routing and summarising

Unanswered Question
Sep 24th, 2008
User Badges:

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 network, and the router at the other end sees this, If I then put another router at another site and advertise the network, will it be routed correctly, as routers should use the longest match criteria ?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
cowetacoit Wed, 09/24/2008 - 07:31
User Badges:

that will not work, because those two networks overlap. One router is advertising and the other is advertising The first router already has included in its summarized address space.

carl_townshend Wed, 09/24/2008 - 07:42
User Badges:

but we could achieve this using static routes, why would it not work in this way?

cowetacoit Wed, 09/24/2008 - 07:53
User Badges:

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 is behind me, and then if you add a router saying is behind me.....that won't work. they overlap.

explain your topology, maybe i am missing something.

bmbreer Wed, 09/24/2008 - 09:38
User Badges:

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 This is a class C network so the summary is The other router is advertising The the summary for this network is These are two completely different networks and that would work.

cowetacoit Wed, 09/24/2008 - 10:07
User Badges:

ah... /24. I was thinking of it summarized as /16.

Giuseppe Larosa Wed, 09/24/2008 - 11:35
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

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: 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 usually means the example I reported above is possible at the area boundary on the ABR router.

Hope to help


carl_townshend Fri, 09/26/2008 - 07:47
User Badges:

Hi there, so would it work then? if I had 1 router advertising the network and the other advertising the network?

cowetacoit Fri, 09/26/2008 - 09:45
User Badges:

It will only work if you advertise /24, not /16.

carl_townshend Sat, 09/27/2008 - 01:17
User Badges:

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?


This Discussion