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RED Interface MTU same as WAN MTU?

I am using Multiple RED tunnels on our UTM.

Obviously they use the Wan interface on our UTM's so do I set the MTU size on the RED interfaces the same as the WAN MTU size?

Wouldnt an 1500 MTU on an 1492 MTU WAN Interface force the UTM to fragment each Packet?

Thanks in advance for your thoughts.

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0 BAlfson over 7 years ago

What result do you see from:

ping -I 10.10.10.1 10.10.10.2 -s 1500 -M do

Where .1 is the IP on your end of the tunnel and .2 is at the other end.

Cheers - Bob

Sophos UTM Community Moderator
Sophos Certified Architect - UTM
Sophos Certified Engineer - XG
Gold Solution Partner since 2005

MediaSoft, Inc. USA
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0 BAlfson over 7 years ago

What result do you see from:

ping -I 10.10.10.1 10.10.10.2 -s 1500 -M do

Where .1 is the IP on your end of the tunnel and .2 is at the other end.

Cheers - Bob

Sophos UTM Community Moderator
Sophos Certified Architect - UTM
Sophos Certified Engineer - XG
Gold Solution Partner since 2005

MediaSoft, Inc. USA
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0 zoidberg over 7 years ago in reply to BAlfson

Hi Bob,

with:

ping -I 192.168.221.1 (UTM) 192.168.221.2 (Remote PC) -s 1500 -M do

i got:

From 192.168.221.1 icmp_seq=10 Frag needed and DF set (mtu = 1500)

1472 is the only MTU that works.

Interface settings: WAN MTU 1492; RED MTU 1500

Is there a general recommendation for this?
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0 Alexander Busch over 7 years ago in reply to zoidberg

1472, I guess there is a VDSL line related? Maybe on the RED side?

My feeling would be to adjust the RED MTU to that value.

But the question is interesting.

Best

Alex

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0 zoidberg over 7 years ago in reply to Alexander Busch

Hi Alex,

thank you for your post.

I am still wondering why nobody came direct with a clear recommendation.

Kind of a VDSL is included in this construct.. a PPPoE connection.

Maybe the 1472 is resulting from 1492 WAN MTU - 20 byte IP Header.

"My feeling would be to adjust the RED MTU to that value."

->Same here

It would be nice to hear more ideas
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0 Alexander Busch over 7 years ago in reply to zoidberg

Maybe interesting too

So it seems the MTU setting affects only UTM side!?

Best

Alex

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0 zoidberg over 7 years ago in reply to Alexander Busch

Interesting feature request!

I cant say that without further testing.

But it can be, that the RED MTU interface settings in UTM are affecting the local RED. Like for example the vlan interface you can configure on top of an RED interface.
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