Packet Scheduling during interface transmit interrupt???

WRKNonCCNP

I've been reading "Inside Cisco IOS Software Architecture" and so far it has been very helpful. I am slightly confused about one point, however. So here goes:

When an interface is congested (i.e. no room in the Tx ring) and a packet is transmitted from the interface onto the physical medium, freeing up a space in the Tx ring for the descriptor to the next packet's buffer location, the interface media controller sends a transmit interrupt to IOS telling it that a packet has been transmitted and it can de-allocate the packet buffer space. During this interrupt the packet buffer is de-allocated for the packet that was just transmitted and IOS fills the vacant Tx ring descriptor with that of the next packet from the queues allocated to that interface.

My question is: Is the next packet moved into the Tx ring determined by the QoS policies applied to the interface at this point in the packet switching process, during the Tx interrupt?

Thanks.

liquid6

I think so, the interface basically has 2 queues...the software queue which is controlled by congestion avoidance and management tools and the hardware (Tx ring). So depending on the policies that you are using in the software queue will determine the next packet that is placed on the Tx ring. Once on the Tx ring it is FIFO (first in first out) so however long it takes to serialize the packet onto the physical media.

I hope this helps...i'm in the middle of studying QoS right now so I could be off base.

Cheers,
liquid

networker050184

liquid6 wrote: »

I think so, the interface basically has 2 queues...the software queue which is controlled by congestion avoidance and management tools and the hardware (Tx ring). So depending on the policies that you are using in the software queue will determine the next packet that is placed on the Tx ring. Once on the Tx ring it is FIFO (first in first out) so however long it takes to serialize the packet onto the physical media.

I hope this helps...i'm in the middle of studying QoS right now so I could be off base.

Cheers,
liquid

This is my understanding as well.

WRKNonCCNP

Thanks for the responses. I am studying QoS for CCIP/CCIE written prep and wanted to make sure i was understanding that correctly. Understanding those intricacies makes understanding the rest a lot easier.