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Negative side effects of higher MTU on WAN?

Does anyone know of any negative side effects of setting a higher MTU on a WAN?
Say ~1540 instead of 1500.

I'm having an issue where the default 1500 MTU on my WAN causes OSPF to not work, lower MTU causes apps that can't fragment to not work.
It seems like a higher MTU makes everything work.

Outside of small differences in latency or throughput, are there any other negative consequences I should think about?


In case it matters, I'm setting the higher MTU on the WAN interface on my routers(the ISP is matching my settings on their routers), the inside interfaces on my routers are default 1500 MTU.

Thanks,
Jon

2 ACCEPTED SOLUTIONS

Accepted Solutions
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Beginner

One thing to keep in mind is

One thing to keep in mind is MTU has to be enabled end-to-end. Setting MTU on inside interface to 1500 means larger packets will be fragmented even though you have 1540 on WAN side. 

View solution in original post

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Rising star

Looks like your ISP-side is

Looks like your ISP-side is adding some overhead to your packets for transport. You can leave LAN MTU to 1500.

HTH.

View solution in original post

3 REPLIES 3
Highlighted
Beginner

One thing to keep in mind is

One thing to keep in mind is MTU has to be enabled end-to-end. Setting MTU on inside interface to 1500 means larger packets will be fragmented even though you have 1540 on WAN side. 

View solution in original post

Highlighted
Rising star

Looks like your ISP-side is

Looks like your ISP-side is adding some overhead to your packets for transport. You can leave LAN MTU to 1500.

HTH.

View solution in original post

Highlighted
Enthusiast

I am curious to know what

I am curious to know what causes OSPF to not work when the MTU is 1500.

CF

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