From: Victor Cappuccio (vcappuccio@gmail.com)
Date: Mon Mar 24 2008 - 15:05:14 ART
Vinoth,
Based on this link,
http://www.cisco.com/en/US/docs/ios/12_2/qos/command/reference/qrfcmd1.html#w
p1098249
WFQ is automatically enabled on all interfaces that have a default bandwidth
of less than 2 Mbps. The fair-queue command is used to enable WFQ on
interfaces where it is not enabled by default or was previously disabled.,
the configuration of WFQ is based on the following command fair-queue
[congestive-discard-threshold [dynamic-queues [reservable-queues]]]
The number of dynamic queues WFQ uses by default is based on the interface
bandwidth. Using the default settings for an interface WFQ uses 256 Dynamic
queues. the number of queues can be from a configured range in between 16
and 4096
R1(config-if)#do show run int s0/0
Building configuration
Current configuration : 107 bytes
!
interface Serial0/0
bandwidth 32
ip address 10.2.6.1 255.255.255.0
encapsulation ppp
fair-queue
end
R1(config-if)#do show int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 10.2.6.1/24
MTU 1500 bytes, BW 32 Kbit, DLY 20000 usec,
reliability 253/255, txload 214/255, rxload 1/255
Encapsulation PPP, LCP Open
Open: CDPCP, IPCP, loopback not set
Last input 00:00:01, output 00:00:02, output hang never
Last clearing of "show interface" counters 1d09h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 28974
Queueing strategy: weighted fair > Here is the Q Algo
Output queue: 0/1000/64/28974 (size/max total/threshold/drops)
Conversations 0/1/16 (active/max active/max total) - Number of Queues for
conversation
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 24 kilobits/sec
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 59000 bits/sec, 38 packets/sec
6315174 packets input, 1263781475 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
3 input errors, 0 CRC, 3 frame, 0 overrun, 0 ignored, 0 abort
7319381 packets output, 1497005310 bytes, 0 underruns
0 output errors, 0 collisions, 3 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions
DCD=up DSR=up DTR=up RTS=up CTS=up
R1(config-if)#band 32
R1(config-if)#band 127
R1(config-if)#do show run int s0/0
Building configuration
Current configuration : 116 bytes
!
interface Serial0/0
bandwidth 127
ip address 10.2.6.1 255.255.255.0
encapsulation ppp
fair-queue 64 16 0
end
R1(config-if)#int s0/0
R1(config-if)#no fair-queue 64 16 0
R1(config-if)#fair-queue
R1(config-if)#do show int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 10.2.6.1/24
MTU 1500 bytes, BW 127 Kbit, DLY 20000 usec,
reliability 255/255, txload 98/255, rxload 1/255
Encapsulation PPP, LCP Open
Open: CDPCP, IPCP, loopback not set
Last input 00:00:00, output 00:00:01, output hang never
Last clearing of "show interface" counters 1d09h
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 28974
Queueing strategy: weighted fair
Output queue: 0/1000/64/28974 (size/max total/threshold/drops)
Conversations 0/1/32 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
Available Bandwidth 95 kilobits/sec
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 49000 bits/sec, 33 packets/sec
6315178 packets input, 1263781795 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
4 input errors, 0 CRC, 4 frame, 0 overrun, 0 ignored, 0 abort
7319385 packets output, 1497005698 bytes, 0 underruns
0 output errors, 0 collisions, 5 interface resets
0 output buffer failures, 0 output buffers swapped out
0 carrier transitions
DCD=up DSR=up DTR=up RTS=up CTS=up
R1(config-if)#
in WFQ, packets are classified by flow. For example, packets with the same
source IP address, destination IP address, source TCP or User Datagram
Protocol (UDP) port, destination TCP or UDP port, and protocol belong to the
same flow, if there are a large number of concurrent flows, it is likely
that 2 flows end up in the same queue.
Probability = 1 - [(Number of Queues!)/((Queues^flows) * (Queues - flows)!)]
The following is the sample calulation of the probability value for 5 flows
and 64 queues
Probability = 1 - ((64!) / ((64^5) * (59!))) == 0.147894 or almost 15% of
that a flow can end in the same queue as another flow.
Now, WFQ has two modes of dropping: Early dropping when the congestion
discard threshold is reached and Aggressive dropping when the hold-queue out
limit is reached (HQO), you must remember that WFQ always drops packets of
the most aggressive flow, also take in consideration that WFQ has 2 Drop
mechanism exceptions
A.- Packet classified into an empty sub-queue is never dropped
B.- The packet precedence has no effect on the dropping scheme
The other parameter that you can configure in WFQ are
CDT, is Number of messages allowed in the WFQ system before the router
starts dropping new packets for the longest queue. The value can be in the
range from 1 to 4096 (default is 64)
dynamic-queues: Number of dynamic queues used for best-effort conversations
(values are: 16, 32, 64, 128, 256, 512, 1024, 2048, and 4096)
reservable-queues: Number of reservable queues used for reserved
conversations in the range 0 to 1000 (used for interfaces configured for
features such as RSVP - the default is 0),
an additional WFQ Configuration parameter is the HQO which Specifies the
maximum number of packets that can be in all output queues on the interface
at any time, 1000 by default and the configuration is hold-queue max-limit
out on the onterface configuration mode
Cheers
Victor Cappuccio
www.vcappuccio.wordpress.com
On Mon, Mar 24, 2008 at 6:46 PM, Vinoth Kannan Ganapathy <
ccieprep4vinoth@gmail.com> wrote:
> Hi All,
>
> This is regarding WFQ, in which I understood that
>
> hold-queue is the absolute limit on the no of packets.
>
> CDT is like max-queue length for each flow.
>
> DOes this mean hold-queue = CDT * no. of dynamic queues...
>
> And also what is the reserved queue ?
>
> Thx and regds
> Vinoth G
>
> _______________________________________________________________________
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