EoMPLS Problem

Unanswered Question
May 29th, 2008

Would like to seek your help to resolve one EoMPLS problem.

My EoMPLS topology is-


I had configure Ethernet over MPLS on subinterface between PE1 & PE2. P1 and P2 connected via E1 (Encap PPP). On LDP path, MTU is 1530. MPLS L2 VC is up, I can ping CE - CE upto ping size 1480 byte, but not able to transfer any file. In FTP session between CE-CE, can receive only file size upto 1024 byte. I had also try MTU 1560 on the LSP, but result is same.

If I use FastEthernet between P1 and P2, everything works fine with MTU 1530.

How to resolve the issue of the above topology?



I have this problem too.
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amitbatra Sun, 06/01/2008 - 01:47

HI Mate,

i had same issue in my network. well i dont know if this solution helps you or not. after you change the MTU restart the router.

please let me know how did it go ?



qmesbah Sun, 06/01/2008 - 04:07

Hi Amit,

Today i found that, if i use Windows Vista on both CE side, FTP working fine from both end.

Windows Vista in one side and Windows XP in other side - XP can FTP from Vista but Vista cannot FTP from XP. Vista doing some Auto Tuning. I had also change MTU (1400, 1360)for VPN and PPP in Windows XP, but no result. I think the issue is the packet segmentation.

amitbatra Sun, 06/01/2008 - 04:18


well that tricky. if possible capture the traffic and see if the DF bit is on.

shivlu jain Sun, 06/01/2008 - 23:32


Only thing what you have to do is create a route map in which you have to map the traffic and fragment it and bind it to the ethernet or incoming port.

access-list 101 permit ip any any

route-map JUMBO

match ip-address 101

set df 1

interface x/x

route-map JUMBO in



amitbatra Mon, 06/02/2008 - 00:32

Hi Shivlu,

my friend. i dont think he should be doing this .This will set DF for all the packets . which make packet ineligible for fragmentation.

is there any particular reason why you are recommending this ?


mohammedmahmoud Mon, 06/02/2008 - 22:51

Hi Mesbah,

First lets agree that your core needs to accommodate the extra bytes added for labels allover the LSP, secondly are you filtering ICMP any where within your path. By default PMTUD is operational on Windows platforms, this means that all outbound TCP traffic will have the DF bit set, but if you are not filtering ICMP any where along your path thus PMTUD should work with no problem. One thing to note, the difference between Vista and XP regarding this issue is that Vista has a feature called PMTUBHDetect (Black Hole Router Detect) enabled by default (it can be manually enabled on XP, and more over SP3 enables it by default).





Mohammed Mahmoud.

biddut_link3 Tue, 06/03/2008 - 07:42

Hi Mahmoud,

your solution is pretty close. When we set PMTUBHDitect and PMTUDiscovery in registry of XP. It is working only when we are using router as a CE. Inside the CE routers Windows XP are working with these registry settings. But when we are using direct PC with PE router it does not work.


F.M.Rashed Amin


qmesbah Tue, 06/03/2008 - 20:05

Hi Mahmoud,

Thanks for your post reply. We had enable LDP, TE and L2 MPLS. All over the LSP we had set "MPLS MTU 1530". Can we use MSS on PE router to reduce the packet size?



mohammedmahmoud Tue, 06/03/2008 - 23:21

Hi Mesbah,

Actually manipulating the MPLS MTU allover the path should do the job for you, anyway please do check the attached document, i've tried to have a single document that covers all the MTU, TCP MSS, PMTUD stuff covered, it is still in progress but i believe it'll help you understand most of the story.


Mohammed Mahmoud.

qmesbah Wed, 06/04/2008 - 07:29

Hi Mahmoud,

Thanks for your document.

We had change MPLS MTU 1546, 9192, 9196 allover the LSP.


But no result.

FTP working in one way, FTP from both side is not working.



mohammedmahmoud Wed, 06/04/2008 - 10:47

Hi Mesbah,

You are very welcomed, can you please post your configuration.


Mohammed Mahmoud.

kevin.shi Thu, 06/05/2008 - 02:55

try to add 'ip tcp adjust-mss 1460' in subinterface on each PE to see if it helps.

Or use DrTCP on PC to small MTU size.

qmesbah Fri, 06/06/2008 - 09:59

Hi Kevin,

MSS value in subinterface on each PE does not help.



mohammedmahmoud Tue, 06/10/2008 - 23:07

Hi Mesbah,

Sorry for the late reply, i was very busy the last couple of days, i'll do test your configuration in my lab as soon as possible and feed you back.


Mohammed Mahmoud.

mohammedmahmoud Fri, 06/20/2008 - 11:49

Hi Mesbah,

Sorry for the late reply, i just had very urgent things to take care of, please do accept my apologies. The configuration you've posted is very straight forward and seems to have no problem, can you please describe the specific symptoms and the configuration done on the switches.


Mohammed Mahmoud.

qmesbah Sun, 06/22/2008 - 22:11

Hi Mahmoud,

On switch we had configure Vlan same as PE router, port connected to PE configured as Trunk and port connected to PC configured as access for that Vlan.

Problem is, when we try to transfer file between both end PC, can transfer file smoothly from one side only, from other side transfer speed is very slow and ftp connection drop after transfer of 1024 byte.

There is no ACL and Rate-limit on the LSP.

mohammedmahmoud Mon, 06/23/2008 - 06:30

Hi Mesbah,

I've labed your exact setup and it worked perfectly (the only difference is that i've used ATM interfaces rather than the serial interfaces, but its not a big deal), can you elaborate on the systems part.

[Sorry for the late replies i am currently in the middle of an important project, which is keeping me extremely busy]


Mohammed Mahmoud.

qmesbah Mon, 06/23/2008 - 21:56

Dear Mahmoud,

Thanks a lot for your effort and time.



a.alekseev Mon, 06/23/2008 - 23:30


in any case you have a problem with MTU.

You break MTU1500 along the path.

Try to determine what is the maximum ip packet size with df-bit set you can transfer along the path.

your 7206VXR,NPE300 and 3640 are very old.

They cannot preserve MTU on the path. Ethernet controllers have restriction in hardware. They were created to support (accommodate) ethernet frames + 802.1q or ISL. But it's not enough for EoMPLS tunnel.

As alternative you can use L2TPv3 - which will let you do fragmentation.


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