Hi Igor,
Thanks for your help and time!
> First question: does the router A see the response coming from
> router B and then drop it or does the router B drop the packet?
Router A sees the packets coming in (I see them when I run tcpdump).
> Second question: why don't you configure router A to go through
> router B as a prefered route to access the internet and use VlanY
> as a fallback in case router B gets down?
That is what I think is the nature of using BGP: you get asymmetric
routes, where each router uses the shortest path to the internet, and
the return traffic may come via any router.
> Anayway, as far as I understand your problem, the packet would be
> returned to router A on a different interface from where it was sent.
That is correct.
> So the interface of your router A will receive a packet from
> router B but destination IP address is the one of VlanY which
> is not matching the interface getting it and causes the DROP.
Router A should just route the packet to the correct interface/socket.
It does so for ping6...
> You can probably setup an NDP proxy to solve this but it is an
> ugly solution .
NDP may be necessary when router B does not know where to send a package
(which you can detect by seeing the Neighbour Sollicitation packets).
But router B knows where to send the packages: router A and B share
routes via OSPF, and I see the packages coming in on router A.
The problem must be in router A...
Thanks for that link, it's more complete than other lists I have found.