From: Sam Munzani (sam@munzani.com)
Date: Wed Mar 05 2003 - 16:16:57 GMT-3
My understanding was a full T1 is 1540000 bps. This is full duplex. That
means up to that speed if router keeps putting bits to the serial
interface's output bucket, the line will pick it up and forward it to the
other end. When the router starts buffering data more than that rate, the
serial interface buffers will be full and hence start dropping bits to the
bit bucket. I didn't see any issue until the bucket gets full. The drops are
added to the "output queue drop" counters on "sh int" command. My logic is,
when the T1 is at 80% utilization, the line can still forward every single
bit of it so nothing will go to the queue drop. Is my thinking wrong?
It's different issue that router's bus can handle only 75000 pps. So if my
3640 router has 2 Fast E ports and the traffic between those 2 interfaces is
high, it will consume router's bus speed to 75000pps and hence the queue
drop will happen on Fast Ethernet's input queues. In this case the router's
back plane is the bottleneck.
In short, if you are not hitting back plane limitation, your users should
not see any performance degradation either T1 is at 80% or at 100%. Because
every single packet/bits is getting forwarded properly and there is no need
for retransmission because of collision or CRC.
Am I thinking right or wrong?
Thanks,
Sam Munzani
CCIE # 6479(R&S, Security)
> 75,000 packets per second with each packet 64 Bytes is a throughput of
> around 38Mbps. How many of your packets are going to be just 64 Bytes? If
> packets are larger then the throughput goes up.
>
> What does happen though on a slow serial link that is utilised above 80%
is
> there is not enough bandwidth to cope with bursts of data without
buffering,
> and maybe dropping, packets. This increases latency and also could causing
> problems with routing updates/hello packets being dropped. This is why
Cisco
> recommend a maximum average of 80% utilisation on a serial link.
>
>
> Colin
>
> -----Original Message-----
> From: Sam Munzani [mailto:sam@munzani.com]
> Sent: 05 March 2003 15:51
> To: ccielab@groupstudy.com
> Subject: At what percentage of T1 utilization you should see performance
> degrade?
>
> Hi,
>
> Today we got in to a debate about T1 saturation and performance
degradation.
> Below is 2 different opinions from my self and other engineer.
>
> What I say.
> T1 is a full duplex mechanism. So until it hits 100% utilization on it's
> Serial(Full T1) interface it will forward to packet and no packet drop
will
> happen. Only the time it will do a queue drop is when the traffic bursts
> above
> 100%. This queue drop is because of Serial interface short on forwarding
> buffers. Same behavior will happen on inbound. Only the time this would be
a
> bottleneck is if router CPU is a bottleneck. This is on 3640 with Fast
> switching turned on. I am assuming 3640 can handle more than 1 T1
bandwidth.
>
> The other engineer's opinion.
> 3640 supports only 75000 pps with 64 bytes packets. His opinion is, your
> users
> will see performance problems even before T1 is hitting 100%(aroung 70 to
> 80%).
>
> Any opinions with enough supporting proofs are most welcome.
>
> Regards,
>
> Sam Munzani
> CCIE # 6479(R&S, Security)
>
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