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From: jqj@duff.uoregon.edu (JQ Johnson)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: backplane backbone, routing or bridging?
Message-ID: <1991Jun28.171157.12990@ns.uoregon.edu>
Date: 28 Jun 91 17:11:57 GMT
References: <9106261402.AA12717@ccci>
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Organization: University of Oregon Network Services
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A couple more minor points on variable subnet masks and OSPF:

1/ multiple different address masks on the same physical cable make
handling of ICMP address mask request/reply quite difficult.  RFC1122 as
written (pp. 45-46) does not allow for this case, and would need to be
rewritten to allow multiple masks on the same cable (dropping the "first
received" rule, disallowing processing of received mask replies until the
host's IP address is known, using the SOURCE of the mask reply to test
whether the given reply is appropriate to me, guaranteeing that senders of
mask replies always use the appropriate source address, etc.).  Worse, it
isn't clear to me that a receiver of a mask reply can always determine
whether it is appropriate.  I believe multiple different address masks for
different subnets of the same net on the same cable should be illegal.

2/ the OSPF spec restricts the possible set of subnet masks used in a
variably-subnetted network routed by OSPF.  P. 17 of RFC1131 states
"Subnet masks must be assigned so that the best match for any IP
destination is unambiguous".  This is a fairly weak restriction and in
particular is satisfied by any hierarchical design of subnet masks.

3/ some algorithms for routing to variably-masked subnets are linear cost
in the number of different subnet masks on a network.  I recommend that in
thinking about variable subnet masks people keep the set of subnet masks
small (say no more than 3 or 4 canonical sizes of subnet on a class B CAN).

Conclusions:

(a) don't use subnet masks like:
	subnet		mask		(three "important" bits)
	128.185.2.0	ffff0300	x10	(x = don't care)
	128.185.1.0	ffff0500	0x1
	128.185.4.0	ffff0600	10x
Even if they work, they'll confuse you.

(b) Greg is right.  Variable-sized subnets introduce substantial new
complexity into the subnetting model, and should be carefully documented
before people go out and start implementing them widely.

-- 
JQ Johnson
Director of Network Services		Internet: jqj@oregon.uoregon.edu
University of Oregon			voice:	(503) 346-1746
250E Computing Center			BITNET: jqj@oregon
Eugene, OR  97403-1212			fax: (503) 346-4397