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QoS. Understand "priority" command.

In a two-switch environment, with two sources fighting for one destination:

1. Does QoS configuration have to be identical on two switches?

2. What does the configuration below actually do in a source switch and what does it do in the destination switch? In the source switch, does it simply only mark EISL header of the FC frame? I think that field is 3 bits (1-7) in the EISL header. There are only 3 queues, how does it set that field?

In the source switch, queuing doesn't do anything because two ingress interfaces have separate VOQs, and central arbitrer would service both interfaces equally, correct? So in the source switch, it only marks EISL header?

In the destination switch, do you need to have QoS classes and policies defined? If the EISL header already set with the priority, shouldn't the switch automatically know which queue to use?

qos policy-map POLICY

class HIGH

priority high


New Member

Re: QoS. Understand "priority" command.

1. I'm pretty sure, but not positive.

2. In the ingress port of the source switch, this configuration would mark the frames for the high-priority queue. You are correct that it uses 3 bits in the header for marking. There are actually 4 queues, but only 3 are usable by you. The highest-priority queue is reserved for control traffic.

3. "...because two ingress interfaces have separate VOQs..." You would run into the VOQ if the traffic shared the same egress port on the source switch. They obviously don't share the same ingress port. But yes, the ingress port on the source switch only marks the priority in the frame header.

4. The destination switch will know which queue to use, but what about the return traffic? Do you care about that? I still think you need the same QOS config on each switch in the VSAN... Not 100% positive.


Re: QoS. Understand "priority" command.

Thank you! It's much more clear now!

Follow-up questions:

1. Four queus (PQ, H, M, L), but EISL has 3 bit header (000 through 111, 0 through 7). Do you by any chance know the mapping? I'm pretty sure PQ control traffic is set to 7, but what about DWRR queus?

2. Documentation says that QoS configuration should be the same. But I think they mean "similar". Because if my source switch specifies one source-address "match source-address", the other switch would have to specify a totally different "match source-address".

Re: QoS. Understand "priority" command.

I found the answer to my question.

With QoS on:

PQ = 7

High = 5

Medium = 2

Low = 0

With QoS off:

PQ = 7

Low = 0

The first switch in the path, where you have QoS enabled will mark priority field in the EISL header. This is why the next switch knows which queue to put the frame into in the ingress port.

New Member

Re: QoS. Understand "priority" command.

actually - this is ToS field you talk about. You still have full DSCP field 0-63 support (that's the reason for having byte field for QoS in Vsan_header, and can allocated QoS based on DSCPs .

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