2821 throughput for ISP

Unanswered Question
Nov 15th, 2009

I have 2 providers for my customers, 1st is 20 meg and they provide the routing and a 2nd 100 meg link which we provide routing. Currently I have a 3550 w/EMI routing to one provider and a vlan for the other. Will a 2821 w/ max memory give me the full 100 meg and also pass the other 20 meg? We don't use NAT or other services. We will be doing some Qos soon. Thinking of using a 4 HWIC to Vlan the other 20 megs to our 1st provider. We give our customers public static addresses. Thanks.

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Overall Rating: 5 (1 ratings)
Nagendra Kumar ... Sun, 11/15/2009 - 22:26


Positioning 2821 platform to handle 120 Mbps of traffic is not advisable.

You may be able to reach more than 120 Mbps when you have only IP traffic forwarding (without any services configured). But when you start running features like QoS, throughput will be degraded.

As you are planning to run QoS feature and SVI interface which are highly CPU intensive, I doubt you will be able to reach 120 Mbps throughput.



Mohamed Sobair Sun, 11/15/2009 - 23:51


The throughput for the routing capacity of the 2821 is 87Mbps capacity maximum.

and the QoS is done in a software, this does not fit with your reuirment + the capacity requirment.

If your budget is sufficient, why dont you go for 3825 or 3845 integrated service routers ?

you can then install the required HWIC and provide up to 256 Mbps throughput for routing capacity. + some other features.



Joseph W. Doherty Mon, 11/16/2009 - 04:34

"The throughput for the routing capacity of the 2821 is 87Mbps capacity maximum. "

Assuming this from the routing performance sheet, keep in mind that CEF PPS number is for minimum sized packets so with larger packets, higher throughput is possible.

Paolo Bevilacqua Mon, 11/16/2009 - 05:08


Historically, PPS is tested with the smaller packet size, throughput with the largest, unless specified otherwise.

Joseph W. Doherty Mon, 11/16/2009 - 07:53

Your "No" comment is unclear, at least to me. As I specifically noted, I'm assuming Mohamed's ". . . 2821 is 87Mbps capacity maximum.", is from the router performance reference sheet (more precisely the "Portable Product Sheets - Routing Performance", attached) which lists 87.04 for "Fast/CEF Switching" and further notes "Mbps calculated by pps * 64bytes * 8bits/byte" in the explanation header section. What specifically are you saying "No" to?

Paolo Bevilacqua Mon, 11/16/2009 - 08:15

To the correctness of assuming a throughput figure is measured with the smallest packet size.

But in the case you mentioned, it was.

Joseph W. Doherty Mon, 11/16/2009 - 04:50

For 120 Mbps (duplex) and QoS, you probably would want to go no smaller than a 3845, and even it might be too small. The 7301 (or 7201) might be safer choice as might the newer 3945 (150 Mbps w/services) (or the 3925 [100 Mbps w/services]).

sjonessha Mon, 11/16/2009 - 06:37

Thank you for the helpful answers. I don't think my budget will handle a 3845 right now. I'll look into the other choices as well or change around my network, maybe keep the 3550 for the 20 megs. Does Netflow reflect traffic that goes across the 4 HWIC?

Paolo Bevilacqua Mon, 11/16/2009 - 10:25

Netflow works on any interface.

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