MTU - MPLS DS3 Setup Question

Answered Question
Feb 13th, 2008
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I'm not sure if I am in the correct section.


We have an MPLS network with DS3s. From what I understand (and correct me if I am wrong), MPLS puts 4 bytes on the packet. Our serial has an MTU size of 4470. Our Ethernet has an MTU size of 1500.


Do I need to change the MTU size, and if so on which interface do I need to make this change. My thinking is to change it on the serial interface (MTU => 4466).


Am I right in my assumption?

Correct Answer by Paolo Bevilacqua about 9 years 1 month ago

When doubts with mtu size, try pings with DF bit set, size 1500 end-to-end.



Correct Answer by Paolo Bevilacqua about 9 years 1 month ago

Hi,


first of all, if you are an MPLS user, there is no four bytes, just regular IP packets.

Differently if you were an MPLS SP.


Then, you don't need to change anything anyway, the large MTU on the serial is never used, but leave it as it is anyway.


Hope this helps, please rate post if it does!

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Correct Answer
Paolo Bevilacqua Wed, 02/13/2008 - 10:01
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Hi,


first of all, if you are an MPLS user, there is no four bytes, just regular IP packets.

Differently if you were an MPLS SP.


Then, you don't need to change anything anyway, the large MTU on the serial is never used, but leave it as it is anyway.


Hope this helps, please rate post if it does!

dphills18 Fri, 02/15/2008 - 07:22
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Thanks for the reply. We are having issues with a database query across the MPLS cloud, and I can't seem to put my finger on the issue. The query works fine on the LAN, but not via the MPLS cloud. I was thinking that it may have something to do with the MTU size.

Correct Answer
Paolo Bevilacqua Fri, 02/15/2008 - 07:36
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When doubts with mtu size, try pings with DF bit set, size 1500 end-to-end.



dphills18 Fri, 02/15/2008 - 11:42
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hey man, that is awsome. thanks for the reply. never really knew about that. i always ENTER through that part of the extended ping. thanks again.


by the way, my problem is not with the MTU. kind of hoping that it was though. :(

Paolo Bevilacqua Fri, 02/15/2008 - 11:55
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Yep, there's even a short way;

ping xxx df size 1500 (requires recent ios)


now, about the strange problem, does the application works a little and slow, or not at all ?



dphills18 Fri, 02/15/2008 - 13:23
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only on this huge once a month query. i checked all the ports in the path (1000 full & DS3), and everything looks good to me. i'm not sure what i'm missing.

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