QoS: Feedback on 2 ways to configure QoS on a Cisco Router

Unanswered Question
Jan 26th, 2009

There are two ways for configuring QoS that accomplishes the same thing. I have configured both methods and both work without any issues.

I would like to know from the two simple examples below, which one if more recommended and why???

Method 1: Classify, Mark DSCP, and do LLQ on WAN interface.

Method 2: Classify and Mark DSCP on LAN interface. Then do LLQ on WAN interface matching the previously marked packet from LAN.

>>>>>>>> METHOD 1: <<<<<<<<<<<<

class-map match-any class-smtp

match protocol smtp

policy-map qos-policy

class-map class-smtp

priority percent 20

set dscp af21

interface serial0

service-policy output qos-policy

>>>>>>>> METHOD 2: <<<<<<<<<<<<

class-map match-any class-smtp-lan

match protocol smtp

class-map match-any class-smtp-wan

match ip dscp af21

policy-map qos-lan

class-map class-smtp-lan

set dscp af21

policy-map qos-wan

class-map class-smtp-wan

priority percent 20

interface serial0

service-policy output qos-wan

interface fastethernet0

service-policy input qos-lan

Thank you!

I have this problem too.
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Jon Marshall Mon, 01/26/2009 - 10:06


Although they both have the same effect i don't think they are achieving it in quite the same way.

Method 1 allocates the traffic to the priorirty queue based on it nbar matching smtp traffic in the packets. It also sets the DSCP value to af21.

Method 2 allocated the traffic to the priority queue based on it matching DSCP af21 in the packets.

Method 1 involves 1 match on the router.

Method 2 involves 2 matches on the router - the first on the LAN interface with nbar and the second on the WAN interface with af21.

Generally if possible you should mark packets as close to their source as possible and then simply match on the packets later.


ryabutler Mon, 01/26/2009 - 10:17

I agree Jon, however, there are environments especially with SMB customers that may have unmanaged switches for their LAN Collapsed Core/Access and maybe a SonicWall firewall on the Internet Edge .

So this example would be applying QoS for controlling, Internet usage where there is no previous classification or markings anywhere on the LAN.

Any other points?

Jon Marshall Mon, 01/26/2009 - 10:30

If there is no way of marking before packets reach the WAN router then i would use Method 1 simply because there is one less action that i can see ie.

Method 1 - match SMTP and mark as af21

Method 2 - match SMTP, mark as af21, match af21

To be honest it probably doesn't make that much difference :-).


Joseph W. Doherty Mon, 01/26/2009 - 11:34

We're assuming your router only has two interfaces (LAN/WAN) and the IOS supports method 1. My preference is method 1 since it keeps the QoS configuration smaller and easier, I think, to understand.

Method 1 also allows bypassing the need for marking the packet with the same QoS results (except for tagged packets).

There are many other factors, though, that might merit one approach vs. another. For instance, if you want to police traffic, just as with ACL, you might want to do that upon interface ingress rather than interface egress; method 2 could have an ingress policer. If there were other interfaces on the router, they might push for a different selection based on other requirements.


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