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Author Topic: MTU MANIPULATION ISSUES NEED YOUR GUIDE  (Read 3078 times)

Offline adecisco

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MTU MANIPULATION ISSUES NEED YOUR GUIDE
« on: March 21, 2014, 06:21:42 AM »
Hi there,

I am simulating an MPLS environment in other to understand how one can allowed desire MTU not to defragment.

From my topology R1 and R8 are CE and want to send minimum MTU size of 1600. Between interface MTU and IP MTU and MPLS mtu I am wondering how the manipulation can be done.

Thanks in advance.
Technology makes life easy but I hope the same technology will not send man back to stone age!

Offline MC

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Re: MTU MANIPULATION ISSUES NEED YOUR GUIDE
« Reply #1 on: March 21, 2014, 11:00:33 PM »
By default Ethernet interface MTU is 1500. If you want to send a packet size 1600 bytes without fragmentation, you need to increase all non-MPLS interfaces using 'mtu' command to 1600 and MPLS interfaces to 1600 + (4 bytes * number of label). Instead, you can also use 'mpls mtu' command (ie. increase MRU) for all MPLS interfaces as well and keep the MTU the same at 1600.

Offline ModSheVit

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Re: MTU MANIPULATION ISSUES NEED YOUR GUIDE
« Reply #2 on: October 09, 2014, 12:56:16 AM »
How can I know Everything same you?

Offline adecisco

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Re: MTU MANIPULATION ISSUES NEED YOUR GUIDE
« Reply #3 on: October 09, 2014, 03:13:03 AM »
After playing around on this for a while my final submission is this rule of thumb for the issues are these:

1. MTU interface is the total MTU of the frame which include MPLS MTU plus IP MTU.

2. MPLS MTU is use to ensure that the total MPLS packet size will not exceed the configured MPLS MTU. This situation is important when you have restriction configuring the ETHERNET MTU. And this include IP MTU.

3. IP MTU is the total size require for IP packet before adding MPLS MTU.

Therefore:

ETHERNET MTU = ETHERNET HEADER + TAIL +MPLS MTU + IP MTU

MPLS MTU = MPLS HEADER + TAIL + IP MTU

IP MTU = IP HEADER AND THE REST..

Hope this clears thing..but if you have contrary opinion I will appreciate your input..all is for the learning..

Thanks,
Technology makes life easy but I hope the same technology will not send man back to stone age!

Offline MC

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Re: MTU MANIPULATION ISSUES NEED YOUR GUIDE
« Reply #4 on: October 10, 2014, 09:53:25 AM »
Thank you for the explanation adecisco. Although I don't think Ethernet MTU is an official well-known terminology as MTU usually refers to L3 packet size excluding L2 frame header. I think it should be referred to just frame size  :)   

Offline adecisco

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Re: MTU MANIPULATION ISSUES NEED YOUR GUIDE
« Reply #5 on: October 11, 2014, 05:10:57 AM »
Yeah you are right at that brother.. ;)
Technology makes life easy but I hope the same technology will not send man back to stone age!

 

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