Multiplexing and packet loss

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Hi

I have a 1gbit (1000BaseZX) link between two sites that's, due to
large number of small packets, have alot of buffer related packet loss
in one direction. A solution I'm planning to implement is CWDM on this
link and team at least four interfaces into an etherchannel. I'm
however unsure if this will actually solve any of the packer loss
problems. In theory four interface buffers should be better than one
but I wounder if anyone have any real world data on this. Will a CWDM
etherchannel only move the buffer problems from the interface to the
etherchannel buffer?

Regards
Fredrik

Re: Multiplexing and packet loss
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One thing to consider is the path selection algorithm used
by etherchannel. The options vary across cisco platforms and
range from dest mac whereby say all traffic to a  particular router
uses the same path through src/dst mac and also IP addresses.
I have the idea that some platforms can use TCP/UDP ports but
I forget.

Depending on the nature of the traffic you may get no load
distribution at all in the worst case. There is no option
to balance traffic depending on the individual link loads.

If you can change to 4 routed links there may be a per packet
load balancing regiem available that will be supported at
full speed. Beware though that some per packet load balancing
schemes enormously affect the throughput (factor of more than 10).
Behaviour depends very much on platform.

Another alternative may be some sort of packet prioritisation (QoS).

If you provide details of the platform in use, software version,
output that shows the problem, then perhaps someone will come
up with a some suggestions. Oh, and the nature of the traffic.
e.g. Voice will mostly have small packets.





Re: Multiplexing and packet loss
Hoffa wrote:
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What kind of hardware are you using on this link? And if it's 6500
platform - what kind of line card?

Regards,
Andrey.

Re: Multiplexing and packet loss
How did you come to the conclusion that the packet loss is due to a large
number of small packets?  Even if that were the case, I very seriously doubt
using CWDM to multiplex this into 4 interfaces would solve the problem and
would create other issues.  You probably just need to do some simple buffer
tuning.

Post the output of  "show version", "show interface" and "show buffers"


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Re: Multiplexing and packet loss
On Tue, 21 Apr 2009 05:13:52 -0700 (PDT), Hoffa

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you need to understnad the loss mechanism.

If you are running out of bandwidth, then CWDM may help.

if the device driving the link cannot cope with the number of packets,
then giving it more bandwidth to drive is likely to make it worse.

So equipment, traffic profile and details?
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--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl

Re: Multiplexing and packet loss
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Thank you for the input. I'll give as much technical info as possible.
I've done some packet sniffing on the link and it's easy to see the
number of 64byte packets coming in floods on the interface. The source
of the packets is a server application cluster located at both ends of
the link and they are sending updates back and forth and the out on
the Internet.
Switch: 6513 sup720-3B
Line card: WS-X6516A-GBIC

Thrill5: What kind of buffer tuning might provide a solution? I was
under the impression that one should leave outgoing buffers and queues
alone.

Regards
Fredrik

Re: Multiplexing and packet loss
The default buffer allocations are fine 95% of the time, but on some links
you need to adjust them because of your specific traffic, especially on high
bandwidth, highly utilized links.  A "show interface" and "show buffers"
will make it very obvious if buffer tuning is needed.  The fact that you
have lots of small packets makes it very likely that you need to increase
the default buffer allocations.   If you post the above requested outputs I
can give you recommendations as to how to increase the buffer sizes.


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Re: Multiplexing and packet loss
Hoffa wrote:
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According to

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet0900aecd801459a7.html

this line card has only 1MB buffer per port. Recommended replacement -
WS-X6748-SFP or WS-X6724-SFP - has 1.77MB on egress, so it's 6 of one,
half a dozen of the other.

Question is - are you experiencing tail drops in egress queue or fabric
drops? Can you post show interface? Number of output drops and its
ration to total traffic is most interesting here.

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Nothing can be done here - it's hardware based queuing. Thrill5 most
likely assumed 7200 or similar platform.

Regards,
Andrey.

Re: Multiplexing and packet loss
wrote:

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also check the inbound queues.

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you can also get issues on input, since there is buffering needed
between the blade in the switch and the forwarding engine.

i agree the 6724 is a better blade to use, but mainly because it uses
the fabric rather than the shared bus, so it wont contend with other
traffic on the bus.

the fabic tap gives a 20 Gbps channel shared by the 24 GigE ports -
which doesnt sound like much over subscription.

However, the cisco hardware wraps every packet in extra control info
as it crosses the fabric link - so esp with min size packets the
useable bandwidth is only maybe 70 to 75% of that.

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--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl

Re: Multiplexing and packet loss

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http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet0900aecd801459a7.html
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Yup, your correct!!!   A 3750 Metro switch would be a good platform to use
for this application.

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Re: Multiplexing and packet loss
I suggested a 3750 Metro switch because this switch has two gig L3
interfaces (like a router interface).  Gigabit switching interfaces on a
6500 don't have the same queuing and QoS capabilities and a true L3 routed
interface does.  Since you don't need this capabilites, I'm still puzzled as
to why you are seeing traffic drops.   Are you seeing output drops or input
drops?



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