diff --git a/about-mtu.md b/about-mtu.md
index 4f7d0ee..e1582e0 100644
--- a/about-mtu.md
+++ b/about-mtu.md
@@ -2,4 +2,6 @@ udp2raw introduces it's own header for encryption, anti-replay and multiplex. As
 
 Since udp2raw itself doesn't have user space packet splitter, you have to tune the MTU at upper level appliation (for example: openvpn, wireguard, kcptun) to make sure they don't send packet too large for udp2raw.
 
+udp2raw support mtu at most 13xx depends on the upper level application (e.g. what kind of VPN you are using), I suggest to use `1200` as a safe value.
+
 However, if you are not happy with this or your upper level application doesn't allow you to tune MTU, there is indeed a way to by pass the MTU limitation, see [[https://github.com/wangyu-/udp2raw/wiki/bypass-mtu-limitation]]
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