01-26-2009 09:53 AM - edited 03-04-2019 12:58 AM
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!
01-26-2009 10:06 AM
Rashida
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.
Jon
01-26-2009 10:17 AM
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?
01-26-2009 10:30 AM
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 :-).
Jon
01-26-2009 11:34 AM
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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