routing and summarising

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Sep 24th, 2008
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Hi all, I have just been reading up about ospf, what I want to know is, If a have a router that is summarised to advertise the 192.168.0.0 network, and the router at the other end sees this, If I then put another router at another site and advertise the 192.168.1.0 network, will it be routed correctly, as routers should use the longest match criteria ?

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cowetacoit Wed, 09/24/2008 - 07:31
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that will not work, because those two networks overlap. One router is advertising 192.168.0.0 and the other is advertising 192.168.1.0. The first router already has 192.168.1.0 included in its summarized address space.

carl_townshend Wed, 09/24/2008 - 07:42
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but we could achieve this using static routes, why would it not work in this way?

cowetacoit Wed, 09/24/2008 - 07:53
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you might be able to get it to work with static routes but you are talking about using dynamic routing protocol in your first post. Your first router is saying, everything belonging to 192.168.0.0 255.255.0.0 is behind me, and then if you add a router saying 192.168.1.0 255.255.255.0 is behind me.....that won't work. they overlap.


explain your topology, maybe i am missing something.

bmbreer Wed, 09/24/2008 - 09:38
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By default, summary LSAs are not summarized so you would need to provide the mask information to get a proper answer.


If you're asking about classful addressing then those two networks do not overlap. One router is advertising 192.168.0.0. This is a class C network so the summary is 192.168.0.0/24. The other router is advertising 192.168.1.0. The the summary for this network is 192.168.1.0/24. These are two completely different networks and that would work.


cowetacoit Wed, 09/24/2008 - 10:07
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ah...192.168.0.0 /24. I was thinking of it summarized as 192.168.0.0 /16.

Giuseppe Larosa Wed, 09/24/2008 - 11:35
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Hello Carl,

ospf supports classless routing variable length subnet mask because it carries the prefix with the mask.

So in all cases routing will be correct:

for example:

192.168.0.0/23

192.168.1.0/24 the second is more specific and will be used for a destination 192.168.1.x


your understanding of longest prefix match first is correct.


as explained in other thread

net 192.168.0.0 usually means 192.168.0.0/24 the example I reported above is possible at the area boundary on the ABR router.


Hope to help

Giuseppe


carl_townshend Fri, 09/26/2008 - 07:47
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Hi there, so would it work then? if I had 1 router advertising the 192.168.0.0/16 network and the other advertising the 192.168.1.0/24 network?

cowetacoit Fri, 09/26/2008 - 09:45
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It will only work if you advertise 192.168.0.0 /24, not /16.

carl_townshend Sat, 09/27/2008 - 01:17
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so what would happen on the router in between these 2 networks? how would the routing table look, and would it not install the /24 route if there was already a /16 present?

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