Econet bridge question - routing to non-configured networks

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philpem
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Econet bridge question - routing to non-configured networks

Post by philpem » Fri May 22, 2020 1:22 pm

Are there any Econet bridge experts in the room?

One of Acorn's examples is a network like this:

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BACKBONE (1)  ========+========+======== [FILESERVER (1.254)]
                      |        |
                    BRIDGE   BRIDGE
                      |        |
    Net 2 ------------+        +----------- Net 3

The DIP switches on the bridges only identify their "A" and "B" networks. So the bridges would know by means of the switches that 2.nnn and 3.nnn can reach 1.254 in a single hop.

But how does Net 2's switch know that by going over the Backbone, it can reach Net 3?

It strikes me that knowing Acorn it's probably a very simple solution, but the only solutions I came up with were indiscriminate broadcast with network number rewriting, and some kind of RIP broadcast system.

What is the logic inside the bridge?

Cheers,
Phil.

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IanS
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Re: Econet bridge question - routing to non-configured networks

Post by IanS » Fri May 22, 2020 3:29 pm

Sorry, nothing very technical, but the sj research bridge describes it as.
bridge.PNG
My understanding is the each bridge effectively has a list of which networks are on each side. The lists get updated as the bridges "natter". If a bridge seems a transaction that is on the other side, it passes the message on and starts flag filling on the source side until it sees a reply.

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BeebMaster
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Re: Econet bridge question - routing to non-configured networks

Post by BeebMaster » Fri May 22, 2020 3:40 pm

The Bridge listens to data packets on the network so that it can "discover" the network number of other networks. There are two calls a user can issue which the Bridge will respond to - "IsNet" and "WhatNet". IsNet asks the bridge about network n, and if it knows about it, it will pass the data on through the Bridge so that it can continue its journey to the right network, or it ignores it if it doesn't know about network n. WhatNet asks the bridge to report its local network number. If the Bridge receives a data packet addressed to a station on its local network number, effectively it "claims" the data packet by substituting 0 for the network number. Usually all this is transparent to the user without the need to issue calls to the Bridge, as the user can communicate with other stations several networks away by using the appropriate net number.

There's a good summary in the MDFS Manual section 10.18. Also the section "Campus Networks" pp.6-13 of the Econet Design & Installation Guide 1993 (can't just find a download link at the minute).
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