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Webcast-Catalyst9k
New Member

QOS Newbie Needs Help

I need advice and hope this is the right forum. I have a dedicated T1link between one branch office and our home office. I would like all traffic from all users in the branch that go to certain hosts in the home office to get higher priority than other destinations. I do not wish to dedicate a certain amount of bandwidth to these host destinations, just priority. No VOIP on this link, just data. I am confused at this point from all the different options, please help. Thanks.

5 REPLIES
VIP Purple

Re: QOS Newbie Needs Help

Hello,

one way to prioritize your traffic would be to set the precedence or DSCP, depending on your IOS version (the ´ip precedence´ option has been replaced with the ´set dscp´ option, starting with IOS 12.2(2)T).

So, in your case, the configuration could look like this (the IP addresses and interfaces in this example are likely different from yours, just make sure that the access list matches the sourec and destination hosts you want to give priority to):

class-map match-all PRIORITY

match access-group 101

!

policy-map DSCP

class PRIORITY

set dscp cs5

class class-default

set dscp cs1

!

interface Serial0

service-policy output DSCP

!

access-list 101 permit ip 192.168.1.0 0.0.0.255 172.16.1.0 0.0.0.255

Be aware though that the prioritized hosts might starve other traffic, that is why usually a certain amount of bandwidth is guaranteed...

HTH,

GP

New Member

Re: QOS Newbie Needs Help

Thank you but have more stupid questions ---- if I did assign a certain amount of bandwidth to Priority, would priority be able to use more if there were more available and vice versa, would default get more if Priority were not using all its bandwidth or is this a carved in stone value which would never fluctuate depending on link use? Thanks again for any input to my stupid questions.

VIP Purple

Re: QOS Newbie Needs Help

Hello,

with the priority, you set an upper limit, if the upper limit is not reached for your particular traffic type, the bandwidth is available for other traffic.

HTH,

GP

New Member

Re: QOS Newbie Needs Help

As per my response to the other poster, the priority command enables LLQ. LLQ will drop any traffic over the configured bandwidth.

If you want the prioritised servers to use available bandwidth over what's been reserved, then use the "bandwidth" statement in place of the "priority" statement. Be extra careful though. If the prioritised server pumps large amount of data, they WILL drown out the default class.

And, oh, your question ain't stupid. ;-)

New Member

Re: QOS Newbie Needs Help

I don't think the prioritised hosts would starve the traffic. What you suggested above was LLQ. LLQ will NOT use anymore than what has been configured, thanks to its policing feature. Any traffic exceeding the configured bandwidth will be discarded.

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