Re: [OSPF] I have a question regarding forwarding address

"Acee Lindem (acee)" <acee@cisco.com> Wed, 13 July 2016 14:19 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: "ospf@ietf.org" <ospf@ietf.org>
Thread-Topic: [OSPF] I have a question regarding forwarding address
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Date: Wed, 13 Jul 2016 14:10:13 +0000
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Subject: Re: [OSPF] I have a question regarding forwarding address
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Hi Abhinay,

One more question answered inline.

From: OSPF <ospf-bounces@ietf.org<mailto:ospf-bounces@ietf.org>> on behalf of Acee Lindem <acee@cisco.com<mailto:acee@cisco.com>>
Date: Wednesday, July 13, 2016 at 9:59 AM
To: Abhinay R <abhinay.is2006@gmail.com<mailto:abhinay.is2006@gmail.com>>, OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>
Subject: Re: [OSPF] I have a question regarding forwarding address

Hi Abhinay,

Your questions are very implementation specific but since I’m a good guy…

From: OSPF <ospf-bounces@ietf.org<mailto:ospf-bounces@ietf.org>> on behalf of Abhinay R <abhinay.is2006@gmail.com<mailto:abhinay.is2006@gmail.com>>
Date: Wednesday, July 13, 2016 at 8:11 AM
To: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>
Subject: [OSPF] I have a question regarding forwarding address

Hi All,
        I have a question on when forwarding address is set in external LSA?

Suppose I have a route from RIP say 100.1.1.0/24<http://100.1.1.0/24> with nexthop 10.1.1.1 in routing table and I want to redistribute that route into OSPF, then I feel below conditions must be vaild to set 10.1.1.1 as forwarding address in the external LSA that OSPF originates for 100.1.1.0/24<http://100.1.1.0/24> network.


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

    OSPF is enabled on the ASBR's next hop interface AND

Although, it would be a strange topology where it weren’t enabled, it only has to be reachable within the OSPF routing domain as an intra or inter area route.




    ASBR's next hop interface is non-passive under OSPF AND

This should have no bearing on whether the forwarding address is advertised. There could be a better path through the OSPF routing domain independent of whether the interface is passive.



    ASBR's next hop interface is not point-to-point AND

    ASBR's next hop interface is not point-to-multipoint AND

Although the likelihood of the forwarding address providing a better path is increased for multi-access networks, it is not necessary.




    ASBR's next hop interface address falls under the network range specified in the router ospf command.

Again, the next hop only has to be reachable within the OSPF routing domain as an intra or inter area route.

Hope the Helps,
Acee






Any other conditions besides these set the forwarding address to 0.0.0.0.

If I have 10.1.1.0/24<http://10.1.1.0/24> network as intra or inter route in OSPF routing table at the router that imported RIP route, do I originate External LSA with 10.1.1.1 set as forwarding address ?


There is a confusion that if the redistributed network nexthop is present in routing table as intra inter or connected route, we need to set it as forwarding address. Please clarify.

This is implementation specific since if you a have an OSPF local RIB, you can do the lookup considering only OSPF routes. The criteria is that the route to the next hop is reachable in the OSPF routing domain via an intra-area or inter-area path. If you don’t have an OSPF local RIB, you need to handle this case (for other reasons as well).

Thanks,
Acee






While we receive a External LSA with forwarding address set, then we see if there is a connected, intra or inter route to that nexthop and then use it to compute the route nexthop. This is clear.

Thanks & Regards,
Abhinay R