[c-nsp] Maximum traffic on Gigabit Ethernet

[Nick Hilliard]

On 03/10/2011 14:47, Manaf Al Oqlah wrote:
> What is the maximum traffic that a gigabit Ethernet interface can handle
> on Cisco 7600 router RSP720-3C-GE before dropping packets . we are able
> to reach 750 mbps / 125 mbps input/out rate!

The maximum traffic that a gigabit Ethernet interface can handle before
dropping packets on any interface capable of handling line rate
transmission is exactly 1,000,000,000 bits per second - no more, no less.

I think you're confusing "maximum traffic" with "maximum traffic averaged
out over a period of X seconds, where X is larger than the
deterministically longest buffer time available on that specific port".

To understand why your question is meaningless, consider the situation
where a GE port receives exactly 2gbit/sec for one second, then spends the
next 299 seconds with no traffic at all.

Because the port could only handle 1Gbit/sec out of the 2Gbit/sec received,
this will register as a total of 1Gbit/sec of traffic over 300 seconds,
i.e. 3.333 mbit/sec.  However you will see 50% packet loss, because the
port could only handle half the traffic it expected to process.  Actually,
it won't be quite 50% due to packet buffering, but you get the idea...

So from a mathematical point of view, let's say you're using a
WS-6748-GE-TX card which has 1.3M of buffer space per port and that you're
using 5 minute load counters.  This works out as 96ms worth of traffic.  So
you are guaranteed that regardless of the input traffic load, the first
96ms worth of traffic received on the port will not have any packet drops -
note that this is not the same as the first 96ms worth of traffic
transmitted to the port.  If you figure out the maths, it means that
without prior knowledge of the traffic profile, the maximum 5 minute load
average on that GE port where you are _guaranteed_ not to have packet drops
is 320kbit/sec.  Freaky, huh?

This is nit-picking though.  In real life, the number is highly variable,
and usually ranges from about 650mbit/sec upwards, depending on traffic
type.  imix traffic is usually much more forgiving than bursty lan traffic.

Nick