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Setting up BGP on a MikroTik is much much much easier than getting IPIP mesh to run on anything!

I see an opportunity here to learn BGP. I'm for this idea.

I posted an example configuration above.  Of course this is not the same as "learn BGP", but I think many users of the current IPIP mesh also did not "learn IPIP" and "learn RIP" but only copied an example configuration and fiddled with it until it worked.

Of course when you want to setup a regional VPN server you need some configuration for that as well, but when such a system would be deployed of course examples for that can be given as well.

BGP for such a small closed network is not that complicated. Basically every system maintains a TCP connection (port 179) with all its peers, and it sends the networks that it can route. It receives the same information from the other side, and fills the route table.  The active route is selected on a couple of criteria, where the least number of hops is usually preferred. It is possible to send tags along each route (called bgp-communities) that can be used to prefer certain routes over others, e.g. to prefer routes over radio when both a radio and a VPN path exist.

There are some issues with BGP, e.g. the total lack of security in the protocol.  Anyone can claim that they have a subnet and all the others will happily route all traffic for that subnet to them. The routing filters are an attempt to work around the most severe problems, but as can be seen on the internet (which also uses BGP) it is difficult to make it completely failsafe. Also, in our world it is a bit of a nuisance that there is no way to incorporate some form of dynamically determined link quality in the routing decision.  Links are either up or down.  But for this proposed use (replacement of the IPIP mesh) that is not a problem, it mainly affects the use of BGP on the radio links in our network.

Rob