OSPF - How to send ip block's to external router and propagate those routes.

Unanswered Question
Apr 19th, 2010


   How is the best way to send an ip block to a router that is not part of my network,

I have      Area 1   <---->   Area 0  <----> Area3,

An external router is connected to a router that belog Area3, so I need to send an Ip block to that external router and that ip block need to reach from all areas that i have (Area 1 and Area 0,)

what is the best practice (the best configuration) to send ip blocks to routers that are conected to diferents routers into Area3. lilke ISP scheme?

thanks for you help


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Tharak Abraham Mon, 04/19/2010 - 21:15


My bad, not really able to understand the wordings there "send an ip block" to the external routers ?

Did you mean you wanted to originate some subnet in the external router so that the Ospf domain recieves it ?

rpazosvargas Tue, 04/20/2010 - 11:23

Hi Tharak

   I mean,  I am little internet service provider, and i have assinged a Class C ip public address, i have some customer's router connected to my edges routers into area 3, each link to my customers have a public WAN  /30 and I'm routing /29's each one of them (its that i said "send an Ip block''  sorry about that)

and my question is how to send the /29 subnets to my customer from each one of my edge routers that belog to area3, the best practice to do ?

I could send it with a static route, but all my routers into Area3 will be ASBR, i



Giuseppe Larosa Mon, 04/19/2010 - 22:12

Hello Rolando,

if that router is outside the OSPF domain, one router in area 3 will act as an ASBR (autonomous system border router) by injecting in the OSPF domain the routes that are learned or point to that external router.

For communication between the ASBR node and the external router you can use:

static routes

a BGP session

or another IGP to build a neighborship in a common link (including a different OSPF process or RIP or EIGRP or ISIS)

So depending on your choices or the external router simply has a static route for the IP address block that spans over multiple OSPF areas (behind area range) or if a dynamic protocol is used it is the ASBR node that has to advertise this block over the session with the external router.

A typical trick used in this scenarios is the use of a floating static route to null0 on ASBR:

in order to have the desired IP prefix in the ASBR IP routing table.


ip route null0 250

this allows then to have the prefix ready to be advertised in BGP or other routing protocol using the appropriate network command


null0 is a waste bin that is used to silently discard packets so it is important that:

you use an high AD value and that more specific routes exist in OSPF domain in normal conditions (no faults)

Hope to help



This Discussion