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BGP fast external failover doesn't work when received EBGP route NH is not same as peer IP

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TitleBGP fast external failover doesn't work when received EBGP route NH is not same as peer IP
Symptoms
BGP fast-external-failover doesn't work when received EBGP route NH is not same as peer IP. To work BGP fast-external-fallover, route should have been received from EBGP peer:
* eBGP-switch.28 # sh bgp nei

     Peer            AS         Weight State        InMsgs OutMsgs(InQ)  Up/Down
-----------------------------------------------------------------------------------
Ie-- 198.18.4.237   64533      1      ACTIVE       0      0     (0    ) 0:18:28:14
Ee-- 198.18.5.161   64530      1      ESTABLISHED  132    132   (0    ) 0:0:02:03
Ee-- 198.18.5.185   64530      1      ESTABLISHED  130    139   (0    ) 0:0:02:04


* eBGP-switch.29 # sh bgp routes all

Routes:
     Destination         Peer            Next-Hop        LPref Weight MED        AS-Path
----------------------------------------------------------------------------------------
*>i  0.0.0.0/0           198.18.5.161   198.18.5.161   100   1      0          64530    
*>i  10.1.0.0/16         198.18.5.185   198.18.4.244   100   1      0          64530  ---> NH changed through Route-policy.
After disabling the ports:
* eBGP-switch.31 # sh bgp nei

     Peer            AS         Weight State        InMsgs OutMsgs(InQ)  Up/Down
-----------------------------------------------------------------------------------
Ie-- 198.18.4.237   64533      1      ACTIVE       0      0     (0    ) 0:18:29:19
Ee-- 198.18.5.161   64530      1      IDLE         134    136   (0    ) 0:0:00:02
Ee-- 198.18.5.185   64530      1      ESTABLISHED  132    142   (0    ) 0:0:03:10  ---> Didn't go IDLE state. !!ISSUE!!

Flags: (d) disabled, (e) enabled, (E) external peer, (I) internal peer
       (m) EBGP multihop, (r) route reflector client

BGP Peer Statistics
  Total Peers       : 3
  EBGP Peers        : 2                 IBGP Peers          : 1
  RR Client         : 0                 EBGP Multihop       : 0
  Enabled           : 3                 Disabled            : 0
* eBGP-switch.32 # sh bgp routes all

Routes:
     Destination         Peer            Next-Hop        LPref Weight MED        AS-Path
----------------------------------------------------------------------------------------
u i  10.1.0.0/16         198.18.5.185   198.18.4.244   100   1      0          64530    ---> Route is unfeasible as expected.
Environment
  • EXOS 15.x, 16.x
  • All EXOS platforms
  • eBGP
Cause
This behaviour is by current implementation.
BGP fast-external-fallover is working when switch meet below conditions.
  • Route should have been received from EBGP peer.
  • Next-hop address should be same as EBGP peer IP.
Such behaviour will be changed via CR: xos0067488
Resolution
Upgrade to EXOS version where CR xos0067488 is implemented.
As a workaround BFD can be used to detect link down event.
Additional notes

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