Routing

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Hidden Routes

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  • 1.  Hidden Routes

    Posted 02-11-2020 19:24

    Hi, I'm trying to understand routing in Junos. In particular, hidden routes.

    Correct me if I'm wrong, but I understand that a hidden route is a route that can never be selected as active due to some issue.

    The issue could be something like an unresolvable next-hop, or a reject in a policy.

     

    To have a look at this, I tried to create a hidden route using:

    set routing-options static route 169.254.1.1/32 next-hop 10.10.10.10

    10.10.10.10 is an IP that my switch can't reach. I do have a default route, but this is not a real IP on my network.

     

    As I would expect, this route does not appear in the routing table.

    However, it also does not appear when using:

    show route hidden

     

    Have I misunderstood something? Is this not considered a hidden route?

     

    Thanks



  • 2.  RE: Hidden Routes
    Best Answer

    Posted 02-11-2020 20:29

    Hi Luke Robertson, 

     

    yes, hidden routes are due to unresolvable next-hop or a reject in a policy as you mentioned, however, if you don't have any IP configured on the switch within your subnet (10.10.10/24) , the route will not be even created in the routing table, so this won't even be hidden in first place.

     

    if you do "show route  169.254.1.1/32" it won't show anything because the route is not even active in the RT or because your outgoing interface is not configured (as you mentioned before).

     

    hope this helps! 

     

    If this solves your problem, please mark this post as "Accepted Solution".

    If you think that my answer was helpful, please spend some Kudos.

     

    Thank you, 

    Franky



  • 3.  RE: Hidden Routes

    Posted 02-11-2020 20:57

    Yeah, this makes sense, thanks.

    What would count as an unresolvable next-hop in this case? A case where ARP fails to map the next-hop IP to a MAC?



  • 4.  RE: Hidden Routes

    Posted 02-11-2020 21:06

    Hi Luke Robertson, 

     

    it depends of so many factor - ARP failure, route flapping, policies , directly connected next hop not being resolved (you need to use the resolve command after static route command so it perform recursive lookup), misconfiguration , NH not reachable , memory issues, router dropping the arp ... you name it 🙂 

     

    If this solves your problem, please mark this post as "Accepted Solution".

    If you think that my answer was helpful, please spend some Kudos.

     

    Thank you, 

    Franky



  • 5.  RE: Hidden Routes

    Posted 02-12-2020 15:07

    Excellent! Makes full sense now, thanks!