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From: Patrick M. Hausen on 22 Mar 2010 09:44
Hello,

bod43 <Bod43(a)hotmail.co.uk> 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, AS16188)

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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