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Routing Table Clarification

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  • 1.  Routing Table Clarification

    Posted 02-18-2020 13:52

    Hi everyone!

    I just want to clarify the usage of the '>' (caret?) symbol in the routing table.

    I thought it was used to signify which next-hop was pushed to the forwarding-table but after some experimentation, I think I've got it wrong.

     

    I created two dummy routes, one with a regular next-hop and a second with a qualified next-hop.

    169.254.1.1/32 *[Static/3] 4d 18:08:29
    > to 10.16.104.2 via vlan.104
    [Static/5] 4d 18:17:55
    > to 10.16.104.1 via vlan.104

     

    Both appear in the routing table as shown above, and both have the '>' symbol.

    However, only one is appearing in the forwarding-table, which is behaving as I would expect.

    169.254.1.1/32 user 0 10.16.104.2 ucst 1318 3 vlan.104

     

     

    So, my question is, what is the '>' symbol really used for?

     



  • 2.  RE: Routing Table Clarification

     
    Posted 02-18-2020 14:02

    What you are referring to should be * asterisk. 

     

    *—An asterisk indicates that the route is both the active and the last active route. An asterisk before a to line indicates the best subpath to the route.

     

    https://www.juniper.net/documentation/en_US/junos/topics/reference/command-summary/show-route.html

     

    > is for candicate next hops 



  • 3.  RE: Routing Table Clarification

    Posted 02-18-2020 15:22

    Thanks, but I don't think I fully understand...

     

    If I configure a route with two next-hops...

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.1
    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.2

     

    Only one of these is considered a candidate next hop? Even though they're essentially identical?

    169.254.1.1/32 *[Static/5] 4d 19:48:07
    to 10.16.104.1 via vlan.104
    > to 10.16.104.2 via vlan.104

     

     

    But if I configure one of these as a qualified next hop, so the two next hops have different preferences...

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.1
    set routing-options static route 169.254.1.1/32 qualified-next-hop 10.16.104.2 preference 3

     

    Both are considered candidates

    169.254.1.1/32 *[Static/3] 4d 18:08:29
    > to 10.16.104.2 via vlan.104
    [Static/5] 4d 18:17:55
    > to 10.16.104.1 via vlan.104

     

     

    Why does this happen?



  • 4.  RE: Routing Table Clarification

     
    Posted 02-18-2020 19:55

    I believe in the below case you have same preference for both the route with same next hop. Hence you see ">" only for one route.

     

     

    169.254.1.1/32 *[Static/5] 4d 19:48:07
    to 10.16.104.1 via vlan.104
    > to 10.16.104.2 via vlan.104

     

    In the below case you have different preference and hence you see ">" for both the routes.First route has preference of 3 and second one has preference of 5.

    169.254.1.1/32 *[Static/3] 4d 18:08:29
    > to 10.16.104.2 via vlan.104
    [Static/5] 4d 18:17:55
    > to 10.16.104.1 via vlan.104

     

    So if you have route from the same protocol (static in this case) with same preference then you may not see ">" for all the next hops.

     

    Hope this helps

     



  • 5.  RE: Routing Table Clarification

    Posted 02-19-2020 00:46

    Hi Luke ,

     

    Good day !

     

    As per your question :

     

    1> question

     

    Answer :

    • ">" indication the next hop the packet will be sent to
    • " * " indicates the active route that will be pushed to the forwarding table

     

    2> question

    If I configure a route with two next-hops...

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.1

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.2

     

    Only one of these is considered a candidate next hop? Even though they're essentially identical?

    169.254.1.1/32 *[Static/5] 4d 19:48:07

    to 10.16.104.1 via vlan.104.   >> 1st one

    > to 10.16.104.2 via vlan.104  >> 2nd one

     

    Answer :

    Here when you have configured both static routes both appear in the routing table as candidates  , only 1 with " * " is used , both have default preferences hence there appear in above format where the preference is not explicitly mentioned

     

    [Static/5] - in this 5 is the preference value and it is default

     

    3> question

     

    But if I configure one of these as a qualified next hop, so the two next hops have different preferences...

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.1

    set routing-options static route 169.254.1.1/32 qualified-next-hop 10.16.104.2 preference 3

     

    Both are considered candidates

    169.254.1.1/32 *[Static/3] 4d 18:08:29

    > to 10.16.104.2 via vlan.104  >> 1st one

    [Static/5] 4d 18:17:55

    > to 10.16.104.1 via vlan.104  >> 2nd one

     

    Answer :

    Here when you have configured the preference as 3 for 10.16.104.2

    Which is lower than the default value 5

    Still like in the above table both the routes appear , but the format is different as for each static next hop the preference is different

     only 1 with " * " is used , in this case the one with the lower value 3

     

     

    Regards 

    Sharanya



  • 6.  RE: Routing Table Clarification
    Best Answer

     
    Posted 02-19-2020 10:21

    Hi Luke,

     

    Greetings, let me try to simplify this for you, 

     

    If I configure a route with two next-hops...

    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.1
    set routing-options static route 169.254.1.1/32 next-hop 10.16.104.2

     

    Only one of these is considered a candidate next-hop? Even though they're essentially identical?

    169.254.1.1/32 *[Static/5] 4d 19:48:07
    to 10.16.104.1 via vlan.104
    > to 10.16.104.2 via vlan.104

     

    Yes, and this is expected because the primary criterion for route selection is the route preference since both routes have the same preference then the Junos OS chooses in a random fashion one of the next-hop addresses to install into the forwarding table, in other words, if you don't see the ">" on a route, it means that it is not the next-hop that is forwarding the traffic and if the primary fails it is not necessary the back up for your primary since the new next-hop will be selected through a "randomizer" again, this would make much more sense if you drop in a 3rd static route with the same default preference they are all equal and the randomized will start again for all 3 if you restart the rpd or delete all static routes and add them back at the same time.

     

    Now here is when the qualified next-hop is very useful with the qualified next-hope you can overwrite the default behavior ("randomize") and specify which route will be the back up in case the primary fails by modifying the preference as you did hence you will see ">" on all candite routes. 

     

    Regards,
    Lil Dexx
    JNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB

     

    If this solves your problem, please mark this post as "Accepted Solution" so we can help others too \:)/

     

     

     

     



  • 7.  RE: Routing Table Clarification

    Posted 02-19-2020 19:30

    Thanks, I think I finally get it now.

    I still just find the qualified next-hop a bit confusing, coming from a non-Juniper background.



  • 8.  RE: Routing Table Clarification

    Posted 02-19-2020 10:59

    If you really want to get fancy with this experiment you can make both routes active by using policy statements. By default Junos will only install one active route into the forwarding table...but what if you want to have two active paths to a desitnation over two links so that both are active. This can be done with two steps.

    1. set policy-option policy-statement balance-traffic from route-filter x.x.x.x/y orlonger

        set policy-option policy-statement balance-traffic then load-balance per-packet

    2. set routing-options forwarding-table export balance-traffic

     

    This will install both routes into the forwarding table as active and will balance traffic per flow (as per-packet is inaccurate) and you will see traffic flowing on both links.