From: Paresh Khatri (Paresh.Khatri@aapt.com.au)
Date: Sun Dec 04 2005 - 22:17:37 GMT-3
Thanks Bob,
That does indeed confirm my understanding.
Regards,
Paresh.
-----Original Message-----
From: Bob Sinclair [mailto:bob@bobsinclair.net]
Sent: Monday, 05 December 2005 10:31 AM
To: Paresh Khatri; Cisco certification
Subject: Re: Egress Policing
Paresh,
Here is a link to a technote on the subject of QoS order of operations:
http://www.cisco.com/warp/public/105/qos_orderofop_3.html
Policing happens before queueing, but policing happens after outbound
classificaton, ACLs and marking.
HTH,
Bob Sinclair
CCIE #10427, CCSI 30427
www.netmasterclass.net
----- Original Message -----
From: Paresh Khatri <mailto:Paresh.Khatri@aapt.com.au>
To: Cisco certification <mailto:ccielab@groupstudy.com>
Sent: Sunday, December 04, 2005 7:06 PM
Subject: MQC: Egress Policing
Hi all,
I'm just trying to re-inforce my understanding of how the different elements
of QoS work with MQC and would appreciate any comments on the following:
Outbound Policing
=================
Why use egress policing instead of just ingress policing ?
To limit traffic of a particular class at an egress interface. If only
ingress policers were used, there would be no way to limit traffic of that
class at the ingress interface without setting inflexible hard limits on
ingress. For example, say you want to limit DSCP AF41 traffic egressing out
of an interface to 10Mbps. And say that you have two ingress interfaces, A
and B, from which you are receiving AF41 traffic. If you use an egress
policer on the egress interface to limit all such traffic to 10Mbps, you will
never exceed that amount of traffic in the outbound direction. In addition,
the sum total of all AF41 traffic received over A and B will be limited to
10Mbps. That means that you could support 10Mbps of this traffic from A if B
is not offering any such traffic. However, if you use ingress policers on A
and B and set hard limits of 5Mbps for each, the traffic ingressing each of
these interfaces can never exceed 5Mbps. Therefore, the egress policing
mechanism !
works better in this case.
When is egress policing applied ?
It would seem to make sense that egress policing is applied as packets are
switched to the egress interface i.e. before these packets are placed in the
queue for the class on that interface. It would not make sense to enqueue the
packets before policing them since you would lose the inter-packet arrival
time information for these packets once they are queued (and hence, would not
be able to police them).
Comments appreciated.
Paresh.
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