OSPF Forward Address

In Normal Areas:


http://www.costiser.ro/2013/05/26/ospf-understanding-the-forwarding-address-fa/

In the beginning, let’s see why FA (Forwarding Address) exists in the first place. We all know that packets destined to external destinations are routed through the advertising ASBR. According to RFC 2328 (see page 141), there might be situations when this behaviour is not desirable and they introduced the concept of FA in order to avoid extra hops in the path.
Consider the diagram below, where both RT-A and RT-B are connected to RT-X (a partner company), but only RT-A speaks eBGP with partner company (let’s say that RT-B does not have enough memory to run BGP). RT-A redistributes the BGP routes (ex: 172.16.10.0/24) into the OSPF domain, thus becoming an ASBR.

Without the concept of FA, traffic from RT-B towards those external destinations will go via the ASBR (RT-A), as shown in the traceroute output.
If RT-A set FA = 192.168.1.3, then RT-B would route directly to FA instead of ASBR, as you can see below:

Now, let’s see what are the conditions required to have a non-zero FA. According to Cisco documentation, all of these conditions need to be true:

• 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

Note the AND logical operator between each condition. An easier way to remember this: next-hop interface must be a broadcast interface that is natively advertised in OSPF.

- See more at: http://www.costiser.ro/2013/05/26/ospf-understanding-the-forwarding-address-fa/#sthash.69acUFQA.dpuf

IN NSSA AREAS:

he selection process is to choose the highest IP of a loopback advertised
into the NSSA area. If no loopback is advertised then choose the highest
physical interface IP advertised into the NSSA area.

I hope that I have provide another piece to the OSPF puzzle and you now have
a good understanding of the forwarding address.

http://lostintransit.se/2012/09/20/asbr-in-nssa-choosing-what-ip-to-use-as-forwarding-address/