BGP scan timer

Hi,

Can anyone please explain me the concept of BGP scan timer? I thought the the scan is to be run every minute. But the debug show a scan running every 15 secs. I haven't changed the timer value from default.

R2#
*Mar  1 01:12:15.679: BGP: Regular scanner event timer
*Mar  1 01:12:15.679: BGP: Import timer expired. Walking from 1 to 1
R2#
*Mar  1 01:12:30.679: BGP: Regular scanner event timer
*Mar  1 01:12:30.679: BGP: Import timer expired. Walking from 1 to 1
R2#
*Mar  1 01:12:45.683: BGP: Regular scanner event timer
*Mar  1 01:12:45.683: BGP: Import timer expired. Walking from 1 to 1

Confused!!!

Comments

  • Ooops!! just now realized that default value is 15 sec for scan. [:D]

  • Oops..Just realized that the default timer for Import scanner is 15 sec.. :)

  • what bgp scan-time shows?  if u do not see it, must be default values of 60.

    do sh run full | s router bgp

  • Its 60.

    R2#sh run | sec bgp
    router bgp 200
     no synchronization
     bgp log-neighbor-changes
     neighbor 1.1.1.1 remote-as 100
     neighbor 1.1.1.1 disable-connected-check
     neighbor 1.1.1.1 update-source Loopback0
     neighbor 3.3.3.3 remote-as 200
     neighbor 3.3.3.3 update-source Loopback0
     no auto-summary
    R2#

    But I assume that the one that runs every 15 sec is the import scanner which is related to VPNv4 routes.

  • Its 60.

     

    R2#sh run | sec bgp
    router bgp 200
     no synchronization
     bgp log-neighbor-changes
     neighbor 1.1.1.1 remote-as 100
     neighbor 1.1.1.1 disable-connected-check
     neighbor 1.1.1.1 update-source Loopback0
     neighbor 3.3.3.3 remote-as 200
     neighbor 3.3.3.3 update-source Loopback0
     no auto-summary
    R2#

    I assume that the one that runs every 15 sec in the import scanner which is related to VPNv4 routes.

  • R2#sh run | sec bgp
    router bgp 200
     no synchronization
     bgp log-neighbor-changes
     neighbor 1.1.1.1 remote-as 100
     neighbor 1.1.1.1 disable-connected-check
     neighbor 1.1.1.1 update-source Loopback0
     neighbor 3.3.3.3 remote-as 200
     neighbor 3.3.3.3 update-source Loopback0
     no auto-summary
    R2#

  • But a test done my me shows that BGP detects loss og next-hop in 15 secs.

    R2#
    *Mar  1 02:17:17.683: BGP: Regular scanner event timer
    *Mar  1 02:17:17.683: BGP: Import timer expired. Walking from 1 to 1

    Last scan ran at 02:17:17.683. Just after this I removed the IGP advertisement of next-hop route

    R2#sh ip bgp
    BGP table version is 4, local router ID is 2.2.2.2
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network          Next Hop            Metric LocPrf Weight Path
    *> 100.0.0.0        1.1.1.1                  0             0 100 i
    R2#
    *Mar  1 02:17:26.051: EvD: charge penalty 500, new accum. penalty 500, flap count 3
    R2#sh ip bgp
    BGP table version is 4, local router ID is 2.2.2.2
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network          Next Hop            Metric LocPrf Weight Path
    *> 100.0.0.0        1.1.1.1                  0             0 100 i
    R2#sh ip bgp
    BGP table version is 5, local router ID is 2.2.2.2
    Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
                  r RIB-failure, S Stale
    Origin codes: i - IGP, e - EGP, ? - incomplete

       Network          Next Hop            Metric LocPrf Weight Path
    *  100.0.0.0        1.1.1.1                  0             0 100 i
    R2#

    HERE BGP DETECTED THE LOSS OF NEXT HOP AND MARKED THE ROUTE Out OF PATH SELECTION.

    *Mar  1 02:17:32.687: BGP: Regular scanner event timer
    *Mar  1 02:17:32.687: BGP: Import timer expired. Walking from 1 to 1
    R2#


    Totally confused now!

  • sorry, i meant if u do not see bgp scan-time configured; it means it likely default values of 60;

    default vlaues/configs are hidden; u can see them with sh run full or all.

     

    some new versions of ios have commands to make this event-driven event; other commands can infulence bgs walker to run.

  • Ok..So that means loosing IGP route will create an event driven BGP scan?

  • This command can be used to disable event based BGP scan.

    R2(config)#router bgp 200
    R2(config-router)#no bgp nexthop trigger enable
    R2(config-router)#

  • The BGP scanner is legacy.  Newer platforms use event-driven next-hop tracking as opposed to the scan time, which was used to poll the table every X interval.  This tracking is on by default in any version that supports it.

    http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/feature/guide/sbbnhop.html

     

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