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Can I prevent OSPF from advertising a specific network on aninterface

 
 
ve7eje
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      03-18-2010
Is there a way to tell OSPF to not advertise a specific network on a
spefic interface without stopping the adverts of other networks?
Example
Router has 5 interfaces
F1 is a link to another router
F2 is a link to another router
F3 = netB
F4 = netC
F5 = netD

I need nets BCD to be advertised out F1 and F2 but not each other.
Similarly I need nets learned on F1 and F2 to be advertised on F1, F2
and F3 only.

I've looked at the passive interface command but I don't think it will
do what I want.

Thanks -Rob-
 
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Thrill5
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      03-18-2010
You need to use a distribute list on the interface.


"ve7eje" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Is there a way to tell OSPF to not advertise a specific network on a
> spefic interface without stopping the adverts of other networks?
> Example
> Router has 5 interfaces
> F1 is a link to another router
> F2 is a link to another router
> F3 = netB
> F4 = netC
> F5 = netD
>
> I need nets BCD to be advertised out F1 and F2 but not each other.
> Similarly I need nets learned on F1 and F2 to be advertised on F1, F2
> and F3 only.
>
> I've looked at the passive interface command but I don't think it will
> do what I want.
>
> Thanks -Rob-



 
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bod43
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      03-19-2010
On 18 Mar, 23:20, "Thrill5" <(E-Mail Removed)> wrote:
> You need to use a distribute list on the interface.
>
> "ve7eje" <(E-Mail Removed)> wrote in message
>
> news:(E-Mail Removed)...
>
> > Is there a way to tell OSPF to not advertise a specific network on a
> > spefic interface without stopping the adverts of other networks?
> > Example
> > Router has 5 interfaces
> > F1 is a link to another router
> > F2 is a link to another router
> > F3 = netB
> > F4 = netC
> > F5 = netD

>
> > I need nets BCD to be advertised out F1 and F2 but not each other.
> > Similarly I need nets learned on F1 and F2 to be advertised on F1, F2
> > and F3 only.

>
> > I've looked at the passive interface command but I don't think it will
> > do what I want.

>


Sorry Thrill5, but I suspect that is not the whole story.

In a word - no. Due to the architecture of OSPF it is not
possible to so restrict the advertisment of networks.

Under some circumstances it is possible to filter routes,
e.g. when re-distributing between OSPF processes,
perhaps between OSPF Areas?, however
in general this is not possible.

OSPF works by every router sending out Link State
Advertisments (LSA). These represent the network
topology. All the routers in an area eventually receive
all of the LSAs as sent by every router in the area.
Each router then uses the LSAs to construct a
topology database. The routing tables in each router
are then calculated independently by each router
from its own copy of the topology database.

The key thing is that every router in a area *must*
have the same topology database. Well maybe
ends up with the same topology database is more
accurate

So if your "router" has knowledge of the topology it will send it
to all of the other routers.

You can use EIGRP since it works by sending routes
which you can of course effectively filter.

Perhaps you could find a solution with multiple OSPF
processes and redistribution between them. You can
apply filters to the redistribution process since they are
simple routes. I may have a think about it later. Seems
pretty likely at first glance.


 
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Stephen
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      03-19-2010
On Thu, 18 Mar 2010 07:47:44 -0700 (PDT), ve7eje <(E-Mail Removed)>
wrote:

>Is there a way to tell OSPF to not advertise a specific network on a
>spefic interface without stopping the adverts of other networks?
>Example
>Router has 5 interfaces
>F1 is a link to another router
>F2 is a link to another router
>F3 = netB
>F4 = netC
>F5 = netD
>
>I need nets BCD to be advertised out F1 and F2 but not each other.
>Similarly I need nets learned on F1 and F2 to be advertised on F1, F2
>and F3 only.


as the other posters said - OSPF works by replicating the link state
database to other OSPF routers within the area.

However - advertising the OSPF topology (or not) only matters if there
is another router on net BCD.

so - dont run OSPF on those interfaces (or operate in passive). If
this router doesnt have 2 way OSPF comms with another router it will
not form an adjacency and cannot exchange routes.

>
>I've looked at the passive interface command but I don't think it will
>do what I want.
>

it does sound like a different protocol may be the best way to do this
- BGP?

>Thanks -Rob-

--
Regards

http://www.velocityreviews.com/forums/(E-Mail Removed) - replace xyz with ntl
 
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Patrick M. Hausen
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      03-22-2010
Hello,

bod43 <(E-Mail Removed)> wrote:
> In a word - no. Due to the architecture of OSPF it is not
> possible to so restrict the advertisment of networks.


In general, this is true. With a slightly modified architecture
that is used by all ISPs that I know personally, you can still
achieve the desired effect.

Short version: instead of e.g.

router ospf 1
network 217.29.32.0 0.0.15.255

(which happens to be my RIPE allocation, AS1618

you use

router ospf 1
passive-interface default
no passive-interface <int to neighbour 1>
no passive-interface <int to neighbour 2>
no passive-interface <int to neighbour 3>
...
network <ip of int to neighbour 1> 0.0.0.0
network <ip of int to neighbour 2> 0.0.0.0
network <ip of int to neighbour 3> 0.0.0.0
...
redistribute connected subnets route-map ospf-out
redistribute static subnets route-map ospf-out

This way all prefixes routed to customers are distributed
accross my entire backbone as AS external. With the route
map in place you can prevent prefixes from being redistributed
into OSPF at the originating (i.e. directly connected) router.


Best regards, HTH,
Patrick
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Tel. 0721 9109 0 * Fax 0721 9109 100
(E-Mail Removed) http://www.punkt.de
Gf: Jürgen Egeling AG Mannheim 108285
 
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