AE interface is configured with child interfaces used from two different FPCs, example, FPC slot 5 and 10.When the FPC in slot 10 is restarted, BFD flaps in the AE interface though the child interfaces also belong to FPC in slot 5 which is stable. Example Config:
user@router> show configuration | display set | match ae1set interfaces et-5/0/0 gigether-options 802.3ad ae1set interfaces et-5/0/1 gigether-options 802.3ad ae1set interfaces et-5/0/2 gigether-options 802.3ad ae1set interfaces et-10/0/0 gigether-options 802.3ad ae1set interfaces et-10/0/1 gigether-options 802.3ad ae1set interfaces et-10/0/2 gigether-options 802.3ad ae1
user@router> show configuration | display set | match ae1
set interfaces et-5/0/0 gigether-options 802.3ad ae1
set interfaces et-5/0/1 gigether-options 802.3ad ae1
set interfaces et-5/0/2 gigether-options 802.3ad ae1
set interfaces et-10/0/0 gigether-options 802.3ad ae1
set interfaces et-10/0/1 gigether-options 802.3ad ae1
set interfaces et-10/0/2 gigether-options 802.3ad ae1
BFD is configured for BGP as per the below config.
user@router> show configuration routing-instances VRF protocols bgp | display setset routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection minimum-interval 500set routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection minimum-receive-interval 500set routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection multiplier 3
user@router> show configuration routing-instances VRF protocols bgp | display set
set routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection minimum-interval 500
set routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection minimum-receive-interval 500
set routing-instances VRF protocols bgp group ROUTER bfd-liveness-detection multiplier 3
Single hop BFD is running for BGP protocol:
user@router> show bfd session detailMar 08 14:22:47Detect TransmitAddress State Interface Time Interval Multiplier<IP address> Up ae1.131 1.500 0.500 3 Client BGP, TX interval 0.500, RX interval 0.500Session up time 00:23:44, previous down time 00:00:50Local diagnostic None, remote diagnostic NoneRemote state Up, version 1ReplicatedSession type: Single hop BFDDetect TransmitAddress State Interface Time Interval Multiplier<IP address> Up ae2.132 1.500 0.500 3 Client BGP, TX interval 0.500, RX interval 0.500Session up time 00:23:44, previous down time 00:00:50Local diagnostic None, remote diagnostic NoneRemote state Up, version 1ReplicatedSession type: Single hop BFD
user@router> show bfd session detail
Mar 08 14:22:47
Detect Transmit
Address State Interface Time Interval Multiplier
<IP address> Up ae1.131 1.500 0.500 3
Client BGP, TX interval 0.500, RX interval 0.500
Session up time 00:23:44, previous down time 00:00:50
Local diagnostic None, remote diagnostic None
Remote state Up, version 1
Replicated
Session type: Single hop BFD
<IP address> Up ae2.132 1.500 0.500 3
By default, BGP uses single-hop BFD sessions if the peer is directly connected to the router’s interface. BGP uses multihop BFD sessions if the peer is not directly connected to the router’s interface.Single-hop BFD in Junos OS runs in distributed mode by default.The term “distributed BFD” refers to BFD that runs on the Packet Forwarding Engine.
user@router> show ppm adjacencies protocol bfd detailMar 08 14:18:43Protocol: BFD, Hold time: 1500, IFL-index: 381Distributed: TRUE <<---------------------------------------- PFE distribution is True.ReplicatedBFD discriminator: 16, BFD routing table index: 0Redirection Type: DYNAMIC_FILTER, Rule Term Src: <IP address>, Rule Term Port: 3784, Rule Term Action: 825Num Packets: 5, Absorbed Packets: 0, Rx Packet: 21 C8 03 18 00 00 44 B7 00 00 00 10 00 07 A1 20 00 07 A1 20 00 00 00 00Distribution handle: 363, Distribution address: fpc5 <<------- Session distributed to FPC5
user@router> show ppm adjacencies protocol bfd detail
Mar 08 14:18:43
Protocol: BFD, Hold time: 1500, IFL-index: 381
Distributed: TRUE <<---------------------------------------- PFE distribution is True.
BFD discriminator: 16, BFD routing table index: 0
Redirection Type: DYNAMIC_FILTER, Rule Term Src: <IP address>, Rule Term Port: 3784, Rule Term Action: 825
Num Packets: 5, Absorbed Packets: 0, Rx Packet: 21 C8 03 18 00 00 44 B7 00 00 00 10 00 07 A1 20 00 07 A1 20 00 00 00 00
Distribution handle: 363, Distribution address: fpc5 <<------- Session distributed to FPC5
If FPC5 reboots, the BFD session will flap and the BFD gets distributed to FPC10 which has the other child interfaces. BFD session flap will cause BGP session flap. This behavior is expected and there is no issue with the protocol or specific hardware.