MTU issue (not classic one)

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Feb 23rd, 2009
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I got a LES circuit b/t side A and side B. side A is a trunk port connected to carrier. B is CPE of cisco 2611. all ports are Ethernet with defalt MTU size 1500.

From A ping B, ping test is fine with whatever MTU size, but there is packet loss when doing ping from B to A with MTU higher than 1500.

could anyone shed some light please? thanks

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Giuseppe Larosa Mon, 02/23/2009 - 12:50
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Hello Kevin,

try to use the sweep option in extended pin in this way you can measure the range of sizes without answer.

There is the possibility that if packet is bigger then 1500 bytes but less then 1500+x a device in the middle tries to send it without fragmentation.

in this case you get bad results in the range 1500, 1500+x and then again ping is successful

Let's say is a test that can help.

Hope to help


kevin.shi Tue, 02/24/2009 - 09:29
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the thing is I only got one way packet loss. would I suspect its CPE issue since it seems could not handle fragmentation properly which cause the issue?

Mohamed Sobair Tue, 02/24/2009 - 11:06
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There is lots of factors can cause this issue, but the main concept is:

If you ping with Large MTU packet size (larger than 1500), with (DF) not set, then the packet has to be fragmented , the fragmentation can cause delays.

The default timeout in Cisco router is (2) seconds, and if the delay is larger than those (2) seconds you will see . instead of !.

This doesnt necessarily mean the packet is not transmitted at all, it just means there is no reply within thoses 2 seconds.

Try to increase the timout or decrease the packet size from that path.



mahesh-gohil Wed, 02/25/2009 - 02:20
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Hi Mohamad,

What I have observed that one of my router is connected to switch and via Media to another switch

Router (MTU=1546 default)-Switch (MTU:1546)-Media-Router (MTU:1500)

In above case packet with datagramsize of 9000 or more can easily pass with few drops.

but when MTU change as below

Router (MTU=9170 default)-Switch (MTU:1546)-Media-Router (MTU:1500)

it seems packet above 1500 datagram is totally dropped.

Please guide as what could be issue

kevin.shi Thu, 02/26/2009 - 07:05
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Thanks Mohamed,

Let me say its delays. but 2 seconds delay would not accept as well.

My concern is why I only got one way packet loss/delay. and it only happens on the PING packet size over 1500,(1600 eg.) would it link to CPE performence problem??


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