Now that I look at your topology, you can just simply configure R1, R2 and R3 in OSPF Area 0.
- For additional Routers connected to R3 (just like R4), that link can be in a non-zero Area for scalability and appropriate OSPF design using several areas instead of just growing Area 0. In that case, R3 will act as ABR and you can configure the link R3-R4 as non-zero OSPF area and (optionally) as either Stub, Totally Stub, NSSA or Totally NSSA. Remember that NSSA type areas allow you to redistribute a route (with the 'redistribute' command under OSPF process configuration section) from within that Area when Stub type areas do not. In any case, any of those areas types allow advertising OSPF networks using just the 'network' command under OSPF process configuration section.
- Additional Routers that can potentially connect to R3 (let's imagine there is a need to add R5 and connect it to R3 in the future) can be configured on a different non-zero Area and be configured similarly to the R3-R4 OSPF adjacency.
- Again, the idea behind the above is have R1/R2/R3 in Area 0 and make R3 behave as ABR to other Routers that can connect to it in the future.
If R1 and R2 reside in Area 0, you can "attract" traffic to R1 & R2 by advertising from them a default route into the OSPF domain using:
R1(config)#router ospf <OSPF_PID>
R1(config-router)#default-information originate
R2(config)#router ospf <OSPF_PID>
R2(config-router)#default-information originate
Note that in that case, for the "default-information originate" command to advertise a default route to R3, the R1 & R2 Routers need to have a default-route in their own Routing Table and be introduced into the OSPF process. If R1 & R2 are already receiving a Default Route via iBGP, you can accomplish this as follows:
R1 or R2 already have a default route in their own Routing table via iBGP
#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override, p - overrides from PfR
Gateway of last resort is 10.1.2.1 to network 0.0.0.0
B* 0.0.0.0/0 [200/0] via 10.1.2.1, 00:00:32
R1 & R2
router bgp <BGP_AS>
bgp redistribute-internal
IMPORTANT: With the above configuration you advertise only the default route into the OSPF domain from R1 & R2. No "redistribute" is needed under OSPF configuration section on those Routers. Now, be careful since iBGP routes can now be redistributed into OSPF since "bgp redistribute-internal" is now enabled. This in some scenarios could lead to loops. It is recommended to always use route-maps to control redistributions in case you need to do so in the future.
Another option to advertise the default Route from R1 & R2 is just with:
R1(config)#router ospf <OSPF_PID>
R1(config-router)#default-information originate always
R2(config)#router ospf <OSPF_PID>
R2(config-router)#default-information originate always
Note the 'always' parameter. With this, R1 & R2 will always advertise a default route to the OSPF domain regardless they do not have the their own default route in their Routing Table which could be lost by virtue of a failure on the BGP adjacency. This could lead to a blackhole of the traffic since R1 & R2 will always be advertising the default route and attracting the traffic of the OSPF domain.
On R1 & R2 you need to prevent the Default Route from being learned via OSPF since it will due to the fact that OSPF Administrative Distance is lower than iBGP's AD.
OSPF = 110
iBGP = 200
R1 & R2
ip prefix-list DEFAULT-ROUTE seq 5 permit 0.0.0.0/0
!
route-map OSPF-LOCAL-FILTER deny 10
match ip address prefix-list DEFAULT-ROUTE
!
route-map OSPF-LOCAL-FILTER permit 100
!
!
router ospf <OSPF_ID>
distribute-list route-map OSPF-LOCAL-FILTER in
!
Where in your topology is the ASA? is it inside the OSPF domain or inside the BGP domain?
If your ASA sits in Area 0 (based on the above explanation), you can just redistribute or advertise its routes into the OSPF domain. If you want to continue using your current Area 2 Stub area design, why don't you change it to an NSSA type area so you can redistribute routes from within this area?
DISCLAIMER:
The configurations discussed in this posts are merely templates and not final configurations that you can copy & paste to your network devices in production. You need to review, evaluate and modify the configurations at your best convenience to make sure the results are the ones you intended and introduce those changes to you network only during maintenance windows. The author of this post is not responsible of unintended consequences by failing to follow this disclaimer note.
