Bandwidth from Access layer to distribution layer

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Dec 24th, 2008
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I know the rule of thumb is that a users connection should have at all times 20:1 of the that would mean if I have 31 users connected to a switch at 1000MB and 8 Users connected at 100MB for a total of 31800MB....for the 1000MB connections I need 50MB per user and 5MB per user on the 100MB connections for a total of 1590MB minimum uplink to the distribution layer switch so I would etherchannel 2 1000MB uplinks for this. Am I correct in this setup?

Now I think that is for just there something else when you add VoIP in to the mix?


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Overall Rating: 3.3 (3 ratings)
ohassairi Wed, 12/24/2008 - 08:12
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users will not use the bandwidth all the time.

using a software like PRTG you can determine how much the uplink is used. then you can take qos decisions

burleyman Wed, 12/24/2008 - 08:31
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I am just looking at a design perspective right now...I know Cisco recommends the 20:1 setup and I wanted to find out if there was something different if VoIP is in the mix.


viyuan700 Wed, 12/24/2008 - 08:42
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for VoIP traffic u r going to define Qos

in case at some point of time data is more ur VoIP traffic will have priority.

dabels Wed, 12/24/2008 - 09:15
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you will always want to define QOS for voice so that the priority queue will get used to give consistent voice quality even if the average amount of traffic is low you may get bursts that need to be worked thru for voice.

richard.m.gilbert Wed, 12/24/2008 - 21:54
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Is that because voice is UDP and data is typically TCP? TCP can be, and will be, retransmitted as it is windowed while UDP traffic is gone if it is lost?

Joseph W. Doherty Fri, 12/26/2008 - 08:41
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The 20:1 rule of thumb is very rough; much depends on the nature of what the hosts are doing. I.e. how much traffic is being passed back and forth and how time sensitive it is. Also, normally as you increase host edge bandwith, since the host's actually traffic amount usually doesn't increase with edge port bandwidth, you don't always need to increase the uplink bandwith (e.g. 48 FE ports with gig uplink that's performing well will likely perform just as well or perhaps even better with 48 gig ports and gig uplink).

As another poster noted, VoIP bandwidth is best dealt with using QoS, although it too has its own bandwidth requirements that depend on the codec being used and how the users are using it (e.g. normal business phone usage vs. sales/help service/support phone usage).

Etherchannel is an attractive option for increasing link bandwidth but it doesn't really perform the same as a link of the same bandwidth. For instance, assume a dual Etherchannel provides, on average, 50% more bandwidth not 100% more bandwidth. (NB: if channel hashing isn't adjusted for the link ends, Etherchannel might not even use its multiple links.)


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