And that indeed is the trick, and the point I am trying to make.
WARNING: BEGIN LECTURE - OSPF and OTHER NETWORK STATEMENTS
Although it is sometimes hard to discern this from the various study
materials ( I have not read Tom Thomas' Designing OSPF Networks, so I can't
comment there ) the NETWORK statement in OSPF does something a bit different
than it does in RIP, IGRP, and EIGRP. In these latter protocols, the network
statement places the classful network into the routing process. With OSPF,
an interface is placed into the OSPF routing process.
What I believe happens is that the IOS code is written in such a way that it
looks at the network statements, compares the contents to the contents of
the interface address/mask table, and if there is a match, takes the
appropriate information, and places it into one of the OSPF tables. Recall
that OSPF is a classless protocol, supporting CIDR and VLSM, and that it
carries subnet mask / prefix information as a part of the network
advertisements.
Therefore, the following network statements accomplish the exact same thing,
as far as the OSPF process is concerned
Network 172.16.1.1 0.0.0.0 area 0
Network 172.16.1.1 0.0.255.255 area 0
Network 172.16.1.1 0.0.1.1 area 0 ( why? )
Network 0.0.1.1 255.255.0.0 area 0 ( this is the kind of thing Howard warns
about :-> )
Network 172.16.1.1 172.16.1.1 area 0 ( interesting pneumonic, is it not? )
There are a whole lot more ways to do this, as you can see.
And these all work, whether the subnet mask is /16 or /24 or /20 or /30 or
whatever.
In terms of best practice, one consideration is maintenance and ease of
troubleshooting. Now in a core routers with 72 interfaces, al of which are
in the OSPF process, in terms of ease of management and troubleshooting,
which is it better to do - manually enter 72 network statements? Devise some
clever mask or series of masks? Or just place all of them into the process
with an all inclusive statement? Where are errors more likely to occur?
The point about troubleshooting is one well worth considering. In terms of
simplicity, the more apparent something is, the more apparent something is
wrong, if it is wrong. But one day, sooner or later, we will all run into
the situation where some clever person has done things cleverly, and left no
documentation. Without a thorough understanding about how binary works, how
the bits match up, what they do and do not match, we will be unable to
quickly determine if things or processes or access-lists or whatever are
functioning as they are intended.
So yes, this might be considered showboating. It might be considered a silly
trick. Or it might be considered another means of broadening one's
understanding. It all depends upon one's outcomes.
The books I've read all say to do something like network 172.16.0.0
0.0.255.255 area 0, using the inverse network mask. This leads one to
believe one is placing entire logical networks into the process. But if the
mask were /21, it might be far to easy to incorrectly determine the
appropriate mask ( 0.0.7.255 0 and thus create an evening of
troubleshooting.
END OF LECTURE
-----Original Message-----
From: Fred Ingham [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, July 04, 2000 9:09 PM
To: Chuck Larrieu
Subject: Re: More puzzles - OSPF Network statements
Since OSPF uses the network statements to decide which
interfaces are included in the area the wildcard mask doesn't have to
match the interface SNM.
network 0.0.0.0 255.255.255.255 area 0.0.0.0
will get all interfaces in area 0.0.0.0. I assume you had
something else in mind?
Chuck Larrieu wrote:
>
> Gee, the recent wildcard mask puzzle was so much fun, let's do it again.
>
> The problem revolves around the OSPF network statement. Given an OSPF
> routing process, what are the fewest number of network statements that
will
> incorporate the following networks into the given OSPF process?
>
> For example:
>
> router OSPF 100
> Network a.b.c.d w.x.y.z area 0.0.0.0
> Network b.c.d.e u.v.w.x area 0.0.0.0
> etc
>
> Assume that all networks are in the same area ( for simplicity sake )
> You may also assume ( for simplicity sake ) that the particular interface
on
> which each particular network resides is the first host address on that
> network. For example, on network 10.10.3.0/24, the first host address
and
> therefore the interface address ) is 10.10.3.1
>
> Networks are as follows:
>
> 10.10.3.0 /24
> 10.25.17.0 /16
> 10.51.100.192 /26
> 172.16.0.0 /16
> 172.16.100.0 /24
> 172.16.210.240 /28
> 172.27.32.0 /19
> 192.168.0.0 /24
> 192.168.1.0 /30
> 192.168.207.56 /29
>
> remember - accolades go to the one who can enter all of these networks
into
> a single ospf process with the fewest possible network statements.
>
> Have fun!
>
> Chuck
>
> Please check out my new footers for a new age
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> http://www.thehungersite.com/cgi-bin/WebObjects/HungerSite
> Please help feed hungry people worldwide. A few seconds a day can make a
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>
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> http://www.certificationzone.com
> An excellent study focal point for all levels of certification, as well as
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>
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