Ethernet Switching
Ethernet Switching

QoS Confusions?

10.26.10   |  
‎10-26-2010 06:22 AM

Hi Everyone


I would like to ask few questions on QoS configuration in JUNOS:


1- When we classify the traffic to forwarding class through Multifield Classifier (Firewall Filters) then in order to tag the traffic (Marking) we have to define the rewrite rules (Mapping of code points to forwarding class). My question is that the code points are 6 digit binary number like 101 11 0, where 101 represents the Priority, 11 represents the drop probability (loss priority) and zero is unchanged. But when we classify the traffic to forwarding class then why we give explicitly loss-priority although loss-priority is part of code point.


2- When we define the schedular-map for queues then where exactly we have to apply the schedular-map on the ingress interface or egress interface.


3- In Schedular, could any one explain the buffer-size in simple words.


4- When we assign the multiple forwarding classes for example two forwarding class to same queue and the transmit-rate is 10mb exact then how this transmit-rate is distributed to both forwarding class? 50-50 or what?


Looking forward to positive response.



Ethernet Switching

Re: QoS Confusions?

10.26.10   |  
‎10-26-2010 06:02 PM

1. It is possible that certain applications may share the dscp code point. Say there are two applications http and ftp and the data packets of both are coming with default dscp code point which is 000000 getting classified to same queue and under congestion, we would like to give certain drop priority to one among the both.


2. Scheduler map will be applied at the egress interface.


3.Every port is given few port buffers. The number we specify under scheduler is the % of total buffers of port buffers  that we would like to allocate for this queue or this is the % of total buffers that is guaranteed for that queue.


4.It will be not be 50-50. The minimum guaranteed will be 10mb for both together under congestion.

Ethernet Switching

Re: QoS Confusions?

11.07.10   |  
‎11-07-2010 06:18 AM

1 - Don't confuse the actual diffserve code point breakdown with the functions inside of the JUNOS based router/switch.  You can create a COS configuration that mimics the DSCP values for Priority|Loss Priority|Unused - which I recommend, but the configuration and codepoint are not dependent on each other.


Normally, I would configure AF11 into the Assured-Forwarding queue with a loss-priority of low in JUNOS - which mimics the actually DSCP itself.  You could also put AF21 traffic into the same Assured-Forwarding queue but with a loss priority of high - which is contrary to the DSCP values it represents.


The automatic mapping of DSCP to JUNOS queues are not automatic or rigid - even though they have the same look and feel.


2 - Unlike our main competitor, our QOS encompasses the whole box.  There is a classifier and scheduler on every interface on every Juniper router or switch.  You can see the defaults by issuing the command - show class-of-service interface xxxxxx.  If you want to change the behavior of COS on the box you will need to (a) customize schedulers and classifiers for every interface (b) manipulate the defaults to add your functions.


3 - Buffer-size is the amount of "memory" you have assigned to that particular queue for packet buffering.


4 - Check out this whitepaper to see if it helps answer these questions in greater detail.