[HECnet] One more for the Cisco GRE mesh: Seeking Tunnel End-Points

[Paul Koning]

> On Feb 26, 2019, at 7:19 PM, Frank Wortner <frank at wortner.com> wrote:
> 
> I’m connected via VDE to Supratim’s network.  Must I also move to area 31? Just curious 

The critical requirement is that an area must not be partitioned.  So all nodes in area X need to have connections among them that don't pass through non-X routers.

The reason is that DECnet area routing looks for the closest area router that is attached to area X, and sends traffic for X there.  If X is partitioned, then only the part of X that's reachable via that area router is accessible.  And note that "closest area router" depends on where you start from, so a partitioned area will not necessarily look the same to everyone outside.  Some may see partition 1, some may see partition 2.

DECnet Phase V includes the ability to repair partitioned areas by going from one partition to the other via the L2 network, but that isn't possible in Phase IV.  It can only be done if you have what amounts to a map -- the "link state routing" that is the distinguishing feature of Phase V.

By the way, the limitations of area routing that Johnny described are not specific to DECnet.  You'll find them in any hierarchical routing scheme, because the essence (the purpose) of hierarchical routing is to summarize the fine detail of the network map in far away places.  The details of exactly how that hiding works and how (if at all) the system deals with cases that aren't well handled by a summary route vary depending on the specific protocol.  For example, the ATM network routing protocol P-NNI is somewhat like Phase V in that it uses link state routing, but it uses more than two levels, and it has the ability to describe "exception routes" for places for which the summary is not a good approximation.

	paul