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From: kwe@bu-it.bu.edu (Kent England)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Help designing address allocation in a metronet
Message-ID: <77489@bu.edu.bu.edu>
Date: 21 Mar 91 17:34:30 GMT
References: <1991Mar20.150615.18929@m2xenix.psg.com>
Sender: news@bu.edu.bu.edu
Reply-To: kwe@bu.edu
Organization: Boston University Information Technology
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> From: news@m2xenix.psg.com (Randy Bush)
> Newsgroups: comp.protocols.tcp-ip
> Subject: Help designing address allocation in a metronet
> Date: 20 Mar 91 15:06:15 GMT
> 
> We have read the paper by Tsuchiya, "On the Assignment of Subnet Numbers",
> which describes a scheme for managing a net in which subnet masks differ in
> length.  But are confused by the following question.  If all the neighbors of a
> router have a subnet mask <= n bits, how does this router send to a distant
> site whose subnet mask is > n bits. 
> ...

	The router only has to worry about variable subnet masks for
the subnetted net that it participates in.  For distant nets it uses
the netmask that it derives from the net class.

	Within the variably subnetted net, the router must have the
mask for each route.  This is most easily gotten from the routing
protocol and most easily stored in the routing table.

> In general, we are unsure whether the varied software (PC-Route, SysV/386,
> Xenix, NeXT, VMS/TGV) in RAINet will be able to handle varying subnet masks.
> 
	They will all have to support OSPF or some other protocol with
variable length subnet support or else don't vary the subnet mask
length.  (Try gated.)

> We also have another concern.  As many of the inter-site links are or will be
> V.32 or so, we are worried that RIP will eat up bandwidth.
> 
	Another argument for OSPF, which is link state and not
distance vector.  Milo Medin has documentary evidence of reduced
routing bandwidth in the NASA internet to prove that link state is
better in this regard.

	The way most people do this sort of thing is to have one
network for the WAN and each site has its own independent network
space.  Is Class C big enough for RAINnet itself?  Set the subnet mask
all the way down to two nodes per subnet.  That's two bits for the
host part, 0 and 3 are reserved, 1 and 2 are the endpoints of the
link.  Make every subnet in the RAINnet net the same length.  Every
host or router not directly on RAINnet simply routes to the netmasked
route.  That should work and it won't waste subnet space.  If the mask
size is constant you don't need OSPF.  If you are passing around all
RIP routes, try judicious use of default routes.  Prefer dynamic
default via RIP.  If that isn't good enough then try OSPF or some
other link state algorithm to reduce traffic levels.

	--Kent