[c-nsp] ASR9k Bundle QoS in 6.0.1

Saku Ytti saku at ytti.fi
Wed Jun 8 06:44:01 EDT 2016

On 8 June 2016 at 11:53, Robert Williams <Robert at custodiandc.com> wrote:


> My understanding is that the central arbitration has a few benefits, such as zero packet loss when there is an RSP failure (because every request is sent to _both_ arbiters at the same time, so when one RSP fails there is only one answer back, not two). Thus, the packet in flight is still transmitted without loss. Critical when you have very sensitive BFD over MPLS tunnels traversing the chassis. There may not be a quick enough 'detection' of the failed RSP/arbiter on the core device to maintain data plane forwarded BFD relationships between up/downstream nodes.

But is that really so? Why can't we do this with 100% separated
fabrics with distributed arbiters? The linecard could resend fabric
request if it does not receive reply in n microseconds, to handle
failover with no loss?

> The second benefit, relating to my original question :) - is that backpressure from a congested egress port can be applied 'globally' to all ports on the chassis, via the virtual output queues. This should (in my opinion) open up the door for centralised egress queueing because it seems to me that the pieces are all there, and, maybe, it is now in place from 6.0.1 onwards so that bundles may support it across all member ports.

I don't think the arbiter experiences the traffic in higher precision
than NPU, I don't think it can discriminate between actual WAN ports.


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