Hello all. I can't for the life of me seem to understand this, or find someone who can explain this.
If I am doing some LLQ and have a priority queue with 33% bandwidth guaranteed (3.33 mbps), but the queue limit is 64 packets - how does this really do me any good? I'm terribly confused! I know queuing only occurs during congestion (can anyone explain how I can determine when that point is?), but let us pretend the 10 mbps circuit is maxed out. Say a boat load of traffic comes into the router while this thing is maxed out, and packets start to get queued. Well, if all queues are 64 packets at most, and a packet is at most 12000 bits, that means a queue holds roughly 750000 bits of data at most (or 0.75 mb - but in reality 64 voice packets is only going to be around 41000 bits). If there's just 3 queues, that's only 2.25 mb totally accounted for. These queues can be dumped at sub-second rates, so they can be filled multiple times within a second I imagine - but I am having a hard time understanding this. Especially when a default queue gets 61% remaining BW, and a priority queue only gets 33% guaranteed - how the hell does this actually work out when the queue limit is exactly the same size? If everything starts getting queued up and the default class should use 61% of the remaining bandwidth, shouldn't it need to hold more packets?
I'm really hopeful someone can make sense of this - it's got me going crazy!
The only way I can see this making sense is that the link is maxed out, the router knows 80% is default class currently in the hardware queue, and it is willing to empty out the software priority queue enough so that additional voice traffic can get sent out (up to 3.33 mbps). I don't know - maybe I've gone off the deep end here lol. It's just kind of funny that queues are measured in packets, but packets vary in size and you are really trying to just gurantee bandwidth, and 3.33 mbps essentially equates to 38 people making outgoing calls with G.711 encoding guranteed, and that's going to equate to 1900 packets per second but the hardware queue is only 1000 packets? But obviously that isn't 1000 packets per second, the 2911 can forward way more packets per second (up to 183,000 if all packets are 64 bytes I believe) which allows for 180 mbps, which is why we need to use much beefier ASR 1000 series routers on the MPLS head end.
This stuff just happens so fast (milliseconds) that I guess it's hard to imagine/comprehend?