From: Peter van Oene (pvo@usermail.com)
Date: Thu May 06 2004 - 13:25:05 GMT-3
At 11:59 PM 5/4/2004, Howard C. Berkowitz wrote:
>At 9:53 PM -0400 5/4/04, Kenneth Wygand wrote:
>>Howard,
>>
>>Thank you for your complete and very comprehensive analysis. My
>>question, however, was theoretical in application and is not intended for
>>a production environment. Technically the bits in the TOS field can be
>>set to anything provided they support the overall implementation of the
>>network policy in effect.
>
>Correct. I did want to emphasize the point that network/internetwork
>control protocols MUST have absolute priority, and, in production
>environments, it's often wise to have either out-of-band or prioritized
>telnet for remote consoles.
>
>>The real basis of my question was if a question says "set a specific
>>priority or DSCP for a specific type or subset of traffic and use "best
>>effort" for everything else", do I retag all other traffic with all
>>zero's for the TOS byte or do I trust them the values of the TOS byte for
>>this traffic. What does "best effort" actually imply, all zero's or unchanged?
>
>It's a little more subtle than that. Under the differentiated service
>model, there are two main kinds of traffic: guaranteed service (GS) and
>best effort (BE). Controlled load is a special case and not worth
>considering in a general discussion.
>
>BE is a somewhat misleading term when one looks at the Cisco RSVP
>implementation of GS. Since you can reserve only 75 percent of an
>interface bandwidth for GS, leaving 25 percent for BE, the
>highest-priority BE really has a guarantee that it will get 25 percent of
>the total bandwidth.
>
>If this were a question in the CCIE lab, you really have already
>identified the key issue, which, I believe, is a legitimate question to
>ask a proctor: "I need to determine what amount of policy enforcement you
>want. Do I force differentiated service priority, or should I trust the
>data originator? Further, should I follow best practice and not
>reprioritize routing protocols and related network control mechanisms?"
In the lab, this question will be answered by the point value of the
question. If it requires 5 or more steps, its likely not a two
pointer. Also, if its worth 10 points and you added three lines of config,
you likely missed something ;-)
>[If the proctor answered "no" to the last question, I might indeed walk
>away muttering that he needed a thorough thrashing with a large clue
>stick. Nevertheles...]
>
>Given that you'll use a router as data source in the lab, AFAIK, it will
>not set non-routine priority on anything other than VoIP.
>
>Is that answer closer to your question? :-)
>
>>
>>-----Original Message-----
>>From: nobody@groupstudy.com on behalf of Howard C. Berkowitz
>>Sent: Tue 5/4/2004 5:48 PM
>>To: ccielab@groupstudy.com
>>Cc:
>>Subject: Re: Best Effort Definition
>>
>>
>>
>> At 4:31 PM -0400 5/4/04, Kenneth Wygand wrote:
>> >Hello Group,
>> >
>> >
>> >
>> >I think I know the answer to this one but I just want to get
>> some more
>> >opinions...
>> >
>> >
>> >
>> >If I am performing QoS (whether it be CoS, IP Precedence, ToS
>> Bits, or
>> >DSCP), supposed I would like to "mark all traffic going from
>> router A to
>> >router B as IP Precedence 6 while other traffic receiving "best
>> effort"
>> >service"... obviously through some kind of classification,
>> marking and
>> >queuing I would make sure all traffic from router A to router B
>> receives
>> >the type of service it requires.
>>
>> Why are you using priority 6? Priority 5 is intended as the highest
>> to be used for application traffic.
>>
>> >However, what about all other traffic?
>>
>> Priorities 6 and 7 are reserved for time-criticall routing
>> protocols,
>> network management, etc. Never interfere with the priorities for
>> these services, or you may create a situation where the routers lock
>> up and you cannot get control. Along the same lines, you might want
>> to create fine-grained rules to have telnet from a control
>> console at
>> priority 5.
>>
>> >When requesting that other traffic gets "best effort", should
>> one leave
>> >the QoS markings as-is, or actually remark them back to all 0's?
>>
>> Leave them as-is. I can't say with certainty that any other
>> applications will set priority, but they rarely do without a good
>> reason. If, for example, you use TFTP to reload NVRAM during
>> production hours, I might give it priority 4.
>>
>> To put it in perspective, the original military purposes of the
>> precedences were having the highest for network and internetwork
>> control. The next was used, among other things, for Emergency
>> Command
>> Precedence, which is an order to launch a nuclear weapon.
>> Considering
>> that the sender of such a message may become part of a mushroom
>> cloud
>> at any time, that message HAS to take priority -- but even then, the
>> network/internetwork control had even higher precedence, because if
>> they weren't working, the network might not be there to carry the
>> Emergency Action Message at ECP precedence.
>
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