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vlan vs port based qos


I have a question about vlan based qos. I am happy with qos configuration as applied to ports. However, vlan based qos confuses me somewhat.

Is vlan based qos intended for situations where packets are to cross vlans? In that case, am I correct in assuming that vlan based qos has no effect on packet flows within that vlan? In that case the idea of vlan based qos would be to police/mark traffic leaving/joing that vlan?

Or, does vlan based qos extend queuing (priority queue etc) down to ports that are members of that vlan are configured with vlan based qos? I think not but I'm not absolutely sure.

I can't seem to get to the bottom of this on cco.

Thanks, Steve


Re: vlan vs port based qos

Hi Steve,

Packets do not have to cross VLANs for you to need VLAN-based QoS.

VLAN-based QoS gives you an additional layer of queueing hierarchy. With port-based Qos, there is a set of software queues per physical port. As packets are scheduled from these queues, they are emitted from the port.

With VLAN-based QoS, there is another layer. Each VLAN configured for VLAN-based QoS will have a set of queues associated with it, instead of having a set of queues for the physical port. This comes in useful for providers of Metro Ethernet service who offer multiple classes of service. Such ethernet services are usually sold with a fixed bandwidth per-VLAN. At egress switch ports, the provider will use vlan-based QoS to police/shape traffic in order to conform to the sold rate. Within this shaped rate, queueing will be used to ensure that the higher classes of service get preference.

In answer to your questio, vlan-based qos does have an effect on packet flows within that vlan.

Hope that helps - pls rate the post if it does.



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