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Community Member

IOS Buffer Tunning issue

I found a Cisco 3662 router have so many Large Buffer and Huge Buffer failures, but I just don't known where these packets come from? as you known, Large Buffer match 5024 bytes and Huge Buffer match 18024 bytes.

another queston: we planning to upgrade the DRAM of this router from 64MB to 128MB, if the IOS will automaticlly tunning the size of the buffer itself?

many thanks.

Alex You, CCIE#6138

Community Member

Re: IOS Buffer Tunning issue

Hi, hope the following info will be helpful:

Buffer Tuning

Below is an example from the show buffers command:

Middle buffers, 600 bytes (total 150, permanent 25):

147 in free list (10 min, 150 max allowed)

61351931 hits, 137912 misses, 51605 trims, 51730 created

91652 failures (0 no memory)

In this example, the middle buffers have a lot of failures. This is not a serious problem as it represents only 0.1

% of the hits. These numbers should easily be improved with some buffer tuning.

Depending on the architecture of the router, the buffers you tune generally belong either to I/O memory

(low−end) or main memory (high−end). Before tuning the buffers, first check if you have enough free I/O or

main memory using the first lines of the show memory command.

Here are some general values that you can use:

permanent: take the number of total buffers in a pool and add about 20%. ·

min−free: set min−free to about 20−30% of the permanent number of allocated buffers in the pool. ·

max−free: set max−free to something greater than the sum of permanents and minimums. ·

In the buffer tuning example above, we could enter these commands in the configuration:

buffer middle permanent 180

buffer middle min−free 50

buffer middle max−free 230

Normally, Cisco IOS® software creates buffers dynamically so that these settings should be fine. However, in

case of a traffic burst, the router may not have enough time to create the new buffers, and the number of

failures may continue to increase.

There are two kinds of traffic bursts:

Slow burst: In this case, the router has sufficient time to create new buffers. Increase the number of

min−free buffers. By using free buffers, you can reach the min−free value and then create new



Fast burst: With fast traffic bursts, the router does not have enough time to create new buffers, so you

should use the free buffers. To do this, modify the number of permanent buffers.


Conclusion: If the create counter increases after the initial tuning, increase the min−free (slow burst). If the

failure counter increases, but not the create counter (fast burst), increase the permanent value.

Community Member

Re: IOS Buffer Tunning issue

Well, for detailed info, please check

Community Member

Re: IOS Buffer Tunning issue

hi Hao, thanks. but I think you URL did not answer my questions.

I compared two copy of "show buffer" information about two 3662 routers, one have 64MB DRAM and another have 128MB. I found their buffers have no different. so I think my second question have answer already.

Cisco's SE ken told me, these big packets maybe come from router's ethernet, that's mean they were come from switch <- Server. because of router's interface only fragment output packets to match their MTU. if the applications make a big packets, they can tranfer it into router. please confirm it for me.


Alex You, CCIE#6138

Community Member

Re: IOS Buffer Tunning issue

Not sure about that, maybe Cisco SE's deduction is right. I haven't found any good means to judge exactly where the large and huge buffer failures come from. Well, if it's possible to get any hint from the Output Interpreter?

By the way, from the document, I can find there's an explanation regarding 3600 series router's buffer. Cisco 3600 Series Routers use particles. The interface buffers are atomic buffers, called particles, into which the packets are split. When a packet cannot be fast-switched, the router has to reassemble it in one system buffer because the process switching code cannot handle particles. And private particle pools are used by the interfaces and should not be tuned. When no buffer is available in the free list, the router falls back to the public particle pools. Note that public particle pools cannot be tuned.System buffers are used for process switching. On the Cisco 3600, all these buffers are in the I/O memory which is located in DRAM. You can specify the amount of I/O memory using the memory-size iomem command.

I am not sure if the character of 3600 mentioned above will have any effect on the allocation of system buffer.

So i am looking forward to get any suggestions on this topic too. Thanks!

VIP Purple

Re: IOS Buffer Tunning issue

Hello Alex,

can you hook a sniffer into the switch ? Maybe then you can trace the source of these large packets. What Ken says sounds ok, the router fragments the packets to I think 1544 in order to be able to send them out.

There is a free sniffer available at which works well.



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