From: dstine@cisco.com (David Stine)
Newsgroups: comp.protocols.appletalk
Subject: Re: Extended- vs. nonextended-net net range aging
Date: 23 Nov 1992 08:58:38 GMT
Organization: cisco Systems, Menlo Park, California, USA
Distribution: comp
NNTP-Posting-Host: regal.cisco.com

In article <KAI.92Nov22234334@pyrite.SOM.CWRU.Edu> kai@pyrite.SOM.CWRU.edu (Kai Getrost) writes:
>[...]
>
>The problem: 
>
>   Inside AppleTalk (2nd ed.) gives slightly different algorithms for
>expiring THIS-NET on extended vs. nonextended nets.
>
>   On nonextended nets, it says (p. 5-18/19) that the value of
>A-ROUTER and THIS-NET are gleaned from every RTMP Data packet
>received.  If no RTMP packets are received for 50 seconds, A-ROUTER
>should be expired (set to 0), but *not* THIS-NET.
>  On extended nets, however, it says that *both* A-ROUTER and THIS-NET
>should be reset when the aging timer expires.
>
>  In other words, nodes on nonextended nets will save their link's net
>number if all routers go down, but nodes on extended nets are supposed
>to reset theirs (and set them again once RTMP packets are received
>again, possibly to a new value).  Why do these algorithms differ?

Remember that on extended cables, you can have a _range_ of network numbers.
The router could come back on the net with a different network number in its
address.

Example: let's say that you have an extended AppleTalk Ethernet with a
cable-range of, oh, 10-20.[footnote 1] Let's say that the router's current
net.node address is 10.240. Now, let's say that there is a hiccup in the power
and when the router comes back on-line, all non-reserved node numbers in
network 10 have been taken. The router, while AARP'ing for an open address,
must choose a node in network number 11. Say 11.20.

If it were the _only_ router on the cable, the nodes which were holding an
A-ROUTER value and not timing it out would still be sitting on the wire, deaf
to the world, since they are waiting for the router to re-appear on network
10.

The analgous scenario occurring on a non-extended cable means that the nodes
will have to re-AARP for new node addresses, since the cable configuration has
changed. On an extended net, the nodes' current address is still valid and the
cable configuration hasn't changed; simply the address of the router has
changed.

>   The benefit of expiring the net range on extended nets could be
>that you can reconfigure the net range by taking down just the routers
>for some minimum time, and then bring them up with a new value that
>all nodes will (hopefully) adopt.  But this will affect any current
>connections non-router nodes have, as they change to the new net range
>and adopt new net:node IDs, and Inside AppleTalk is nebulous as to
>when and how these connections should be reset (since it's a rare
>occurence and difficult to deal with effectively).  

Yes, this is one of many instances where "Inside AppleTalk" is rather, well,
vague. Perhaps wishy-washy. A tad economical with the facts, even.

In addition to reading Inside AppleTalk on this issue, you should also be
reading the document from APDA called:

	"AppleTalk Phase 2 Protocol Specification, C0144LL/A"

which will set you back $20. I don't have my copy here right now (I'm at home)
but this document contains the details on router disappearance and
re-appearance that are missing from "Inside AppleTalk." If you are
implementing an AppleTalk node or router, you must have this document. Simple
as that.

No, I'm not connected or getting a kick-back from APDA in any way. ;-)

dsa

[Footnote 1] 
There is a heap of misconception out there as to cable-range configurations on
Phase II routers. I frequently see people with a cable-range 10 to 100 wide
when they have an Ethernet segment with only 100 nodes _max_.

Apple forgot to stress the point that a "wide" cable range (ie, a cable range
which would be 10-11, rather than 10-10) is required _only_ when you will have
more than 253 nodes on that particular cable/media segment. This usually only
occurs in large bridged LAN's. If there are note going to be more than 253
AppleTalk nodes on said cable segment, there isn't any point in configuring a
wide cable range.


Newsgroups: comp.protocols.appletalk