From: Scott Morris (swm@emanon.com)
Date: Fri Feb 27 2004 - 12:59:28 GMT-3
You're correct in your understanding there... (flow-wise)
For the drop thresholds, again just think of succinct configurations. The
fact that your threshold maps (essentially what you are creating) say that
the same map can be applied more than one place which SEEMS to imply that
the same DSCP can exist in more than one place is true. But on the other
hand, it's keeping the configuration tight, and if we embrace the reality
that one DSCP will really only be in one queue at any point in time, it's
more like the threshold maps are applied to the interface describing the
total picture rather than you thinking about configuring any one particlar
queue.
>Switch(config)# interface gigabitethernet0/1 <-- egress
>Switch(config-if)# wrr-queue random-detect max-threshold 1 50 100
>Switch(config-if)# wrr-queue random-detect max-threshold 2 70 100
>Switch(config-if)# wrr-queue random-detect max-threshold 3 50 100
>Switch(config-if)# wrr-queue random-detect max-threshold 4 70 100
>Switch(config-if)# exit Switch(config)# interface gigabitethernet0/2
>Switch(config-if)# mls qos trust dscp
>Switch(config-if)# wrr-queue dscp-map 1 0 8 16 24 32 40 48 56
>Switch(config-if)# wrr-queue dscp-map 2 10 20 30 40 50 60
What is happening there is that each specific wrr queue (1-4) is being set
with a two different threshold levels (50 and 100 or 70 and 100). And
across the board to all queues on an interface, you are saying which DSCP
values are considered part of threshold 1 or threshold 2.
You use the cos-dscp maps and other wrr-queue commands to map specific
dscp/cod/prec values into specific queues. But the mapping thresholds is
agnostic of what goes where. It's keeping the config a little bit more
generic.
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, CISSP,
JNCIS, et al.
IPExpert CCIE Program Manager
IPExpert Sr. Technical Instructor
swm@emanon.com/smorris@ipexpert.net
http://www.ipexpert.net
-----Original Message-----
From: Ramasubramanian Sethuraman [mailto:snrmanian@hotmail.com]
Sent: Friday, February 27, 2004 10:29 AM
To: swm@emanon.com; ccielab@groupstudy.com
Subject: RE: cat35550 qos
Scott,
Thanks for the reply.
I also read dscp-to-cos and i was understanding like this
case a ) trust dscp (or) use dscp-dscp mutation at the i/p interface that
results in foll flow for a packet inside 3550
dscp --- dscp --- policed dscp --- cos --- o/p queue selected
case b ) trust precedence (or) use ip-precedence-dscp map at the i/p
interface
prec --- dscp --- policed dscp --- cos --- o/p queue selected
case c ) trust cos (or) use cos-dscp map at i/p interface
cos --- dscp -- policed dscp -- cos --- o/p queue selected
In all above cases, 64 possible dscp's at the end of policing (if its
configured)
will be mapped to one of 8 possible cos values. Even though multiple dscp's
can be mapped to same COS, one dscp(for eg dscp 8) can be mapped to more
than one COS.
In such a case, each dscp will be present in only queue.
Pls clarify if my above understanding is right.
So i was confused when i saw the statment
>As a result of this configuration, when the queues 1 and 3 are filled
>above 50 percent, packets with DSCPs 0, 8, 16, 24, 32, 40, 48, and 56
>are randomly dropped. The same packets are randomly dropped when queues
>2 and 4 are filled above 70 percent. When the second threshold (100
>percent) is exceeded, all queues randomly drop packets with DSCPs 10,
>20, 30, 40, 50, and 60.
The above implies that packets with same DSCP can be spread across the 4
queues and depending upon the threshold for each queue, these packets will
be random dropped.
For eg) packets with dscp 8 will be randomly dropped from queue 1 and 3 if
their queues are above 50 percent and similarly packets with dscp 8 wil be
randomly dropped from queues 2 and 4 if the queue is above 70 percent.
Pls clarify.
thanks,
Subbu
>From: "Scott Morris" <swm@emanon.com>
>To: "'Ramasubramanian Sethuraman'"
><snrmanian@hotmail.com>,<ccielab@groupstudy.com>
>Subject: RE: cat35550 qos
>Date: Fri, 27 Feb 2004 09:32:11 -0500
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>
>There are also dscp-cos maps that you can change along the way.
>Mutation maps are interesting creatures to play with.
>
>Default operation is to map IP Prec directly to COS (each have 8
>possible values). DSCP has 64 possible values, so there's a little overlap
there.
>
>64 --> 8 --> 4 queues There has to be SOME mapping going on!
>
>Any of these commands allow you to start tweaking the default maps as
>well as being able to implement a policy regarding their setup and
handling.
>
>The wrr-queue dscp-map option allows you to map particular DSCP values
>to one of two discard thresholds. So it's not so much that a DSCP
>value can exist in more than one place, but that you have less WRED
>maps than you do queues. Think of it like applying one policy to multiple
interfaces.
>You're setting some values that won't exist in certain places, but it
>makes the overall configuration smaller and more compact.
>
>Check out
>http://www.cisco.com/en/US/tech/tk389/tk813/technologies_tech_note09186
>a0080
>1558cb.shtml#topic4 it has some decent discussions about the mapping.
>
>HTH,
>
>
>Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
>CISSP, JNCIS, et al.
>IPExpert CCIE Program Manager
>IPExpert Sr. Technical Instructor
>swm@emanon.com/smorris@ipexpert.net
>http://www.ipexpert.net
>
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>Ramasubramanian Sethuraman
>Sent: Thursday, February 26, 2004 11:53 PM
>To: ccielab@groupstudy.com
>Cc: snrmanian@hotmail.com
>Subject: cat35550 qos
>
>I have a question regarding 3550 Qos. I read that based on the internal
>COS value of the packet, one of the 4 o/p queues is selected. But the
>COS value itself is based on the DSCP value of the packet after
>policiing, which implies there is a one-to-one correspondence between
>DSCP and the selected queue (each queue has unique set of DSCP's)
>
>The point i don't understand is there is also a option(ingress
>interface) to map dscp to either of the 2 thresholds for each o/p queue
>using the command "wrr-queue dscp-map 1|2 <dscp set>". This tells that
>same DSCP can exist on multiple queues which contradicts my earlier
>understanding that each queue has a unique set of DSCP's.
>
>Pls clarify if i'm missing something.
>
>This is the sample config from CCO
>
>Switch(config)# interface gigabitethernet0/1 <-- egress
>Switch(config-if)# wrr-queue random-detect max-threshold 1 50 100
>Switch(config-if)# wrr-queue random-detect max-threshold 2 70 100
>Switch(config-if)# wrr-queue random-detect max-threshold 3 50 100
>Switch(config-if)# wrr-queue random-detect max-threshold 4 70 100
>Switch(config-if)# exit Switch(config)# interface gigabitethernet0/2
><-- ingress Switch(config-if)# mls qos trust dscp Switch(config-if)#
>wrr-queue dscp-map 1 0 8 16 24 32 40 48 56 Switch(config-if)# wrr-queue
>dscp-map 2 10 20 30 40 50 60
>
>explanation
>---------------
>As a result of this configuration, when the queues 1 and 3 are filled
>above 50 percent, packets with DSCPs 0, 8, 16, 24, 32, 40, 48, and 56
>are randomly dropped. The same packets are randomly dropped when queues
>2 and 4 are filled above 70 percent. When the second threshold (100
>percent) is exceeded, all queues randomly drop packets with DSCPs 10,
>20, 30, 40, 50, and 60.
>
>
>
>
>thanks,
>subbu
>
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