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Here's my show ver IOS version:

Cisco IOS Software, C2600 Software (C2600-ENTBASE-M), Version 12.3(11)T2, RELEASE SOFTWARE (fc1)

It's a Cisco 2621 router providing NAT between fa0/0 and fa0/1. The WAN connection is tested from a device alongside this router as being approx 40mbit. A speedtest of a device accessing the WAN via this NAT router shows almost exactly 20mbit.

Can anyone suggest what might be limiting throughput here?

  • How big is the NAT network behind the router, a /24? – NickW Mar 19 '13 at 9:14
  • You need to troubleshoot. Is the CPU maxed (show proc cpu)? Are the packets being fast switched (show int fa0/0 switching)? How many packets per second are moving in each direction? – David Schwartz Mar 19 '13 at 10:00
  • The NAT is being performed by the CPU, not by the hardware ASICs – jwbensley Mar 19 '13 at 11:54
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It is your routers performance that limiting you. Here is a cisco's routers performance PDF

Your router throughput is roughly 12.80 Mbps.

  • 1
    Also remember that performance sheet is for plain packet forwarding without any kind of NAT or other service(QoS/NBAR/...). – cpt_fink Mar 20 '13 at 6:55

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