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Cisco 2621 router

I have got 2621 router with the following version of IOS:


I have got two subnets created as primary and secondary under the LAN interface of the router. As we got older version of switches I've got this limitation and these subnets need to be under a singla VLAN. This interface is the gateway for the users of both the subnets in the LAN. Now if there is any huge amount of transfer among these two subnets the CPU utilization is growing rapidly high. We enabled ip cef, but still the problem exists. I can't afford to purchase an EMI switch and convert the L3 functionality(i.e, the user traffic to hit the switch rather than hitting the router directly). Any suggestion in this case?

Is the IOS a problem?



Hall of Fame Super Gold

Re: Cisco 2621 router


If you can't afford a L3 switch (these days are quite cheap, look for example at linksys), your only option is to keep the users that do large transfer between themselves, in the same subnet.

Even with secondary address in the same router interface, will overload the router and give poor performances.

Hope this helps, please rate post if it does!

Hall of Fame Super Silver

Re: Cisco 2621 router


I am not clear whether part of your question is about putting both subnets into a single VLAN or whether your question is only about the issue of high CPU utilization.

From your description it sounds like you have configured on the physical interface (not with subinterfaces) a primary IP address (defining one subnet) and a secondary address (defining another subnet). This configuration does put both subnets into a single VLAN. The best configuration would be to connect the router to an access port (not a trunk port) on the switch.

For the issue of high CPU utilization, enabling CEF (if it was not already enabled) is an excellent place to start since CEF is the optimized switching path. I might also suggest that the code that you are running on the router is pretty old code and that a newer version of code might have some performance optimizations which could help you.

The other thing to do is to look for things that are keeping the CPU utilization up. For example do you have access lists configured which use the log function to record information about the traffic. This will increase CPU utilization. You could start with the output of show process cpu and look for processes that are using large amounts of CPU.