Question about OSPF Forwarding Address

[Sean.N]

Below is the examp

In above setup, R3 will reach 192.168.1.1 via R5, R4, R2. This is due to forwarding address is 0.0.0.0. In order to fix this we can run OSPF on R2's Gi0/0. By doing this, the forwarding address for 192.168.1.1 will change to R1's IP address 10.123.1.1 (for example). But R3 will not install 192.168.1.1 into RIB because the forwarding address is not a OSPF intra or inter area route. Instead, it's direct connected route on R3 itself. Till here I can fully understand.

However, in the book R3's Gi0/0 run OSPF as well (see below). By doing this, R3 can install 192.168.1.1 into RIB and reach it directly from Gi0/0.

I dont understand why this can work. Even we run OSPF on R3's Gi0/0, the 10.123.1.0/24 still a connected route in RIB and not meeting the requirement "If the Type 5 LSA forwarding address is not a default value, the address must be an intra-area or interarea OSPF route. If the route does not exist, the LSA is ignored and is not installed into the RIB"

Or my understanding is wrong, as long as the forwarding address is in OSPF DB it will work (not necessary to be in RIB) ?

[MHM Cisco World]

""In these tests, you saw the importance of the forwarding address and how to interpret routing when it is set. Also, the assumption that if the forwarding address is set, it must be used, can be false as it depends on the platform. When the forwarding address is known via OSPF intra/inter area, it is used, otherwise it is populated but not used for recursion. The behaviour on XR gives a level of reassurance, in the case an external LSAs forwarding address becomes known via another source, the traffic can not be blackhole.""

[Meddane]

@Sean.N  in the scenario when you enable OSPF on R3 G0/0, is the same behavior when you have an NSSA ABR connected directly to an NSSA ASBR. Let's say R1 (ABR) 12.0.0.1-----area NSSA-----12.0.0.2 R2 (ASBR). R2 the ASBR advertises a Type-7 LSA with the Forward Address 12.0.0.2. Now on R1 you have an ON2 Route even if it does not have an intra area route to reach the FA 12.0.0.2, instead it has a directly connected route 12.0.0.0/24. But do not forget that this subnet is carried through a Type-1 LSA of R1 and R2.

Now  come back to your topology.

Imagine you dont want to enable OSPF on G0/0 of R3 and G0/0 of R2, so R3 will not install the external route to 192.168.1.1. Because the directly connected route is preferred than the OSPF route advertised by R2. Right.

How to make sure R3 will install the external route while keepling OSPF enabled on G0/0 of R1?

The value of the forwarding address specified by the autonomous system boundary router (ASBR) can be either 0.0.0.0 or non-zero. The 0.0.0.0 address indicates that the originating router (the ASBR) is the next hop. The forwarding address is determined by these conditions:

The forwarding address is set to 0.0.0.0 if the ASBR redistributes routes and OSPF is not enabled on the next hop interface for those routes. This is true in this scenario before enabling OSPF on R1’s F0/0 interface.

These conditions set the forwarding address field to a non-zero address:

1. OSPF is enabled on the ASBR's next hop interface AND
2. ASBR's next hop interface is non-passive under OSPF AND
3. ASBR's next hop interface is not point-to-point AND
4. ASBR's next hop interface is not point-to-multipoint AND
5. ASBR's next hop interface address falls under the network range specified in the router ospf command.
6. Any other conditions besides these set the forwarding address to 0.0.0.0.

The obvious solution is to configure the G0/0 of R1 as a  point-to-point or point-to-multipoint.

Now I give you another intuitive solution to enrich your scenario:

You should remember how a router evaluates routes in the following order:

*Prefix Length - The longest-matching route is preferred first. Prefix length trumps all other route attributes.

*Administrative Distance - In the event there are multiple routes to a destination with the same prefix length, the route learned by the protocol with the lowest administrative distance is preferred.

* Metric - In the event there are multiple routes learned by the same protocol with same prefix length, the route with the lowest metric is preferred.

To make sure an intra area route is preferred than  connected route, configure a secondary IP address 10.1.123.11 in G0/0 interface of R1 with a length prefix greater than 24, R1 will advertise an OSPF route with a mask 27 which is preferred than the connected route /24:

R1(config-if)#int G0/0

R1(config-if)#ip address 10.1.123.11 255.255.255.128 secondary

or another intuitive solution:

configure R1 with the following command : local-rib-criteria forwarding-address

This command specifies that the OSPF local RIB will be used for route validation.

I have similar scenario in my book OSPF with the details above.