Routing

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  • 1.  BGP routes: what triggers a time refresh in show route

    Posted 07-19-2016 09:47

    Hello guys!

     

    I wanted to ask about something that I have never found documentation and it's a general question, not for a specific event that happened. 

     

    What triggers a refresh of the timer seen in the "show route" output? (specifically for bgp routes)

     

    *[BGP/170] 37w5d 11:07:27, localpref 100
    AS path: 65535 I, validation-state: unverified

     

    The "show route" documentation states it indicates how long the route has been known. But I have not been able to detect what causes it, as I have seen different cases over the years. So my question is, what specific event has to happen for it to refresh, for example: 

     

    - change in next-hop

    - next-hop not reachable (route becomes hidden)

    - route refresh

    - etc.

     

    I appreciate any help, specially any official Juniper documentation.

     

    Best regards,



  • 2.  RE: BGP routes: what triggers a time refresh in show route
    Best Answer

     
    Posted 07-19-2016 13:24

    Hi,

     

    This corresponds to the "Age" of the route.

    For BGP routes, a new update message received for that prefix would reset the age. Any changes in the BGP attributes received in an update message would result in resetting the age.

     

    Cheers,

    Ashvin

     

     



  • 3.  RE: BGP routes: what triggers a time refresh in show route

    Posted 07-19-2016 15:42

    Thank you very much 



  • 4.  RE: BGP routes: what triggers a time refresh in show route

    Posted 07-26-2016 16:27

    Hello,

     

    I have seen something interesting regarding this topic. BGP l3vpn routes stored in bgp.l3vpn.0 can have a different route age than the routes in their respective "routing-instance".inet.0 table. It seems that when there is a change in the next hop, like an LSP having a change in route, the route age gets refreshed in "routing-instance".inet.0 table, but the corresponding route in bgp.l3vpn.0 does not refresh it's route age.

     

    This behavior would not correspond to an update message from the BGP neighbor. 



  • 5.  RE: BGP routes: what triggers a time refresh in show route

     
    Posted 08-23-2016 03:27

    Hi,

     

    This is an interesting observation.

    For L3VPN, routes are exchanged between PEs using BGP into bgp.l3vpn.0. MP-BGP routes in the vrf are imported from bgp.l3vpn.0 [primary routing-table] using extended communities [route-targets] as filter conditions.

    > show route table vrf.inet.0 protocol bgp
.....
Primary Routing Table bgp.l3vpn.0

    This would probably explain why the age for MP-BGP routes in bgp.l3vpn.0 does not reset when the transport path [LSP] changes as this does not trigger new BGP update or refresh messages as long as the BGP session is unaffected.

     

    Cheers,

    Ashvin