Hi there,
please refer to the following white paper:
http://www.cisco.com/web/partners/downloads/765/tools/quickreference/routerperformance.pdf
First, be aware that the answer to your question strongly depends on your individual environment, especially the packet mix in your network and used services like NAT, ACL, firewall & encryption services etc.
As outlined in the document, the numbers have been measured under the following conditions:
1. No services are used on the router, thus maximizing the number of packets per second (PPS) in the test (Best-Case).
2. The size of all packets is 64 bytes only, thus minimizing the transmitted data (Mbps) in the test (Worst-Case).
From the table on page 2, you can tell that the ISR 2911 has a raw forwarding performance of 353,000 packets per second. Using the given 64 bytes per packet, this results in a forwarding performance of 353,000 packets/sec x 64 bytes/packet x 8 bits/byte = 180736000 bits/sec, or approx. 180 Mbps.
Again, this is assuming the conditions 1+2 mentioned above, so look at the opposite extreme of 1500 bytes/packet only: 353,000 packets/sec x 1500 bytes/packet x 8 bits/byte = 432600000 bits/sec or approx. 4,236 Gbps.
However, both approaches are the ends of the scale under ideal circumstances and not applicable to a productive environment. Assuming an average packet size of 400 bytes/packet, we get approx. 1,1 Gbps, which could be a realistic number to go for.
At maximum, your router needs to handle 2x 200Mbps links at the same time (assuming link 3 as backup only). From this perspective, the throughput of the C2911 should be sufficient for your demands, but keep in mind that using services will heavily impact the forwarding rate, maybe dropping it to only 100-200Mbps.
So, if you want to make sure that the router can always fully handle your 400 Mbps-links even in worst case circumstances, you should go for a C3925 or C3945 instead (which deliver around 400/500Mbps according to the testing scenario in the white paper).
Hope that helped.
Cheers, Dennis
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