[HECnet] Multinet peerings...?

[Paul_Koning at Dell.com]

> On Jan 14, 2016, at 4:40 PM, hvlems at zonnet.nl wrote:
> 
> ...
>> Can the bridge program detect whether there are area routers for the dame area at both ends and favor the local one, possibly block advertising of the remote area router?
> 
> I tried blocking traffic from a node in one area from getting to another 
> area, with the exception of packets from area routers.
> Unfortunately, it does not work. DECnet can be clever about local 
> ethernet connectivity. If you are on the same ethernet segment, nodes 
> can communicate directly with other nodes on the same ethernet segment, 
> even if they are endnodes, and this exen extends to nodes on different 
> areas. So such filtering in the bridge cause communication to fail for 
> endnodes on the ethernet segment, when the destination is on the same 
> ethernet, even if in a different area.

DECnet expects a "transitive Ethernet" -- if A can talk to B and B can talk to C, A must be able to talk to C.  That's actually a common assumption, other network protocols do the same.  DECnet is a bit unusual in that it explicitly verifies this property, at least for routers -- that's why router hellos have the router list in them.  We put that in because we had run into some defective Ethernets that were non-transitive, causing very strange misbehavior until this protocol mechanism was added.

End nodes have an on-Ethernet cache: if X talks to Y and both are on the same Ethernet, they will do it directly.  From the first packet if there are no routers; after the initial round-trip if there are.  If you create a non-transitive Ethernet -- which is what filtering does -- this will fail.  There is no workaround.  If you don't want all the nodes on an Ethernet to have direct communication, the only solution is to split it into two separate Ethernets, interconnected by a router (not a bridge).

	paul