From: Lee Carter (l2carter@yahoo.com)
Date: Sat Jun 25 2005 - 01:25:13 GMT-3
Ok,
I have a comment for task 3
"3) provide at most this amount of band during times
of congestion = priority
Priority means put this stuff at the top of the queue,
this only takes effect when there is congestion, as of
course, without a queue, you have no packets to
re-order. As part of the priority command, there is a
natural policer action, such that if there is
congestion, no more than the specified rate will be
treated with priority (ie jumped to the front of the
queue) and any excess is dropped."
Doesn't priority take affect at ALL times.. Even if
there is NO congestion. (doesn't it set aside a TX
queue and a priority queue)
My understanding was that priority is a low latency
queue that always moves the packet to the front of the
line before any other packet is allowed in the TX
queue. However, if a pakcet is already there then the
priority queue must wait until the first packet is
de-queued and then it is placed directly into the tx
queue before any other traffic. (hence the importance
of fragmentation on links less than 768k)
This behavior is in affect ALL the time not just when
thre is congestion. This is how you 'guarantee' a low
latency network queue for a particual type of traffic.
Now for the bandwidth parameter.. it doesn't take
affect until the interface is at 75% congestion. Which
can be tricky for production or the LAB because your
documentation may show that you have a 64k link but if
you don't but the bandwidth statement on the interface
the default maximum speed is used and 75% of a T1 is
totally different than 75% of a 64k link on a T1 port.
I suspect you would loose lab points on the lab if you
forgot your bandwidth statements.
HTH
Lee
> Some slightly different perspectives in-line
>
> -----Original Message-----
> From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com] On Behalf Of
> John Matus
> Sent: Friday, June 24, 2005 9:22 PM
> To: lab
> Subject: QOS question/clarification: bandwidth,
> priority, shape, police
>
> ok, i'm pretty sure that i've got this stuff covered
> but i'm sometimes
> suprised at what kinds of answers labs have to
> specific qos quetions.
> here are my pseudo-axioms.
> 1) police coming in, shape going out
> In general, yes, but outbound policiing is also
> possible and could be a
> requirement if the question is worded such tha
> anything over a given
> rate is dropped or something similar.
> 2) provide at least this amount of band during times
> of congestion =
> bandwidth
> 3) provide at most this amount of band during times
> of congestion =
> priority
> Priority means put this stuff at the top of the
> queue, this only takes
> effect when there is congestion, as of course,
> without a queue, you have
> no packets to re-order. As part of the priority
> command, there is a
> natural policer action, such that if there is
> congestion, no more than
> the specified rate will be treated with priority (ie
> jumped to the front
> of the queue) and any excess is dropped.
> 4) provide an average of X band out an interface =
> shape
> 5) only allow x amount of band = policing.....
> <ok, so really only #1 is an axiom>
>
> my question is about "shaping". i cant remember
> back to any labs i did
> as to how things were worded with regards to
> shaping. the only thing i
> can think of
> is "provide an average of x out an interface".
> can anyone think of
> any
> other catch phrases that might indicate the shaping
> is to be used?
>
> Wording can be tricky, but if they are looking for
> shaping, the could
> refer to things like delay, queue, store, anything
> like that.
>
> TIA
>
>
> Regards,
>
> John D. Matus
> MCSE, CCNP
> Office: 818-782-2061
> Cell: 818-430-8372
> jmatus@pacbell.net
>
>
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