Description

This article explains why routes are present in the router as hidden routes, even though the BGP peer from which they are learned is no longer in the Established state.

Symptoms

Routes from a BGP peer are not removed and remain as hidden routes even though peer is no longer up.

As seen below, the BGP peer 20.1.1.2 is in Active state


    lab@M320-RE0# run show bgp summary
    Groups: 1 Peers: 1 Down peers: 1
    Table Tot Paths Act Paths Suppressed History Damp State Pending
    inet.0
    1 0 0 1 1 0
    Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
    20.1.1.2 2 21 20 0 1 5 Active

Prefix (1.1.1.1/32) learned from it is still there as hidden route:

    lab@M320-RE0# ...protocol bgp 20.1.1.2

    inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden )

    [edit]
    lab@M320-RE0# ...l bgp 20.1.1.2 hidden

    inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
     Prefix Nexthop MED Lclpref AS path
     1.1.1.1/32 20.1.1.2 2 I

    [edit]
    lab@M320-RE0# ...0.1.1.2 hidden detail

    inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
    1.1.1.1/32 (1 entry, 0 announced)
    Accepted
    Nexthop: 20.1.1.2
    AS path: 2 I

Solution

The Routes from the BGP peer remain even though it is not in Established state, because damping is enabled. With damping enabled, as soon as the BGP peer goes down, the routes learned from it are stored as history routes to maintain damping parameters for it, as seen below:

Initially BGP peer is up:


     lab@M320-RE0# run show bgp summary
     Groups: 1 Peers: 1 Down peers: 0
     Table Tot Paths Act Paths Suppressed History Damp State Pending
     inet.0
     1 1 0 0 0 0
     Peer     AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
     20.1.1.2 2  41    41     0    1     17:17             1/1/1/0 0/0/0/0

There is nothing in damping history:

     lab@M320-1-RE0# run show route damping history

     inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)

Now BGP peer is moved to Active state:

     [edit]
     lab@M320-RE0# run show bgp summary
     Groups: 1 Peers: 1 Down peers: 1
     Table Tot Paths Act Paths Suppressed History Damp State Pending
     inet.0
     1 0 0 1 1 0
     Peer     AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
     20.1.1.2 2   44   43     0    2     5                  Active

Route learned from it is present in damping history and damping parameters associated with it are maintained there, so that if the peer comes up again or flaps, parameters are modified accordingly:

     [edit]
     lab@M320--RE0# run show route damping history

     inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
     + = Active Route, - = Last Active, * = Both

     1.1.1.1/32 [BGP ] 00:18:33, localpref 100
     AS path: 2 I, validation-state: unverified
     > to 20.1.1.2 via ge-1/0/0.0
     lab@M320-RE0# run show route damping history detail

     inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
     1.1.1.1/32 (1 entry, 0 announced)
     BGP /-101
     Next hop type: Router, Next hop index: 518
     Address: 0x143c00f4
     Next-hop reference count: 1
     Source: 20.1.1.2
     Next hop: 20.1.1.2 via ge-1/0/0.0, selected
     Session Id: 0x18c
     State: <Hidden Ext>
     Local AS: 1 Peer AS: 2
     Age: 45:51
     Validation State: unverified
     Task: BGP_2.20.1.1.2
     AS path: 2 I
     Accepted
     Localpref: 100
     Router ID: 1.1.1.1
     Merit (last update/now): 740/282
     Default damping parameters used
     Last update: 00:20:58 First update: 00:27:30
     Flaps: 1
     History entry. Expires in: 00:07:40

Since these routes are present in damping history, they are seen as hidden routes in the router:

      [edit]
     lab@M320-RE0# ...l bgp 20.1.1.2 hidden

     inet.0: 6 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
     Prefix     Nexthop  MED Lclpref AS path
     1.1.1.1/32 20.1.1.2 2   I

This is  expected behavior with BGP damping enabled. It will not be seen if we disable BGP damping. For instructions, see Example: Configuring BGP Flap Damping .

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