QOS on a 3750 Switch

Answered Question
Oct 10th, 2008
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Hi All,


I just wanted to see if this is even possible.


I have a 3750 switch as the main switch at each of my outlying sites. They connect to the core via 100Mbs fiber, at the Core is a 4507 switch.


On the 3750 I have 4 vlans, Guest, management, users and phone. I want to configure QOS such that all but the phone vlan share 90 Mbs and leave a nice fat pipe for Phones at 10Mbs.

As far as I know policing will not work in the outbound direction on the 3750 so that leaves me with trying to figure out how to get 3 vlan interfaces to share 90% of the bandwidth. What I have now is 45Mbs to user, 40Mbs to management and 1Mbs to guest. If possible I would like the management, user and guest to share 90Mbs so I don't have to police them to half the bandwidth to ensure the phone bandwidth.


At this point I am thinking it can not be done, anyone care to set me straight on this?


Thanks

Brett

Correct Answer by Jon Marshall about 8 years 5 months ago

Brett


The 3750 uses SRR on both the ingress and egress queues. On the egress queues SRR can use either shared mode or shaped mode.


Shaped mode allows you to allocate a bandwidth to each of the 4 egress queues and these queues are then rate-limited. However it is important to note that if one of the queues is not being fully used that spare bandwidth cannot be used by another queue. With shared mode you can.


So you can rate-limit traffic on egress ports with the above provisos.


http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_40_se/configuration/guide/swqos.html#wp1200681


Jon

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Correct Answer
Jon Marshall Fri, 10/10/2008 - 12:29
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Brett


The 3750 uses SRR on both the ingress and egress queues. On the egress queues SRR can use either shared mode or shaped mode.


Shaped mode allows you to allocate a bandwidth to each of the 4 egress queues and these queues are then rate-limited. However it is important to note that if one of the queues is not being fully used that spare bandwidth cannot be used by another queue. With shared mode you can.


So you can rate-limit traffic on egress ports with the above provisos.


http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_40_se/configuration/guide/swqos.html#wp1200681


Jon

blittrell Fri, 10/10/2008 - 12:33
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Great thanks for the info, that is just what I was looking for!!


blittrell Fri, 10/10/2008 - 13:04
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Ok, this may be a stupid question be when I enter the weights and do a ? it shows 1-255, is this just because it is a 1 byte value? From reading the docs it sounds like weights are a percentage of the bandwidth, of course anything over a 100 would not make sense, just wondering if this is just a caveat or do I need to do a percentage of the bandwidth I want from 255 and enter that?


Thanks, I am slowly getting this QoS:)


Brett

Jon Marshall Fri, 10/10/2008 - 14:11
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It's not an absolute value. One thing i forgot to mention is that you can use shaped and shared mode on the same port so in your case you could either specify all the weights or just the weight for the VOIP.


From link provided


Switch(config-if)# srr-queue bandwidth shape 8 0 0 0


This example shows how to configure bandwidth shaping on queue 1. Because the weight ratios for queues 2, 3, and 4 are set to 0, these queues operate in shared mode. The bandwidth weight for queue 1 is 1/8, which is 12.5 percent.


Does this make sense ?


Jon


blittrell Mon, 10/13/2008 - 06:05
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Hi Jon,


That makes sense up to the weight, so all weights are defined as 1/weight=bandwidth? So a weight of 1 is 100% bandwidth?


Thanks for putting up with the ignorance:)


Thanks


schmij01 Fri, 10/10/2008 - 12:40
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If you mark the phone traffic accordingly, you can also just enable priority queuing, which will always give the priority queue the bandwidth it needs, and would allow the other traffic to use all the leftover bandwidth.

blittrell Fri, 10/10/2008 - 13:07
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Thanks for the info, that is the next thing to tackle, trying to take baby steps:) I think all the tagging is correct so once I verify I that I did not just hose the switch and it is working as advertised I will plop on the priority queue as well:)


Thanks


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