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qos question

Hi All,

I have a qos question. Thisnk of a situation when a serial link is congested with 120% load. The interface is configured with an outbound policy map with 6 classes. There is EF priority class and four other classes beside default class. There is traffic in all classes. Since the serial link is overloaded the tx-ring will be full all the time.

What will happen to a packets arriving in default class, CS6 class, AF31 and priority EF class at the same times.

I think IOS will stop filling the tx-ring buffers with non priority class packets as soon as it detects a packet arriving in the priority EF queue, but does it do the same thing as it sees packet arriving in a other class for example AF31 vs default class? I believe it does a round-robin based on percentage allocated for each class other than priority class. For example if AF31 has allocated 20 percent and AF11  has allocated 10 percent it will forward two packets out of AF31 class to tx ring and then one packet from AF11 class but as soon as it sees a packet in EF class it stops forwarding packet to tx ring. Is this correct assumption even when the tx-ring is full?

Just need to clarify my concept here!

thanks!

2 REPLIES

Re: qos question

HI,

A little correction to your understanding.

When router see there is no space in hardware queue (tx_ring) the packet is handed over to software queue and it is treated as per PHB

of marking, like ef class will be treated first and then subsquent class and lastly there is default class.

A logic for hardware queue is always FIFO but for software queue it depends upon your configuration. If it is fair-queue it is treated as per fair-queue

algorithm. But packets are there in hardware queue it is not dropped there but it is allowed to pass and then ef will treated . So you can assume like if there is 1500 byte packet already passing it is allowed to pass and then ef packet.

This is the reason we sometime configure interleaving which breaks 1500 byte packet in different chunk and then ef packet will be inserted in between

which will allow less waiting time for priority packet

Regards

Mahesh

Re: qos question

HI,

A little correction to your understanding.

When router see there is no space in hardware queue (tx_ring) the packet is handed over to software queue and it is treated as per PHB

of marking, like ef class will be treated first and then subsquent class and lastly there is default class.

A logic for hardware queue is always FIFO but for software queue it depends upon your configuration. If it is fair-queue it is treated as per fair-queue

algorithm. But packets are there in hardware queue it is not dropped there but it is allowed to pass and then ef will treated . So you can assume like if there is 1500 byte packet already passing it is allowed to pass and then ef packet.

This is the reason we sometime configure interleaving which breaks 1500 byte packet in different chunk and then ef packet will be inserted in between

which will allow less waiting time for priority packet

Regards

Mahesh

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