From: Jay Hennigan (jay@west.net)
Date: Wed Mar 05 2003 - 20:01:25 GMT-3
On Wed, 5 Mar 2003, Sam Munzani wrote:
> 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?
Your thinking is correct. One danger is to condider the interval over
which you're measuring the 80% utilization. The default interface display
is five minutes, which is a very long time for bits waiting in the output
bucket. If your traffic isn't bursty, you might be OK with this but I'd
want to look over a shorter interval to figure the 80%. As an ISP we see
customers with T-1s flatlined at 1500 Mbps for hours of Internet traffic,
effectively 100% utilization. Clearly these customers are seeing some
performance issues, but for web browsing type of traffic of backups over
a WAN in the middle of the night, it may not be a big enough issue to
cause the customer to buy more bandwidth. Don't try it with mostly VoIP,
though.
At 80%, they should seriously consider buying more bandwidth.
> 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.
This isn't likely to be an issue, again depending on the type of traffic.
If you figure 1000-byte packets, 75000 pps is 600 Mbits/s. This limit
is likely to only affect a network with very small average packet size.
> 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.
Collision isn't an issue with full-duplex links. CRC shouldn't be an
issue on clean WAN links.
-- Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net NetLojix Communications, Inc. - http://www.netlojix.com/ WestNet: Connecting you to the planet. 805 884-6323
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