[HECnet] Re: Decnet connectivity issue

Adjacency up involves different checks depending on the node type. If you're on an endnode, it means simply that a router hello was received (from the designated router). It does not mean the designated router can hear you If you're on a router, router adjacency up happens if you hear from the other router AND that router reports you in its router adjacencies list. In other words, for that particular case, it does tell you there is two way connectivity. If you're on a router, endnode adjacency up means you received an endnode hello. It does not mean the endnode can hear you. All of the above is for Ethernet; the rules for point to point connections (DDCMP) are different. Given how the data link layer works there should always be two way connectivity if any DDCMP traffic gets through at all (if the circuit state is "on" rather than "on-starting"). The assumption with Ethernet is that one way connectivity is very unusual. Ignoring newfangled stuff like filtering switches, it only can happen if there is a strange electrical issue with the cable. A long coax segment with a weak transceiver could give you this behavior. Similarly, there is an assumption of transitivity: if A can talk to B and B can talk to C then A can talk to C. In some limited cases, especially when dealing with router to router connections, DECnet does not fully rely on these assumptions and verifies them instead. But it does not do that in all cases, because the issue is too unlikely to worry about in cases where the network wouldn't utterly fail if it happened. paul