Enabling Path MTU Discovery in MikroTik, or why my PPPoE/6rd was slow

For many years, I’ve stuck with OPNsense, first initially since until a couple of years ago I was a die-hard FreeBSD user. But more importantly, by default Linux-based firealls play poorly with CenturyLink’s 6rd.

I’ve been wanting to use a MikroTik as my core router instead of OPNsense for many years, but whenever I tried, 6rd browsing was just so slow for some reason.

A few days ago, I got myself a MikroTik CCR2004-16G-2S+ and intially went IPv4-only. But felt very guilty about it, and wanted my IPv6 back. I did some research, and found out that unlike OPNsense, MikroTik didn’t enable Path MTU Discovery by default.

To fix it, add the following config (source: forum article above):

/ipv6 firewall mangle
add action=change-mss chain=forward new-mss=clamp-to-pmtu passthrough=yes \
    protocol=tcp tcp-flags=syn

Path MTU Discovery for some reason isn’t enabled on Linux-based distributions, not just MikroTik but also OpenWrt and Netgear Orbi’s stock firmware.

While I won’t be needing this configuration for very long, I’m moving back to NYC and will likely have Verizon Fios without the PPPoE and 6rd nonsense. But in the meanwhile, do we really want more IPv4-only traffic?

But had I known, I could’ve gone with MikroTik all along instead of settling for inefficient x86 boxes. Instead, I once even dealt with a psycho eBay seller for firewall hardware that could’ve been avoided if I read the linked post a year back.