Hi all.

Just an update on this... it did turn out to be an MTU issue which I've been working on since last year, November. The trick was finding the right combination of settings between my Mikrotik home router and one of our Cisco ASR1006 edge routers in my backbone that terminates the 6-in-4 tunnel.

After testing this at the office, I was, then, sure that the problem was MTU-related as it only occurred at my house.

My FTTH service is delivered over GPON, and after a bit of testing, concluded that my MTU for IPv4 is 1,452 bytes. Across the 6-in-4 tunnel, the tested MTU is 1,232 for IPv6.

The Mikrotik will not pass any IPv6 traffic if the 6-in-4 tunnel does not have the minimum default MTU for IPv6, i.e., 1,280 bytes (even if the tunnel cannot actually transport 1,280 byte-sized packets). This is not documented anywhere, so it took a while to figure out.

On the Cisco, you can't configure an IPv6 interface MTU lower than 1,280 bytes... but that is within the standard IPv6 spec., so no major drama.

So the right combination of settings is to have 1,280 bytes on the Mikrotik and enable "ipv6 tcp adjust-mss 1232" on the Cisco (the latter for my specific case - yours may vary depending on your IPv4 conditions).

Just wanted to update this thread in case someone else runs into this issue.

Thanks for the clue, Mikael and all.

Mark.

On 13/Nov/18 12:38, Mark Tinka wrote:


On 12/Nov/18 20:34, Mikael Abrahamsson wrote:

 

Are you doing TCP MSS adjust/clamping? If you don't, try that and see
if it helps. This might be a PMTUD issue.

Otherwise if possible, try lowering the MTU sent in RA to the one you
have on your tunnel (this depends on if this is available to you in
your RA sending device).

Thanks, Mikael.

I'll have a sniff and see of this helps.

Mark.