From: ccie2be (ccie2be@nyc.rr.com)
Date: Fri Jun 24 2005 - 16:36:44 GMT-3
John,
Read this, then read this 2 more times:
WRED
-WRED is an alternative to tail drop that occurs on output queues once the
queue is filled to capacity and is a form of "Congestion Avoidance".
-There are 2 parts to how WRED works: 1st detecting when congestion occurs
and 2nd deciding which packets to drop and how many packets to drop.
-Applies to all packets but only tcp packets respond to packet drops.
-To decide when there is congestion, WRED compares the average queue size to
the min & max threshold. If the ave q size is below min threshold, no pkts
are dropped. If ave q size is above max threshold, all pkts are dropped. If
ave q size is between the min and max thresholds, then the number of packets
dropped is based on mark-probability-denominator (MPD) and ip prec or dscp.
-MPD is equal to how many packets in the "one out of how many packets" as
the ave q size approaches the max threshold, e.g. if MPD = 10, then 1 out of
10 packets or 10% will be dropped as the max threshold is reached. By
default, mdp = 10 for all ip prec & dscp values. Therefore, Max Percentage
of Packets Dropped = 1/ MPD.
-WRED can be based on ip prec or dscp or discard-class. While ip prec and
dscp are values carried within a packet, discard-class is a value (from 0 to
7) set on a packet as it comes into the router based on whatever criteria is
specified in a class-map. The discard-class is then used to define WRED
treatment as the packet leaves the router. A discard-class is like a
qos-group in that it's local to a router. For example, you can assign a
discard-class to packets used by gaming programs and then create a very
aggressive WRED drop policy to packets in that discard-class.
-The difference in treatment based on ip prec comes from the fact that, by
default, the min threshold is lower for lower values of ip prec i.e. packets
of lower ip prec will be dropped sooner because of the lower min threshold
than packets of higher ip prec.
-Both the min and max thresholds and the MPD can be changed on a per ip prec
or per dscp or per discard-class basis. Use random-detect <prec | dscp |
discard-class> <value> <min-threshold max-threshold MPD> where value = ip
prec value or dscp value or discard-class value.
-The exponential-weighting-constant is, by default eq to 9 and is used in
the computation of the average q size. The larger this number, the slower
the ave q size changes as compared to the current q size. To change the
default, use random-detect expon #.
-WRED can be configured on interfaces or within a policy map for any class
including the class default, with random-detect.
-Gotcha: Enabling WRED on an interface disables WFQ on that interface.
-According to IE lab 6, if WRED is configured with MQC, then the class in
question must either have its bandwidth configured (reserved) or must have
fair-queue enabled. Therefore, if the requirement specifies that certain
traffic must be subject to WRED but not reserve any bandwidth, then WFQ must
be enabled for that class.
-After enabling WRED, use show queueing int x to see what random detect
settings are being used.
-FRED was designed to overcome the problem of udp flows starving tcp flows
since tcp flows respond to dropped packets although udp flows do not.
-FRED classifies udp flows into 2 categories: non-adaptive and fragile based
on how much of the queue these flows are using. FRED drops non-adaptive
flows aggressively
-Enable FRED with random-detect flow.
-A new feature of WRED, ECN - Explicit Congestion Notification - tries to
make the dropping of packets unnecessary by explicitly informing the sender
to slow down by marking packets. This is like BECN's used by Frame Relay.
To use ECN, 1st configure random-detect and then configure random-detect
ecn.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of John
Matus
Sent: Friday, June 24, 2005 1:39 PM
To: ccielab@groupstudy.com
Subject: mark probability denomitaor: percent??
does the mark prob denomintor use percent?
Rack1R1(config-pmap-c)#random preced 5 50 70 ?
<1-65535> mark probability denominator
<cr>
TIA.
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