Queuing & Buffers

0 views
Skip to first unread message

Alex Revzin (Telstra BPD)

unread,
Jan 30, 2001, 10:00:59 PM1/30/01
to

In Addition the queueing strategy is FIFO.

On Tue, 30 Jan 2001, Alex Revzin (Telstra BPD) wrote:

>
> Imagine the following scenario:
>
> #-------#-------#
> R1 R2 R3
>
> 3 routers in a row, the link between R1 and R2 is slower than the link
> between R2 and R3. The link between R1 and R2 is heavily utilised.
>
> Assuming the traffic is UDP - what sort of buffering would be occuring
> on R2?
>
> Traceroute from R1 to R2 and back to R1 seems not to be buffered.
Traffic
> from R3 to R2 to R1 seems to be buffered at R2 and there is a great
delay
> in getting to R1. Are there different buffers dependant on which
interface
> the traffic enters the router?
>
> Traceroute to R2 from R1 yielded 180ms.
>
> Traceroute to R3 from R1 yielded 650ms.
>
> Please advise.
>
> Thanks in advance.
> ____
> Alex


Hansang Bae

unread,
Jan 30, 2001, 11:25:08 PM1/30/01
to

>On Tue, 30 Jan 2001, Alex Revzin (Telstra BPD) wrote:
>>
>> Imagine the following scenario:
>> #-------#-------#
>> R1 R2 R3
>>
>> 3 routers in a row, the link between R1 and R2 is slower than the link
>> between R2 and R3. The link between R1 and R2 is heavily utilised.
>> Assuming the traffic is UDP - what sort of buffering would be occuring
>> on R2?

>In Addition the queueing strategy is FIFO.

If you are using FIFO, then you configured it manually or your link is
higher than 2Mbps. WFQ is the defualt queing for serial interfaces
whose bandwidth is less than 2.048Mbps.

In FIFO, you have one pipe where data gets queued. If that pipe (think
of it as a water straw) gets full, you get what's known as a tail
drop. That is, as the straw gets full, it overflows and you lose the
packets. If one person hogs the channel, so be it. FIFO has no
mechanism to stop bandwidth hogs.

Priority and Custom queuing works by defining different levels of buffer.
But this can have draw backs in that high priority buffers need to empty
out before lower priority gets a chance to transmit. Also, you need to
speciy what you consider has high priority or low priority.

WFQ works by identifying different "conversations" An ftp session would
be one conversation and a telnet session would be another. Each gets a
buffer "straw" and each will drain in equal manner. To be more
specific, this is Fair Queuing. I.E. each buffer gets equal treatment
when you drain the straw. In Weighted Fair Queuing, you can assign a
weight so that one will drain faster than the other. E.G. two packet
from ftp will go out, then one packet of telnet will go out. The IP
Precendence is used to determine the weight, where
Weight = 4096/(Precendence+1)

Remember that buffering comes into play when the outgoing interfaces
reaches a threshold. Until then, it's a free for all (no congestion, no
need to buffer).

If your congested link shows long latency, it may be that your FIFO pipe
is taking a bit to drain. Remember, FIFO is first come first serve!


hsb


"Somehow I imagined this experience would be more rewarding" Calvin
********************************************************************
Due to the volume of email that I receive, I may not not be able to
reply to emails sent to my account. Please post a followup instead.
********************************************************************

Ian M

unread,
Jan 31, 2001, 5:42:06 AM1/31/01
to
Alex,

It might be useful to see how busy the line is. Can you post a 'show
interface' for either end of the slow link?

Ian


"Hansang Bae" <hb...@primenet.com.REMOVE> wrote in message
news:958434$she$1...@nnrp1.phx.gblx.net...

Jan T Lindstr|m

unread,
Jan 31, 2001, 6:28:27 AM1/31/01
to
Hansang Bae <hb...@primenet.com.REMOVE> writes:

> If you are using FIFO, then you configured it manually or your link is
> higher than 2Mbps. WFQ is the defualt queing for serial interfaces
> whose bandwidth is less than 2.048Mbps.

Says the documentation but I've seen routers running 2600 -series routers
running IOS 11.2, 12.0 and 12.1 that don't comply. They pick FIFO, so
you'd better check the config.


> from ftp will go out, then one packet of telnet will go out. The IP
> Precendence is used to determine the weight, where
> Weight = 4096/(Precendence+1)

Source for this formula? http://www.cisco.com/univercd/cc/td/doc/product/
software/ios121/121cgcr/qos_c/qcprt2/qcdconmg.htm#xtocid182447 seems to
claim otherwise. Is the maximum number of queues 4096?

Jan Lindström

--
-End certainly exists, but to what end?-

Hansang Bae

unread,
Feb 1, 2001, 8:50:39 PM2/1/01
to

>Hansang Bae <hb...@primenet.com.REMOVE> writes:
>> If you are using FIFO, then you configured it manually or your link is
>> higher than 2Mbps. WFQ is the defualt queing for serial interfaces
>> whose bandwidth is less than 2.048Mbps.


Jan T Lindstr|m <jan...@alpha.hut.fi> writes:
>Says the documentation but I've seen routers running 2600 -series routers
>running IOS 11.2, 12.0 and 12.1 that don't comply. They pick FIFO, so
>you'd better check the config.


That's good to know. I usually never trust the default settings. As
someone once said, "Defaults are the guardian angels for the clueless."


>> Precendence is used to determine the weight, where
>> Weight = 4096/(Precendence+1)

>Source for this formula? http://www.cisco.com/univercd/cc/td/doc/product/
>software/ios121/121cgcr/qos_c/qcprt2/qcdconmg.htm#xtocid182447 seems to
>claim otherwise. Is the maximum number of queues 4096?

Pg 141 of Cisco's "Enhanced IP Services for Cisco Networks"
I'll have to look at the above URL to comment.

Reply all
Reply to author
Forward
0 new messages