-21227-02
Appendix B Cisco MWR 2941 Router Command Reference
random-detect
Usage Guidelines Keywords
If you choose not to use either the dscp-based or the precedence-based keywords, WRED uses the IP
Precedence value (the default method) to calculate the drop probability for the packet.
Availability
The random-detect command is not available at the interface level for Cisco IOS Releases 12.1E or
12.0S. The random-detect command is available in policy-map class configuration mode only for Cisco
IOS Releases 12.1E, 12.0S, and later.
WRED Functionality
WRED is a congestion avoidance mechanism that slows traffic by randomly dropping packets when
congestion exists. DWRED is similar to WRED but uses the Versatile Interface Processor (VIP) instead
of the Route Switch Processor (RSP). WRED and DWRED are most useful with protocols like Transport
Control Protocol (TCP) that respond to dropped packets by decreasing the transmission rate.
The router automatically determines parameters to use in the WRED calculations. To change these
parameters, use the random-detect precedence command.
WRED in a Policy Map
You can configure WRED as part of the policy map for a standard class or the default class. The WRED
random-detect command and the weighted fair queueing (WFQ) queue-limit command are mutually
exclusive. If you configure WRED, its packet drop capability is used to manage the queue when packets
exceeding the configured maximum count are enqueued. If you configure the WFQ queue-limit
command, tail drop is used.
To configure a policy map and create class policies, use the policy-map and class (policy-map)
commands. When creating a class within a policy map, you can use the random-detect command with
either of the following commands:
• bandwidth (policy-map class)
• fair-queue (class-default)—for the default class only
Note If you use WRED packet drop instead of tail drop for one or more classes in a policy map, you
must ensure that WRED is not configured on the interface to which you attach that policy map.
Note DWRED is not supported for classes in a policy map.
Two Methods for Calculating the Drop Probability of a Packet
This command includes two optional keywords, dscp-based and precedence-based, that determine the
method WRED uses to calculate the drop probability of a packet.
Note the following points when deciding which method to instruct WRED to use:
• With the dscp-based keyword, WRED uses the DSCP value (that is, the first six bits of the IP type
of service (ToS) byte) to calculate the drop probability.
• With the precedence-based keyword, WRED uses the IP Precedence value to calculate the drop
probability.
• The dscp-based and precedence-based keywords are mutually exclusive.
• If neither argument is specified, WRED uses the IP Precedence value to calculate the drop
probability (the default method).