establishing bandwidth requirements

Unanswered Question
Apr 21st, 2010

Good Day,

Our network serves a residence hall on a university campus. At present we have 7 100mbps links (and 7 subnets), to the router on campus to serve, on average 350 concurrent users. For budgetary reasons we are considering going with a private ISP and reducing the available bandwidth to 100 mbps.

Although we don't administer the router on campus we do have access to graphs indicating traffic flows (Cricket). At present the 7 subnets max out around 150 mbps, but the average is less than 50. The max usage as represented in the graph typically indicates a sigfinficant spike in traffic up to 40mbps  lasting for may 5-15 minutes. This is helpful, aggregating this info from the 7 networks doesn't provide the precision I'd like.

These 7 subnets are trunked through a switch port with speed set at 1000. It seems to me that if I set the speed on this port to 100, I should get a good approximation of the speed of a 100 mbps wan port.

One question I have is if there might be a significant difference between the speed of a minimally configured router port set at 100 mbps and one on an L2 device.

Also, it looks like I might get some insight into port capacity usage from the "sh int" command output, but I'm not real familiar with it.

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:01, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 28000 bits/sec, 39 packets/sec
     13753322 packets input, 2494700497 bytes, 0 no buffer
     Received 20916 broadcasts (0 multicasts)
     5 runts, 0 giants, 0 throttles
     5 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 6438 multicast, 0 pause input
     0 input packets with dribble condition detected
     410392180 packets output, 41365037522 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

Any other insight or approaches to this question would be appreciated. And if you have extensive experience and think 100 mbps should be fine for 350 students watching videos, skyping etc. should be fine, just say "don't worry".

Thanks,

William Doyle

I have this problem too.
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Giuseppe Larosa Wed, 04/21/2010 - 12:11

Hello William,

>> think 100 mbps should be fine for 350 students watching videos, skyping  etc. should be fine, just say "don't worry".

Yes, I would say so.

You have evidence that :

>> At present the 7 subnets max out around 150 mbps, but the average is  less than 50.

So you have already the answer

About emulating the effects of a 100 Mbps WAN interface by creating a bottleneck inside the campus on a trunk link I agree it can be a good idea to emulate the impact of the change.

Note: without advertising the change to  the students

Hope to help

Giuseppe

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