- Conjestion Avoidance tool
- Drops packets randomly before que is full
- prevents tcp synchronisation (dropping window size retrainsmitting and all host get in synch retrainsmitting)
- Drops packets on marked probability
- Typically levels are based on dscp or ip prec
- Non ip traffic is treated as precendence 0 non ip traffic will be dropped more often than ip traffic
- You can not configure wred on subinterfaces
- Wred is not supported with ATM
- CEF must be enabled to use wred
- min threshold is the point we will start dropping we do this based on value less than marked probabilty
- the closer we get to the max threshold we will increasingly drop getting closer to the mark probability we will be at the marked probabilty at the max threshold
- default is 1 out of 10 packets
- IF we go beyond max threshold we are into using tail drop
- This gives us better control on what is dropped precedence would have lower min and max threshold then precendence 3 so if we run into conjestion precedence 1 traffic will be radomly dropped first
- WRED tracks average que depth smoothened based on weight factor (usually ip prec or dscp)
Commands
random-detect dscp-based- specify random detect based on dscp values
random-detect dscp-based cs1 3 10 10- specfying max and min threshold for dscp value so min is 3 max is 10 and max probabilty is 1 out of 10 packets.
sh policy-map int-view stats in relation to random detect
ECN
- THis is extension wred which allow the router mark packets signalling to the reciever that there is conjecstion rather than dropping on recieving these marked packets when they get to reciever marked the reciever will slow down the transmission rate.
- The end host like pc needs to be ecn capable
random-detect ecn
LAB
We will have a little scenrio here with WRED that HTTP traffic is to be given top priority
DNS have wred profile of 3 10 10 so at 3 packets it will start randomly dropping as it move
closer to 10
ok so first things first we need to classify the traffic as it comes in and mark
then outgoing we will run wred
So i created a class map to match the 2 protocols
r5(config)#class-map MATCHHTTP
r5(config-cmap)#match protocol http
r5(config-cmap)#exit
r5(config)#class-map MATCHDNS
r5(config-cmap)#match protocol dns
r5(config-cmap)#exit
Then a policy-map to set dscp value so http gets expedited which is the highest priority
and dns is getting cs1 which is similar to precendence 1 so not much priority
r5(config)#policy-map MATCH
r5(config-pmap)#class MATCHHTTP
r5(config-pmap-c)#set ip dscp ef
r5(config-pmap-c)#exit
r5(config-pmap)#class MATCHDNS
r5(config-pmap-c)#set ip dscp ?
<0-63> Differentiated services codepoint value
af11 Match packets with AF11 dscp (001010)
af12 Match packets with AF12 dscp (001100)
af13 Match packets with AF13 dscp (001110)
af21 Match packets with AF21 dscp (010010)
af22 Match packets with AF22 dscp (010100)
af23 Match packets with AF23 dscp (010110)
af31 Match packets with AF31 dscp (011010)
af32 Match packets with AF32 dscp (011100)
af33 Match packets with AF33 dscp (011110)
af41 Match packets with AF41 dscp (100010)
af42 Match packets with AF42 dscp (100100)
af43 Match packets with AF43 dscp (100110)
cs1 Match packets with CS1(precedence 1) dscp (001000)
cs2 Match packets with CS2(precedence 2) dscp (010000)
cs3 Match packets with CS3(precedence 3) dscp (011000)
cs4 Match packets with CS4(precedence 4) dscp (100000)
cs5 Match packets with CS5(precedence 5) dscp (101000)
cs6 Match packets with CS6(precedence 6) dscp (110000)
cs7 Match packets with CS7(precedence 7) dscp (111000)
default Match packets with default dscp (000000)
ef Match packets with EF dscp (101110)
<cr>
r5(config-pmap-c)#set ip dscp cs1
r5(config-pmap-c)#
This will be applied inbound on the incoming interface from the traffic source R4
r5(config)#int s0/1/0
r5(config-if)#service-policy input MATCH
r5(config-if)#
Now we configure the queing methods and wred first we need to match the dscp values assigned
r5(config)#class-map HTTP
r5(config-cmap)#match ip dscp ef
r5(config)#class-map DNS
r5(config-cmap)#match ip dscp CS1
r5(config-pmap)#class HTTP
r5(config-pmap-c)#priority percent 98
r5(config-pmap-c)#random-detect dscp-based
Must deconfigure priority in this class before issuing this command
r5(config-pmap-c)#
it will not allow us configure wred on the priority que
NOw we create a class for DNS
r5(config-pmap)#class DNS
r5(config-pmap-c)#bandwidth percent 1
r5(config-pmap-c)#random-detect dscp-based
r5(config-pmap-c)#random-detect dscp cs1 3 10 10
r5(config)#int fa0/0
r5(config-if)#service-policy output RANDOM
r5#
*Feb 16 23:48:19.439: %SYS-5-CONFIG_I: Configured from console by console
r5#sh policy-map int
FastEthernet0/0
Service-policy output: RANDOM
Class-map: HTTP (match-all)
30 packets, 1728 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: ip dscp ef (46)
Queueing
Strict Priority
Output Queue: Conversation 264
Bandwidth 98 (%)
Bandwidth 98000 (kbps) Burst 2450000 (Bytes)
(pkts matched/bytes matched) 0/0
(total drops/bytes drops) 0/0
Class-map: DNS (match-all)
40 packets, 2920 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: ip dscp cs1 (8)
Queueing
Output Queue: Conversation 265
Bandwidth remaining 1 (%)
(pkts matched/bytes matched) 0/0
(depth/total drops/no-buffer drops) 0/0/0
exponential weight: 9
mean queue depth: 0
dscp Transmitted Random drop Tail drop Minimum Maximum Mark
pkts/bytes pkts/bytes pkts/bytes thresh thresh prob
af11 0/0 0/0 0/0 32 40 1/10
af12 0/0 0/0 0/0 28 40 1/10
af13 0/0 0/0 0/0 24 40 1/10
af21 0/0 0/0 0/0 32 40 1/10
af22 0/0 0/0 0/0 28 40 1/10
af23 0/0 0/0 0/0 24 40 1/10
af31 0/0 0/0 0/0 32 40 1/10
af32 0/0 0/0 0/0 28 40 1/10
af33 0/0 0/0 0/0 24 40 1/10
af41 0/0 0/0 0/0 32 40 1/10
af42 0/0 0/0 0/0 28 40 1/10
af43 0/0 0/0 0/0 24 40 1/10
cs1 5/365 0/0 0/0 3 10 1/10
cs2 0/0 0/0 0/0 24 40 1/10
cs3 0/0 0/0 0/0 26 40 1/10
cs4 0/0 0/0 0/0 28 40 1/10
cs5 0/0 0/0 0/0 30 40 1/10
cs6 0/0 0/0 0/0 32 40 1/10
cs7 0/0 0/0 0/0 34 40 1/10
ef 0/0 0/0 0/0 36 40 1/10
rsvp 0/0 0/0 0/0 36 40 1/10
default 0/0 0/0 0/0 20 40 1/10
we can see all the stats so we are cs1 there is no conjestion so there is no dropping at the
moment
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