[HECnet] DECnet et al

[Johnny Billquist]

Well, there is really no reason why areas would be tied to geographical separation. You can spread out nodes that are in one area over the whole world as well.
There are both L1 hello messages, and L2 hello messages.
These are used to build the connection topography matrix for the routers.
What areas really brought to the table was a hierarchy where you could also more easily separate administration of the network.

All L1 routers within one are must be able to communicate with all other L1 routers within the area, by just talking directly, or via other L1 routers in the area.

All L2 routers must be able to talk to all other L2 routers, either directly or using only other L2 routers.

Both are obvious requirements when you think of the hello messages, which are not routed, but broadcast on all interfaces. So a scenarion of

A(L2) - B(L1) - C(L2)

would mean that A and C could not exchange L2 hello messages, which means they will not know of each other, or what connectivity the other node have.

Also worth mentioning is that all L2 routers also are L1 routers.

	Johnny


On 2011-07-17 00.03, hvlems at zonnet.nl wrote:
Exactly my point Johnny. The original function for areas was to identify different geograhical locations. Same LAN area routing just wasn't part of the design. The rule is that area routers must always be adjacent to make the connection. In those days (30 years ago) that always meant poiint-to-point connections.
  ISTR that there is a L2 hello message for that purpose.
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-----Original Message-----
From: Johnny Billquist<bqt at softjar.se>
Sender: owner-hecnet at Update.UU.SE
Date: Sat, 16 Jul 2011 23:52:42
To:<hecnet at Update.UU.SE>
Reply-To: hecnet at Update.UU.SESubject: Re: [HECnet] DECnet et al

On 2011-07-16 23:46, Paul Koning wrote:

On Jul 16, 2011, at 5:40 PM,<hvlems at zonnet.nl>     wrote:

Ok, so the convention that the output of the SHO NET command always lists the area router with the highest address is just a display rule. It has nothing to do with an "active" area router then?

I can't find the rule for what L2 adjacent router to pick if you're an L1 router going out of area.   It may be that this is where "highest" kicks in.

For non-adjacent L2 routers, the L1 router doesn't have any idea which router is represented by the "nearest L2 router" pseudo-address zero.   It can't know that.

I don't know all the details here, and instead of starting to read
through documentation, perhaps you know, Paul, if L1 routers knows
*which* nodes in the area are L2 routers?

Because L1 routers knows the shortest paths to all nodes within it's
area, if I don't remember wrong.

However, an L1 router can never know which L2 router is the better for
reaching a specific area, since L1 routers have no idea of the area
topology.

But I *think* that L1 routers knows the type of nodes of all nodes in
the local area, so they should easily be able to figure out where the
closest L2 router is. But that is based on the assumption that they know
the type of each node in the area.

	Johnny


	paul

------Origineel bericht------
Van: Paul Koning
Afzender: owner-hecnet at Update.UU.SE
Aan: hecnet at Update.UU.SE
Beantwoorden: hecnet at Update.UU.SE
Onderwerp: Re: [HECnet] DECnet et al
Verzonden: 16 juli 2011 23:25


On Jul 16, 2011, at 3:42 PM, Johnny Billquist wrote:

On 2011-07-16 19.18, hvlems at zonnet.nl wrote:
I'm not sure what the router rules are. There are 63 areas, each with
one actove area router. There may be more routers configured as an area
router in one area; the one with the highest (?) DECnet address is
selected as the active one.

As far as I know, there can be more than one active area router. Just look at what the next hop are for different nodes in your node list... :-)

The way it works is that address 0 in the level 1 routing data corresponds to "nearest L2 router".   Any L2 router contributes to that.   The L1 routers don't know or care who is the nearest L2 router, they only care which direction to send to get there.

	paul


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