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Can you configure the forward address in Juniper for Type 5 (external) LSAs?

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  • 1.  Can you configure the forward address in Juniper for Type 5 (external) LSAs?

    Posted 02-29-2020 17:02

    Does Juniper allow you to configure the Type 5 (External) LSA forward / forwarding address?

    I can't seem to find it anywhere in the documentation (granted the documentation library is huge, as is the forum).

    I think I read somewhere that someone tried to do it via routing policy, but the option was actually meant for BGP, so nothing happened.

    It's also noted from another post on the internet, that Cisco devices require caveats for the forwarding address to take - is there any such caveats for Juniper devices? (i.e. forwarding address interface can't be P2P, can't be passive in OSPF etc).

    Some old posts on SSGs (ScreenOS) refer to it - but only as the route has been advertised to it and the lack of a next-hop in the routing table due to some route filtering - but that has nothing to do with JunOS and whether or not it's possibly to  modify.

     

    It may be worth noting that I'm asking this out of curiosity rather than trying to solve a real world problem at this time - always get in depth when studying for exams, even though I know a topic wouldn't be on there. But what happens if you really did want to use this in a real world scenario 🙂 



  • 2.  RE: Can you configure the forward address in Juniper for Type 5 (external) LSAs?

    Posted 02-29-2020 18:52

    Hello,

    Yes. since 17.1

    https://forums.juniper.net/t5/Routing/OSPF-routing-issue-with-static-to-same-next-hop/m-p/461851/highlight/true#M19505

    As the knob implies, FA cannot be P2P and cannot be passive.

    HTH

    Thx

    Alex



  • 3.  RE: Can you configure the forward address in Juniper for Type 5 (external) LSAs?

    Posted 03-01-2020 15:21

    Hey,

     

    Thanks for the reply.

     

    So in this instance, when you configure this, it seems to send it to the broadcast address (although, in the example you linked, it looks like it would actually set it to the interface IP of the neighbour router) - but what if you wanted to manually configure the address? I'm assuming that's not possible since it's not really documented. I suppose 9/10 it's not really necessary, because all you need to do is get the packet to the router that originated the external route in the first place.

     

    An example of when this behaviour is used was if two routers were facing the same broadcast network and an external next hop was reachable via that same broadcast network, it would make more sense to send it directly to the next hop, rather than the router that originated the external route. The example is here



  • 4.  RE: Can you configure the forward address in Juniper for Type 5 (external) LSAs?
    Best Answer

    Posted 03-01-2020 21:40

    Hello,

     


    @Purplezorz wrote:

     

    So in this instance, when you configure this, it seems to send it to the broadcast address


     

    Incorrect. The FA in Type-5 LSA is the same IP@ that is configured as static route' next-hop when this static is exported into OSPF. The "broadcast" in the knob name "forwarding-address-to-broadcast" is referring to OSPF broadcast adjacency, not LAN broadcast IP.

     

     


    @Purplezorz wrote:

    what if you wanted to manually configure the address? I'm assuming that's not possible


     

    It is possible. Set the static' route next-hop to an IP@ covered by subnet/netmask of OSPF broadcast adjacency link (say, 198.51.100.128/29 subnet with OSPF broadcast adjacency + static route 203.0.113.0/24 next-hop 198.51.100.133), export this static into OSPF and voila, Your FA would be set to 198.51.100.133.

     

     


    @Purplezorz wrote:

     

    An example of when this behaviour is used was if two routers were facing the same broadcast network and an external next hop was reachable via that same broadcast network, it would make more sense to send it directly to the next hop, rather than the router that originated the external route. The example is here




    The authoritative example is in RFC 2328 section 2.3, and it also gives 2 use cases for FA. The second use case is dangerous though as it implies redistribution of BGP into OSPF which is not a good practice at all in 2020.

     

    HTH

    Thx

    Alex



  • 5.  RE: Can you configure the forward address in Juniper for Type 5 (external) LSAs?

    Posted 03-07-2020 12:58

    Ahhh,

     

    Makes sense now. So to confirm, by default, when you redistribute a static route, the next-hop to OSPF peers would be the address of the ASBR and then the ASBR would send it to the next hop configured in the static route.

     

    With the forwarding-address-to-broadcast command added, the next-hop address is used as the forwarding address in the Type-5 LSA and therefore routers in the OSPF domain will forward packets directly to the next hop.

     

    Thanks a lot for the confirmation, it makes it much clearer!

     


    #type-5lsa
    #type-5LSAs
    #externalroutes
    #forwarding-address


  • 6.  RE: Can you configure the forward address in Juniper for Type 5 (external) LSAs?

    Posted 03-07-2020 23:28

    Hello,

     


    @Purplezorz wrote:

    Ahhh,

     

    Makes sense now. So to confirm, by default, when you redistribute a static route, the next-hop to OSPF peers would be the address of the ASBR and then the ASBR would send it to the next hop configured in the static route.

     

     

     

    There is no "next-hop" information in the OSPF LSA5, only "forwarding-address". By default it is set to 0.0.0.0 which means "this router-id".

    When a static is redistributed and "forwarding-address-to-broadcast" is enabled, then OSPF LSA5 FA is set to that static' next-hop only if this next-hop points to a LAN with established broadcast adjacency. 

    HTH

    Thx

    Alex