My last blog covered class-map’s, which were used to identify traffic to our device, the next part of QOS covers what the device does with the traffic after its been identified. This is called a policy-map.

So you create your policy with the following command

R1#conf t
R1#(config)policy-map QOS_Egress

Next you need to reference each class-map then tell the device what to do with that traffic. Here’s an example

class voice_traffic
priority percent 7
set ip dscp ef
class voip_signal
bandwidth percent 3
set ip dscp af31
class SQL_App
bandwidth 20000
set ip dscp 26
class telnet_traffic
bandwidth percent 3
set ip dscp af21
class citrix_traffic
bandwidth percent 10
set ip dscp af41
class my_desktop
bandwidth percent 40
set ip dscp 18
class web_traffic
police rate percent 15
set ip dscp 10
class class-default
set ip dscp default
shape average percent 80

I’ve used some different actions on my policy map, “priority” and “bandwidth” are worth some discussion.

The idea with priority is just as it sounds, that’s your most important traffic class hence why it’s used for voice, so in my policy-map I’ve reserved 7% bandwidth, on a 20mbps circuit that would mean when the link is saturated, any voice traffic upto 1400kbps would be given priority over all other traffic. The difference that priority has over bandwidth is built-in policer, provides low latency, manages delay and jitter.

For your environment you may need more or less than 7% depending on what codecs you use between your IP phones.

Bandwidth defines the minimum bandwidth guarantee to that class, this can be specified in percentage of the interface bandwidth or in kbps. With CBWFQ the excess bandwidth is divided up amongst the remaining classes if it’s not already being used by the default-class.

Traffic shaping can be applied to the default-class as a way of ensuring your link is never fully utilized by unimportant traffic, what it does in this case is only ever allows maximum outbound traffic for the default-class to reach 80% of the interface rate.

Police means the maximum rate that is always applied to that class, the difference between police and shape is that police will drop packets and shape will buffer them, policing the traffic is alot more complicated than what I’ve explained, this is just simple terms.

DSCP values are set on my policy-map for  inbound traffic to my WAN, so in advance I would speak with my WAN provider and break up all the different DSCP values into classes like gold, silver, bronze then once the WAN provider receives a layer 3 packet with a DSCP value assigned it can decide how to process the packet when the link is congested.

There are lots of variations you can use with policy-maps, you can use nested policy-maps aswell, but this is just a brief overview of what you can achieve with QOS.

Remember you need to have end to end QOS, that means determining trust boundaries. For example from IP phone to IP Phone would be end to end QOS, if you don’t mark the packets coming from your IP Phone then the router will classify them as default traffic, by default your IP phone will send voice traffic marked as EF, You need to connect it to a switchport that at the very least has the command “trust cos”

RH

Advertisement