From: Chris Lewis (chrlewiscsco@yahoo.com)
Date: Sun Nov 27 2005 - 14:20:13 GMT-3
Venkat,
There are several concepts represented in your mail that need to be separated in order for a full understanding to be possible. I also think that some of this is not relevant to the R&S exam, but anyway, here is what I can tell you right now.
First, in terms of deciding which packet to drop, FIFO is mutually exclusive from RED. One either decides to tail drop packets in the case of FIFO, or randomly drop packets before the tail drop case is reached via RED.
If your question is why do I need to configure bandwidth in order to configure RED the answer is you dont J
RED can be applied to an interface or a class. Looking at the interface application first
interface Serial0/0
no ip address
random-detect dscp-based
end
Then looking at the interface gives the following:
sho int s0/0
Queueing strategy: random early detection(RED)
So here we have no bandwidth or fair queueing defined in the configuration and RED can be applied. So the next question is, what happens when I try to apply RED to a class
class-map match-all map1
match protocol icmp
Router(config)#policy-map test1
Router(config-pmap)#class map1
Router(config-pmap-c)#random-detect
bandwidth on the class is required to issue this command
So now we have different behavior. The reason for the difference is that an interface has a given throughput capability, and when that is reached, a queue will build up and RED can act upon that queue. By contrast, a class can have any throughput capability configured on it, so there is no natural throughput level (other than the interface capability) that will build a queue to enable RED to act. So think of a class as something virtual within the router that you need to define the capabilities of in order for RED to do its thing.
Now the situation changes slightly for the default class.
Router(config-pmap)#class class-default
Router(config-pmap-c)#rand
Router(config-pmap-c)#random-detect
fair-queue or bandwidth on the class is required to issue this command
The difference here is that the default class uses the remaining unallocated bandwidth (unallocated to other classes), so it does not strictly need the bandwidth statement, as it knows the amount of bandwidth left and can work with that.
Now RED is in fact a dropping mechanism, and it needs a queue to operate on. As classes are virtual within the router (as opposed to a physical interface) some queueing strategy needs to be defined for the class to define how to build these queues, and for the class-default you are given the additional option of defining fair-queueing.
There are two other RED options to be familiar with. Flow based (for interfaces only) and the ECN option. Flow based was created for non-TCP flows that do not back off in the presence of dropped packets. The ECN option is where you can set ECN in the packet instead of dropping it, so that you do not have to rely upon the dropping of a packet to signal congestion (and by implication hosts will know to back off without experiencing packet loss).
Chris
"Venkataramanaiah.R" <vramanaiah@gmail.com> wrote: Read the last but one para as,
"Why can't random detect work on a FIFO queue. What i mean here
is why i cant configure RED on a FIFO queue (w/o bandwidth statement,
assuming bw
command changes the queue to WFQ)."
TIA
-Venkat
On 11/27/05, Venkataramanaiah.R wrote:
> Hi Chris/all,
>
> Is there any theory behind, why CBWFQ class does not accept
> random detect command w/o bandwidth command?
>
> Might be a silly question, but just want to get the basics clear..
>
> Does the queuing strategy get changed for the class when
> bandwidth command is configured.. May be WFQ..?
>
> Why can't random detect work on a FIFO queue. What i mean here
> is why i cant configure RED w/o bandwidth statement (Assuming bw
> command changes the queue to WFQ).
>
> Warning : I may have more questions based on your response.
> Hope this warning does not prohibit you from answering the above
> questions :-)
>
> Regards
> -Venkat
>
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