Re: [nsp] output buffers swapped out

From: Randall S. Benn http://www.clark.net/pub/rbenn (rbenn@clark.net)
Date: Sat Mar 07 1998 - 10:42:21 EST


At 03:21 PM 3/6/98 +0100, you wrote:
>
>Does anyone know what a very high rate of "output buffers swapped out"
>on a serial link (E1) means?
>
>(Router is a 7507 with FSIP controller and IOS RSP-JSV-M Version 11.2(9))

Pulled the following out of Cisco's Open Forum:

Question: What is the meaning and significance of "nnnn output buffers
swapped out" in the following example?

  nett-br2#sh int s5/0/0:1

  [snip]

  0 output buffer failures, 313684 output buffers swapped out

                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Answer:

It is a feature of the RSP-based Cisco 75xx systems. If the outbound
interface

transmit queue is full, then the packet is copied from a hardware buffer to

DRAM, then copied back to the transmit queue when there is room. If you are

using fancy queuing (such as fair, custom, priority) then you cannot disable

this feature; however, when not using fancy queueing you can disable the
buffer

swapouts with the following interface command:
 

  no transmit-buffers backing-store.

-----------------------------------------------------------------------
Question: What does the "500 output buffers swapped out" mean when doing a
show interface on serial ports?

Answer:

Buffers swapped out is good. That means that when the interface went to

hand off the packet to MEMD (RSP packet buffer memory for the interfaces)

and the interface was at its TX queue limit, the packet was placed into a

system buffer and queued on the interface output queue instead of being

dropped. This process is counted as output buffer swaps. This can be
disabled

with the interface command no transmit buffers backing store.

------------------------------------------------------------------------

Question: In sh int serial, what does "output buffers swapped out" mean?

Answer:

The Cisco 7500 series routers will copy the packet into main memory instead

of dropping the packet. If you want it to behave more like a 7000 series
router,

put the command no transmit-buffers backing-store under the interfaces.

----------------------------------------------------------------------

Question: What does the "transmit-buffers backing-store" message mean?

At the loopback of a Cisco 7000, I get a "transmit-buffers backing-store"
message.

Answer:

The 'backing store' feature was designed to buffer temporary transmit

overloads for an interface. It was requested for network service providers.

 

If 'backing store' is disabled on an interface, we drop the current

interrupt level output packet when we find the output hardware transmit

queue full. This is how the AGS+ and a Cisco 7000 work.

 

In contrast, if 'backing store' is enabled on an interface, and we find the

output hardware transmit queue full, we move the current packet out of

expensive MEMD into a system buffer allocated in DRAM, and then enqueue it

on the output hold queue for later transmission. If the output hold queue is

full, we have to drop a packet anyway. This is how the low end and midrange

products work, except that they don't have to copy the packet because it is

Direct memory accessed (DMA) directly into a system buffer.

 

To disable 'backing store', use the following configuration:

 

        int eth 0/0

        no transmit-buffers backing-store

 

It is on by default for all interfaces.

------------------------------------------------------------------------

Regards,

Randy
http://www.clark.net/pub/rbenn/cisco.html



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