MTU Cisco and Juniper

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Sep 20th, 2010
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Hello we have site to site connetions on a layer2 domain.


cisco3560<----Layer 2 Hardware Encrytor-->ME3400-2CS-A<---Dot1Q tunnel--->ME3400-2CS-A<---LAyer 2 Hardware Encryptor---->Juniper EX 4200

I-am receiving Latencies from copynig from clients on juniper to clients to Cisco. The other side is ok.

MTU Jumbo on ME 3400 is 1800. Link is Gigabit. I havent changed the mtu on Encryptor and on Cisco nor Juniper .

As I googled and saw Juniper and Cisco have diffrent MTU 1518 on Juniper and 1500 on Cisco.

On Wireshirek I see TCP Previous segment lost, TCP Duplicate and TCP Fast retranssmission.

Any advice for this situation you think this is Fiber problem or MTU.

To be Clear everything worked k isnce two weeks before


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Giuseppe Larosa Mon, 09/20/2010 - 07:03
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Hello Enkli,

on EX 4200 1518 refers to the L2 MTU that is the ethernet frame size, so this shouldn't be a problem as ip mtu is 1500 bytes for both

Hope to help


enkli Sat, 09/25/2010 - 11:05
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Giuslar thank you very much for you reply.

To be more clear. The design is like that

(A)C3560<-->Hardware encryptor<--->dot1Q ME3400G<--DarkFiber-->dot1Q ME3400G<--->Hardware encryptor<--->C3560(B)






                                                                                                         Dot1Q ME3400G


                                                                                                    Hardware encryptor


                                                                                                 Juniper EX4200(C)

Everything was fine before

Now I am able to copy in full speed from any computer at site C to/from site B and vice versa

Now I am able to copy in full speed from any computer at site A to/from site B and vice versa

Bu I can copy form site C and A but I am not able to cop[y to site A. I see loss of packets even inside VLAN

Have you any idea her might be the problem in the interconnecting switch ME3400 at junipers might be some loop cause i dont see packets on trunk interface of Juniper outgoing just 1 or 2 Mbps. I have done test on fiber from telecom and they say it is fine.



Reza Sharifi Sat, 09/25/2010 - 11:14
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Hi Enkli,

What type of encryption device are you using?


enkli Sat, 09/25/2010 - 12:13
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I dont think there might be a problem. we are using Cipherop

tics layer 2 encryptors

Ali Norouzi Wed, 12/22/2010 - 23:02
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Do you have problem from site A to C? Have you bypassed encryptors to make sure about them? Have you measured path MTU by ping and setting DF value?


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