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DMVPN MTU Problem

TatianaWarby
Level 1
Level 1

We have been seeing some errors related to the MTU 1400 setting on the tunnel interface - websites hanging (not all, but some, especially those with flash, like camera interfaces), linux workstations unable to establish ssh connection, remote desktop dropping randomly. Manually setting a lower MTU (1286 did it for the linux boxes) on the client side fixes those issues. However, its not realistic to manually set MTUs on all devices.

How should we go about a permanent fix? What is the recommended MTU setting for the DMVPN tunnel? 

1 Reply 1

MARTIN CHONG
Level 1
Level 1

I'd recommend using the following on your tunnel interface:

 

ip mtu 1372

ip tcp adjust-mss 1332

 

These are generally good for most networks. If you are still experiencing oversized packet drops or excessive fragments, you can discover the correct MTU for yourself quite easily using ping and the DF bit.