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Cisco Employee

When disable Fast-Switching

Good Morning all,

I am not understanding the reasons for the following information.

In the Cisco Web sites I have find the following statement:

" In some cases, fast switching is inappropriate, such as when slow-speed serial links (64K and below) are being fed from higher-speed media such as T1 or Ethernet. In such a case, disabling fast switching can reduce the packet drop rate to some extent."

I have got a 7200 with a very old IOS version (11.2). I do not have CEF in the IOS. Can I enable the Fast-switching on the interface of the routers?

Will I face this problem?

What is the reasons behind this suggestion?




Re: When disable Fast-Switching

Fast switching is a technique in which, the Router populates a switching cache, based on initial traffic flows, and used to switch traffic directly from inbound to outbound interface, without much intervention from the CPU. This helps in faster switching of packets. In case you have a high speed fast ethernet (100 Mbps) interface, on one end, while a very slow, 56k link on the other end, with fast switching enabled, packets coming on the ethernet (at 100 mbps) will be immediately switched to the 56k outbound interface buffer. The buffer will be soon outrun because of the faster rate at which packets are being switched. The wan interface running at 56k , wont be able to send all these packets at the same rate at which it is being stored in its interface buffer.

So disabling Fast switching in this case, might help reducing packet drops to some extent.

Fast switching is enabled by default on most of the interfaces. Give a "show ip interface <> to find out whether FSwitching is enabled or not.