Pros and cons:
Cons of 3G: The 3G dongle is very slow, only about 2Mbps downstream, I can’t remember the upstream, and the low MTU of 1440 bytes is an annoyance. The rare failures where it just disappears are bad.
I don’t notice these because there are no notification messages from AA; clueless.aa.net.uk assumes that all mobile network connections are indeed mobile so loss of signal is assumed to be normal and it is assumed, wrongly, that users do not want to be pestered with useless notifications. This is completely wrong for stationary users and Firebrick dongle users in particular so I have suggested that AA make this a per-link flag/option in clueless. There are already options to control the behaviour of notifications but these are not line-specific. Because of this lack of notifications, my current 3G dongle can fail and I don’t know about it for weeks, until I just happen to inspect things.
The low MTU of the 3G dongle means that during failover, 1500-byte long IPv4 packets will get fragmented or worse, and that is not good. That’s for existing flows; new flows started up after the failover point will presumably be set off with the correct MTU, which will be a mere 1440 bytes, but that should be fine. I have I think asked about this before, but I wonder how bad in practice IPv4 fragmentation can be?
Pros of 3G: it routes my existing IPv4 /26 straight out to the internet as it should. IPv6 works through a tunnel.
Cons of 4G:
All things considered, I am thinking about which way to go should it turn out that the 4G dongle does not handle straight routing of my current global routable IPv4 /26 directly to the internet, and I find that it’s mangling source addresses of transmitted packets.
Pros of 4G: speed, possibly; full IPv4 MTU 1500, so I’m told. Hopefully 100% reliability?
If the 4G dongle misbehaves with fiddling about with tx packets, then I might even consider sticking with the 3G one despite the slowness and the low MTU as long as its reliability remains fairly reasonable.
This is all just me fretting in advance; there might be absolutely no problem.
Suggestions?