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Inssomniak
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MPLS over GRE MTU question

Sat Jun 22, 2019 7:28 am

So if Im using MPLS over a GRE Tunnel, do I adjust the GRE MTU to match the MPLS MTU ? or does it use the L2MTU like any other interface? I need 1522 byte MPLS MTU over a GRE tunnel without fragmentation, which states the L2MTU at 65535. So How do I know the GRE Tunnel isn't fragmenting the MPLS packets? What L2MTU do I go by?
 
sup5
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Re: MPLS over GRE MTU question

Sat Jun 22, 2019 2:26 pm

How is MPLS over GRE supposed to work anyways?

MPLS relies on injecting labels between MAC and IP.
That's why it is called Layer-2,5 sometimes.
GRE-payload is IP only, so I don't see a possibility to transport MPLS labels to establish an LSP.

However it will work with proprietary EoIP, which is some kind of Layer-2-over-GRE.
L2TP with BCP might work, too.
 
Inssomniak
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Re: MPLS over GRE MTU question

Sat Jun 22, 2019 5:18 pm

Well it seems to work ok, and I asked around about it first before trying, and many others said it works fine. Just unsure about the MTU.
 
mducharme
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Re: MPLS over GRE MTU question

Sun Jun 23, 2019 1:03 am

Well it seems to work ok, and I asked around about it first before trying, and many others said it works fine. Just unsure about the MTU.
MPLS works fine over GRE or EoIP. Only the L2MTU matters, and for GRE the L2MTU is 65535 and you can't change it.

The only disadvantage of GRE vs EoIP for MPLS is that you can't do MPLS QoS with GRE.
 
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Re: MPLS over GRE MTU question

Sun Jun 23, 2019 1:45 am

can we see you config?
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Inssomniak
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Re: MPLS over GRE MTU question

Tue Jun 25, 2019 12:56 am

Well it seems to work ok, and I asked around about it first before trying, and many others said it works fine. Just unsure about the MTU.
MPLS works fine over GRE or EoIP. Only the L2MTU matters, and for GRE the L2MTU is 65535 and you can't change it.

The only disadvantage of GRE vs EoIP for MPLS is that you can't do MPLS QoS with GRE.
But there is a physical L2MTU, depending on the parent interface that the traffic is leaving from, correct? When does GRE start fragmenting the L2 stuff? It's long before 65535, obviously.
 
mducharme
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Re: MPLS over GRE MTU question

Tue Jun 25, 2019 1:19 am

But there is a physical L2MTU, depending on the parent interface that the traffic is leaving from, correct? When does GRE start fragmenting the L2 stuff? It's long before 65535, obviously.
GRE is 24 bytes overhead - assuming your GRE tunnel is running over 1500 IP MTU, subtract 24 bytes for the GRE overhead.
 
Inssomniak
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Re: MPLS over GRE MTU question

Tue Jun 25, 2019 1:34 am

But there is a physical L2MTU, depending on the parent interface that the traffic is leaving from, correct? When does GRE start fragmenting the L2 stuff? It's long before 65535, obviously.
GRE is 24 bytes overhead - assuming your GRE tunnel is running over 1500 IP MTU, subtract 24 bytes for the GRE overhead.
So you're saying the MPLS L2MTU is the GRE tunnels' 1500-24? So I have to use the IP MTU of the GRE tunnel to determine the L2MTU of MPLS?
 
mducharme
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Re: MPLS over GRE MTU question

Wed Jun 26, 2019 7:45 am

So you're saying the MPLS L2MTU is the GRE tunnels' 1500-24? So I have to use the IP MTU of the GRE tunnel to determine the L2MTU of MPLS?
A GRE packet is never going to grow above the IP MTU of the interface that it is sent over. I am assuming you are probably using IP MTU 1500 on most interfaces that GRE tunnels would be sent over, and therefore 1500-24 is the largest payload that a GRE packet could have before requiring fragmentation.

You can use a very large L2MTU if you like for your VPLS tunnels that run over the GRE tunnel - don't feel like you are limited to below 1500. It is simply the case that if you go above 1500 when you add together the VPLS overhead and the GRE 24 byte overhead that it will be fragmented and recombined on the other end.

We use GRE and EoIP tunnels all the time over layer 2 connections that don't have sufficient L2MTU for us to run MPLS properly. Running MPLS over the EoIP tunnel instead of the Layer 2 connection itself means that we can provide our customers a full 1500 MTU instead of a reduced MTU. Although there is a slight performance degradation due to the fragmentation and recombination of the frames, we do not generally have very high speed packages over such links.

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