From: Bob Sinclair (bsin@cox.net)
Date: Sat Sep 25 2004 - 12:31:57 GMT-3
Drew,
Vegesna's book is a bit dated (2001), and could use some editing. I would
recommend the book Cisco DQOS by Wendell Odom. While not perfect, it is
quite up-to-date and is well-edited. Please see comments in line below.
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Drew Whitaker" <drew.whitaker@gmail.com>
To: <ccielab@groupstudy.com>
Sent: Saturday, September 25, 2004 9:22 AM
Subject: Vegesna's IP QoS Book (rate-limits)
> I've been reading through Vegesna's IP QoS book and had some questions
> regarding its content that I thought I'd pass on to the group.
>
> 1) On page 40, listing 3-4, he lists the rate-limit syntax as:
> rate-limit access-group 1 input 45000000 ...
> Shouldn't this be: rate-limit input access-group 1 ... ?
You are correct: The CAR syntax is rate-limit input/output access-group 1
>
> 2) On page 43, he states that CIR is measured in bits per second, but
> on page 46 it says its in bytes. Which is it?
The reference on page 46 is incorrect. I think he is trying to destinguish
between how Bc an Be are measured and got mixed up. In Shaping Bc and Be
are in bits-per-second. In Policing Bc and Be are in Bytes-per-second.
>
> 3) Page 48 example is designed to set all traffic except HTTP traffic
> with an IPP of 4. Syntax given is:
> access-list 101 permit tcp any any eq www
> access-list 101 permit tcp any eq www any
> int hssi1/0/0
> rate-limit input 30000000 15000 15000 conform-action continue
> exceed-action drop
> rate-limit input access-group 101 150000 10000 10000 conform-action
> set-prec-transmit 4 exceed-action set-prec-transmit 0
> rate-limit input 30000000 15000 15000 conform-action set-prec-transmit
> 4 exceed-action set-prec-transmit 4
>
> Because of the permit in the ACL, I would read this to be all HTTP
> traffic gets an IPP of 4, not all traffic except HTTP. What am I
> missing?
I interpret his requirement as: mark all traffic IPP 4, including http
below 15 Mbps. The only traffic marked to IPP 0 is http over 15 Mbps. I
think he is right there.
>
> 4) Syntax on page 51 gives:
> rate-limit input access-group 2 rate-limit 5000000 ...
> Yet, other examples give syntax of:
> rate-limit input access-group rate-limit 2 ...
> Which is it, or does it matter?
>
see above. my routers do not take the command as he writes it here.
> 5) Page 58, listing 3-19 DTS example.
> policy-map MYPOLICY
> class MYCLASS
> shape average 256000 16384 0
> Why is Be set to 0? Wouldn't setting it to 0 be performing traffic
> policing, and not traffic-shaping?
>
Be of zero says you cannot save tokens beyond Bc. Does not send bursts
above CIR, but will still attempt to buffer them if it can.
> 6) Page 60, listing 3-22.
> policy-map FROMPOLICY
> class FROMHOSTA
> shape peak 128000 8192 1280
> The Be is 1280. Yet the FAQ at the end of the chapter says Be should
> be 2 X Bc, which would be 16384. What am I not understanding here?
>
A bit confusing. the rate used in the shape peak command is the CIR. It
automatically creates a Bc = Be and sends Bc + Be per interval. The syntax
recommends you not configure Bc or Be, but let the algorithm figure it out.
> 7) Speaking of the FAQ on page 63, he writes that Bc = CIR x (1 byte /
> 8 bits) x 1.5 seconds
> This is a little different than the way I have learned this. I
> learned Bc = CIR x TI, with TI being set to .125. Given his example,
> 1 byte / 8 bits is .125, which is similar to the equation I learned
> but then he adds multiplying by 1.5 seconds. However, when I do the
> math, I do not come up with the same values as in his examples. For
> instance, on page 62 he gives a DTS example where:
> CIR = 256000
> Bc = 16384
> Be = 1280
> However, 256000 x .125 x 1.5 = 48,000, not 16384. I'm confused.
As above, shaping bursts are in bits, policing bursts are in bytes. The
1.5 seconds worth of traffic figure is a rule of thumb.
BTW, CAR is generally an older, less powerful method of policing than
class-based. Here is a link that compares them:
http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a00800d7276.shtml
>
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