OSPF is one of the protocols where the details are very important. It has lots
of bits and pieces to make it run in a proper way. I have described the forwarding
address in an earlier post and this time I want to show how the IP that is used
as the forwarding address is selected. We start out with this simple topology.

It’s a very basic config where R1 is redistributing a route and running in a
NSSA area.

Which IP will R1 use for its forwarding address? We look at R3.

It has chosen its interface address towards R2. What if we enable OSPF on the other
Ethernet interface of R1?

We check R3 again.

The forwarding address has changed. It selected the IP of the other Ethernet interface
of R1. We can see that it prefers to choose a higher IP address. What if we announce
the loopback of R1 in the NSSA area?

Now the loopback IP is chosen instead. So since the loopback has a lower IP but still
is preferred we can see that loopbacks are preferred in the selection. To see this
clearly defined in words we reference RFC 3101 section 2.3.

So the 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.

ASBR in NSSA – Choosing what IP to use as forwarding address
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7 thoughts on “ASBR in NSSA – Choosing what IP to use as forwarding address

  • July 20, 2014 at 11:21 am
    Permalink

    “So the 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.”

    This is true as long as the “classic” condition to set the FA to a non-zero value isn’t met:
    ASBR’s next-hop interface is
    – OSPF enabled and non-passive
    – no point-to-[multi]point interface

    Otherwise, evaluation the next-hop interface would provide the optimal path, so this condition is checked first.

    Reply
  • September 19, 2014 at 5:05 pm
    Permalink

    is this true only when redistributing static null routes? I can’t figure out other cases

    Thanks,

    Reply
  • September 21, 2014 at 2:12 am
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    I’ve found myself the answer: the rules for setting the FA apply. If OSPF is enabled on the interface connected to the next-hop of the redistributed networks, the FA value is set to the next-hop IP. However, the interface type seems not to apply.

    Reply
  • August 6, 2015 at 12:39 am
    Permalink

    Hey Daniel. I’ve done a bunch of testing on this and had similar results. But I realised something. The forwarding address does not even have to be an IP on the ASBR at all. So I guess this is applicable if the forward address turns out to be an IP on the ASBR. But the forward address could just be on a completely different router, EVEN THOUGH the ASBR is the one who dictates what that IP actually would be.

    Reply
  • July 6, 2016 at 8:49 am
    Permalink

    I had this issue during migration at work. Very good explanation for this behaviour. Its a pity that I didn’t find it earlier :), it would save my day.

    Reply
  • April 28, 2017 at 11:33 pm
    Permalink

    So the 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 performed practical and that is not right you said……It’s taking latest configured ip address (in same NSSA Area ) as a forward address regardless of highest number.

    Reply

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