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I have confusion regarding Juniper CoS. In Juniper CoS , when we define scheduler we can mention the queue priority to that forwarding class like strict high , high , medium low etc which effects the order in which queues will be serviced. But what happens when we assign same Queue priority to more than one forwarding class.

For example , suppose that we have four forwarding classes like Best-Effort , business , Video and Voice. We assign Voice to the strict-High priority but Best-Effort , business and Video all have same priority which is High . They (3 x Queues) have different transmit rates and buffer sizes.

How in this case scheduler will take the packets from the Queue and put them on interface? Or How will the Queue be serviced?

What is difference between strict high and high?

As per Juniper documentation , strict-high and High Queues share precedence to transmit packets. If that is the case then whats the purpose of defining strict high and high queues separately.

Thanks

This snapshot from the Juniper Study Guide (CoS) says that strick-high and high priority share same precedence.

Juniper CoS -- Strict High vs High

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What is the difference between strict high and high?

The difference comes down to bandwidth. It's true that strict-high and high queues transmit traffic in a weighted round-robin fashion based on the configured bandwidth values, but strict-high queues can consume unlimited bandwidth, where as high queues cannot.

As per Juniper documentation, strict-high and High Queues share precedence to transmit packets. If that is the case then what's the purpose of defining strict high and high queues separately.

strict-high and high will take turns transmitting traffic until the high queue reaching its bandwidth limit. For example, you may have strict-high for voice traffic and high for network control. You wouldn't want voice traffic to starve network control and cause other issues.

For example , suppose that we have four forwarding classes like Best-Effort , business , Video and Voice. We assign Voice to the strict-High priority but Best-Effort , business and Video all have same priority which is High . They (3 x Queues) have different transmit rates and buffer sizes.

Using your example of:

  1. Voice (Strict-High)
  2. Business (High)
  3. Video (High)
  4. Best-Effort (High)

In your case the applied transmission rates would dictate how packets are queued under congested scenarios. Packets would be serviced weighted round-robin style based on those maximum transmission rates.

This is a much better explanation of how things work.

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  • Thank you very much for answering the question. Regarding your point about documentation issue , I was reading study guide from Juniper Networks . Its about CoS and in it mentions this thing. This is a stand alone study guide about CoS. I am attaching snapshot to my original question. Dec 18 '19 at 7:19
  • I found some better documentation, I'll be updating my answer shortly as part of my understanding was actually wrong. Dec 18 '19 at 16:19
  • @JordonHead. Thank you for sharing detailed answer Dec 18 '19 at 18:51

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