After a first read, it looks like a pretty complete implementation of
RFC 6126.  I have't checked in detail, but it looks like you got both the
loop avoidance and the blackhole avoidance mechanisms right.  You're also
doing bidirectional reachability detection and fairly reasonable route
selection.

> - I implemented it like ‘zeroconf IS-IS’ works, that is, without useful
>   cost function. It fully interoperates with more comprehensive
>   implemenations though.

You're doing a little better than that -- you're taking txcost into
account, so if the neighbour is performing link quality estimation, you're
going to use his value.

For consistency with Appendix A.2.2 of RFC 6126, please change your cost
function to:

  cost = rxcost * MAX(txcost, 256) / 256

where rxcost is some value no smaller than 256, instead of the current

  cost = rxcost + txcost

which will cause pybabels to be too strongly preferred.

> - IPv6 only (who wants legacy IP?)

Well, you already parse and honour the Next-Hop TLV, so adding IPv4
support would be trivial.  (NH is needed to advertise an IPv4 route when
speaking over IPv6.)

> - No source [specific] routing

Since the source-specific protocol interoperates with plain RFC 6126,
that's probably not as big a deal as it sounds.  We'll want to modify the
default OpenWRT configuration to make sure that it announces a high metric
(non-specific) default route towards each source-specific edge router.

You should be using a multiplier of the interval as a hold time, rather
than a fixed value (is Section 3.4.1 unclear?).  Interestingly, you
already do the right thing with IHUs (is Section 3.4.2 any clearer than
3.4.1?).  See also Appendix B.

I think you should always be preferring redistributed routes to learned
ones.

Oh, one last thing -- I don't understand the comment on line 367.  How can
the source table entry be a historic copy?

-- Juliusz

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