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nichky
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MPLS MTU Calculations

Tue Dec 26, 2017 1:37 am

Just im wondering how can i calculate MTU on different case and scenarious.

there two tags of MPLS, what a different? (on the picture)

Eth header 14, MPLS 4, VPLS ID 4, VPLS 4, IP header 20, data 1480 = 1526 bytes.
Whats a different between VPLS and VPLS ID?

How to check MTU size?
ping 10.88.88.2 size=1472 do-not-fragment
- VPLS tunnel between 2 routers.
The question is ping from the second VPLS tunnel or from the clients side, connected on that tunnel?

Thanks a lot :)
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tomaskir
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Re: MPLS MTU Calculations

Tue Dec 26, 2017 11:10 pm

Check out this presentation for an in-depth discussion of MTU (and in particular in regards to MPLS/VPLS).

https://youtu.be/Q8AF-Srulmk
 
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nichky
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 12:30 am

Thanks Tomas :)

okey....one of tham is MPLS main tag, other one is QoS tag.

What about VPLS and VPLS ID, and how can i calculate? by difficult is 1500, so just because you have PPPoE add + 8 to be 1508
What is VPLS ID?



Thanks
 
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tomaskir
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 12:34 am

VPLS ID is the VPLS tag (it contains the tunnel ID).

A VPLS tag is just another type of MPLS tags - so also just 4 per VPLS tag.
 
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nichky
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 12:47 am

So when i'm making decision about MPLS MTU i have to collect all thoese values MPLS 4, MPLS 4, VPLS 4, VPLS ID 4.
That's why you got 1550.

Thanks
 
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tomaskir
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 1:07 am

You need to calculate how much you need at every layer.
(like on slide 19 of the presentation)

If you have 4 tags, then you need to calculate that into the MPLS layer MTU, and MTUs on all underlying layers.
 
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nichky
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 1:32 am

Just one thing what will be if the MPLS MTU is overflowing.

Let sey in my case will be enough 1526, because i'm not using pppoe, vlans..etc. but for safety in playing with 1550 at all devices.

It will works like that as we are expecting?
 
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 1:54 am

It will work if MTU is sufficient, or higher.
I can be higher, that will not hurt.

But it MUST NOT be lower than required.
Last edited by tomaskir on Wed Dec 27, 2017 4:17 am, edited 1 time in total.
 
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nichky
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 4:12 am

Does it means same as VPLS,no problem with higher L2MTU?
 
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tomaskir
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Re: MPLS MTU Calculations

Wed Dec 27, 2017 4:18 am

Yes, but do not forget to properly calculate all other MTUs so MTU is sufficient on every layer.
 
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Re: MPLS MTU Calculations

Mon Nov 26, 2018 4:40 pm

I have done like this with two SXT G-5HPacD but MTU is wrong.
C:\Users\Jean-Pierre>ping www.yahoo.com -f -l 1530

Pinging atsv2-fp-shed.wg1.b.yahoo.com [87.248.98.8] with 1530 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.


C:\Users\Jean-Pierre>ping www.yahoo.com -f -l 1534

Pinging atsv2-fp-shed.wg1.b.yahoo.com [87.248.98.8] with 1534 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 87.248.98.8:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),


C:\Users\Jean-Pierre>ping www.yahoo.com -f -l 1522

Pinging atsv2-fp-shed.wg1.b.yahoo.com [87.248.98.8] with 1522 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 87.248.98.8:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

Who can explain that. How that works.
Thanks
 
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Re: MPLS MTU Calculations

Mon Nov 26, 2018 6:36 pm

in Windows ping -f -l 1472 = 1500MTU
1500 bytes (Ethernet mtu) - 20 byte (IP header) - 8 byte (ICMP header) = 1472 byte
 
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Re: MPLS MTU Calculations

Mon Nov 26, 2018 11:06 pm

Do I have to set 1472 mtu on the SXTs?
Thank you for your quick response
 
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 10:28 am

1472 is IP interface MTU.

Question about a picture of topic starter: why there is two mpls header + vpls header(???)

In wireshark first mpls header is label of LDP protocol and second of pseudowire of vpls interface, why third label called vpls? vpls traffic contains only two mpls header 4 bytes each.(without counting ethernet and vlan before pseudowire)
 
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 11:55 am

+ control word (if it is enabled)
 
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Anumrak
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 11:59 am

+ control word (if it is enabled)
It's a part of mpls header, isn't it?
 
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mrz
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 12:09 pm

 
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Anumrak
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 12:21 pm

Control word is additional 4 bytes
https://wiki.mikrotik.com/wiki/Manual:V ... _CW_Format
Oh, I see. I didn't use it, that's why I didn't see it in dump. Thanks.
 
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Re: MPLS MTU Calculations

Tue Nov 27, 2018 6:41 pm

What is the correct MTU for this setup pleace?
How do I check in Wireshark that the packages are not fragmented ?
Thank you very much for this explanation
 
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Anumrak
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Re: MPLS MTU Calculations

Wed Nov 28, 2018 10:32 am

What is the correct MTU for this setup pleace?
How do I check in Wireshark that the packages are not fragmented ?
Thank you very much for this explanation
I think that mpls packet will be dropped simple because there is no fragmentation offset in it's header. Just same as ppp.

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