From: Godswill Oletu (oletu@inbox.lv)
Date: Mon Nov 07 2005 - 22:08:01 GMT-3
Leigh,
There are various way, to arrive at the various values to be used, this is
the way I understand it based on your scenario:
CIR=4meg=4000000 bytes.
Normal rate=8th of CIR=500000
Bc=normal burst*1.5 Sec =750000
Be=Bc*2=1500000
(1.5 Sec is the average round trip time, you can use a value more suitable
to your situation)
interface fastethernet0/0
rate-limit input 4000000 500000 1500000 conform-action transmit
exceed-action drop
Meaning:
1. All traffic from 0 to 4000000 bytes will be allowed normal passage.
2. Traffic above 4000000 bytes will result in token been removed from the
token bucket and the bucket will become empty after 500000 bytes has been
removed.
3. When a packet is transmitted, the token will be released, but it will not
be returned to the bucket until after Tc, which is by default 125ms.
4. When the token bucket is empty, tokens will be borrowed upto 1500000
bytes and placed in the bucket.
5. If the token bucket become empty again, after borrowing the excess tokens
in step 4 above, the 'exceed-action' will kick-in and tail drop all excess
traffics.
You can see that, steps 1 - 4 will trigger the confirm action and the packet
will be transmitted but under different conditions and step 5 will trigger
the exceed action and the packet will be dropped.
When in doubts, you only need to configure the CIR.
#show interface fa0/0 rate-limit
will use the default recommended Cisco values to calculate the other values
and apply then for you and you can note the values down and then manually
add them to your configs..
Remember that, you can also use 'traffic-shape rate' to achieve the same
objective, only that 'rate-limit' is more granular and give one more control
and you will be correct if you say more confusion:
#interface fastethernet0/0
#traffic-shape rate <CIR> <Bc> <Be>
Bc=CIR*Tc
Be=Bc*2
If the interface will not support burst..
Be=Bc
(Where recommended Tc value = 125ms).
Note:
Your ISP is only committed to transmitting 4000000 bytes.
HTH
Godswill Oletu
----- Original Message -----
From: "Leigh Harrison" <ccileigh@gmail.com>
To: "FORUM" <ccielab@groupstudy.com>
Sent: Friday, November 04, 2005 11:00 AM
Subject: Rate-Limit - I just don't get it !!
> All,
>
> I'm having a hard time getting my head around some of the bits of QoS.
> No matter how much I read about it - it doesn't seem to sink in.
>
> I had a requirement to set the bandwidth on an incoming interface to 4
meg.
>
> No problem I thought and did it thusly:-
>
> policy-map traffic_in
> class class-default
> police 4000000
>
> int f0/0
> service policy input traffic_in
>
> But then I thought to myself - hang on, what if in the lab I couldn't
> use a policy map, what else could I do? Ah-ha, I thought,
> Rate-Limiting. Then it hit me - I've no idea why there are so many
> variables for rate limiting!! To be honest - I think I'm going mad. I
> got quite angry with my telnet session when it wanted me to specify
> normal burst size and extended burst size.
>
> Could someone put this into simple(ton) terms for me?
>
> Many thanks in advance,
> LH
>
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