🚀 go-pugleaf

>And this result, in fact explains why the performance is so poor when the
>cisco router is entered into the loop: because the short-circuiting of
>same-Ethernet circuits between different-area nodes ONLY HAPPENS when the
>nodes are END NODES!
Yes, this is quite correct. Depending on software version, end-nodes
keep either a cache of the nodes on the same cable, or the previous
hop to get to a node they have been in contact with. Old (pre VMS V5.0
nodes do the former), new do the later.

When a DEC system originates a packet onto the network, it sets a bit
in the message header which means "originated on this cable". If a router
switches a packet on to a different circuit, it clears this bit - if it
is staying on the same cable, it doesn't touch the bit. 

When a receiving end-system gets a message with the bit set, it creates
a cache entry indicating that messages to that node can be sent directly,
bypassing the designated router for that LAN. If it gets a message without
the bit set, it either (for old software) does nothing, or for new software,
it looks at the MAC address of the previous source, and caches that for use
as a route to the source.

Routers do not contain this cache. The DEC philosophy is that routers should
rely only on information learned via routing protocols, and not make any
decisions based on data path optimizations.

>This brings up, then, another question: if all my DECnet connections
>into my ciscos are to direct-attached ethernets, then is there any
>reason at all to run DECnet routing on the ciscos?
DEC end-systems produce a large amount of background traffic (periodic
hello messages). If you run routing, this information is basically collected
into a single control message sent to other routers. If you run bridging,
all of the hellos get bridged. This is considerable traffic if you have
a lot of end-systems.

Kevin