From: Bob Sinclair (bsin@cox.net)
Date: Sun Dec 21 2003 - 21:38:14 GMT-3
Alec,
For an in-depth discussion of these issues I highly recommend the book DQOS
by William Odom (Cisco Press). He has years of experience teaching this
material, and he explains it well an in depth.
Regarding your point 1, I would say you are correct.
Regarding your point 2, the "shape average" command permits bc+be burst only
after a period of quiet has built up a surplus. The "shape peak" command
allows bc+be in every interval.
Regarding your point 3, the shaping command makes a decision to queue
out-of-profile packets, whereas the bandwidth command is a scheduling
process that determines which queue will next be serviced.
Mr. Odom puts it this way:
"the shaper must make the decision bout whether to put a packet into a
shaping queue, and then the shaper decides when the next packet can be taken
from a shaping queue, and then the queuing scheduler decides which packet
to service next from the shaping queues." page 367.
In your example, I would say that class cust2 traffic is rate-limited, but
that traffic is guaranteed a minimum of 384 Kbps in competition with other
traffic on that interface.
HTH,
-Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Alec" <clapun@graduate.hku.hk>
To: <ccielab@groupstudy.com>
Sent: Sunday, December 21, 2003 12:28 PM
Subject: CAR / Shaping question
> Hi group,
>
> 1) The Bc & Be calculation is different between CAR and shaping ??
> In CAR, setting Be=Bc effectively disables excess burst size whereas in
> Shaping, Be=Bc will NOT disable Be but will assign a burst size = Bc. Am
I
> correct ?
> 2) what's the difference between shape [average | peak] ? The command
> reference doesn't explain clearly.
> 3) What's the purpose of using both bandwidth and shaping in the
policy-map
> simutaneously ? As far as I know, both bandwidth and shaping are talking
> about egress queueing. Isn't that when we apply shaping to certain BW,
that
> BW is already reserved in the egress queue. The following example seems
to
> imply the shaping value always greater than the bandwidth value. Any
input
> is welcome!
>
> The following example is copied from the configuration guide :
>
> Router(config)# policy-map shape-cbwfq
> Router(config-pmap)# class cust1
> Router(config-pmap-c)# shape average 384000
> Router(config-pmap-c)# bandwidth 256
> Router(config-pmap)# class cust2
> Router(config-pmap-c)# shape peak 512000
> Router(config-pmap-c)# bandwidth 384
> Router(config-pmap-c)# configure terminal
> Router(config)# interface Serial 3/3
> Router(config-if)# service out shape-cbwfq
>
>
> thanks
> alec
>
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