[PTX] Micro BFD limitation with Link Aggregation Control Protocol (LACP)

Summary:

When a PTX device is rebooted/upgraded, the micro BFD that is running over the AE interface tears down the Bidirectional Forwarding Detection (BFD) session and does not appear in the active sessions on the device. Also, Link Aggregation Control Protocol (LACP) for the child interfaces transitions to a "Detached" MUX state. This behavior can be seen when an aggregated interface IP address is used to establish the micro BFD session.

This article explains that micro BFD establishment on PTX Series devices can happen seamlessly only when a loopback address is used for micro BFD session formation.

Symptoms:

The following events are seen:

BFD session tears down after session re-establishment.
LACP transitions to Detached MUX state after link flap.
Micro BFD session and LACP remain in Down state unless configuration changes are made.

Example configuration on PTX1000

set interfaces ae103 mtu 9192
set interfaces ae103 aggregated-ether-options bfd-liveness-detection minimum-interval 100
set interfaces ae103 aggregated-ether-options bfd-liveness-detection multiplier 5
set interfaces ae103 aggregated-ether-options bfd-liveness-detection neighbor 172.17.103.0
set interfaces ae103 aggregated-ether-options bfd-liveness-detection local-address 172.17.103.1
set interfaces ae103 aggregated-ether-options link-speed 100g
set interfaces ae103 aggregated-ether-options lacp active
set interfaces ae103 aggregated-ether-options lacp periodic fast
set interfaces ae103 unit 0 family inet address 172.17.103.1/31
set interfaces et-0/0/0:0 gigether-options 802.3ad ae103
 
jtac@ptx # run show bfd session
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
172.17.103.0             Up        et-0/0/0:0     0.500     0.100        5 
 
jtac@ptx# run show lacp interfaces ae103

Aggregated interface: ae103
    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      et-0/0/0:0     Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      et-0/0/0:0   Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
    LACP protocol:        Receive State  Transmit State          Mux State
      et-0/0/0:0                Current   Fast periodic Collecting distributing

After Router Reboot

jtac@ptx # run show bfd session
 
0 sessions, 0 clients
Cumulative transmit rate 0.0 pps, cumulative receive rate 0.0 pps
 
jtac@ptx# run show lacp interfaces ae103

Aggregated interface: ae103
    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      et-0/0/0:0     Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      et-0/0/0:0   Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
    LACP protocol:        Receive State  Transmit State          Mux State
      et-0/0/0:0          Port Disabled   No periodic            Detached

Cause:

PTX Series devices have a limitation as compared to MX Series devices in this aspect. Although devices running Junos OS versions later than 16.1 have the flexibility to configure micro BFD with AE interface addresses for BFD session establishment, this feature does not hold good for PTX Series devices. Micro BFD establishment on PTX series is seen to happen seamlessly only when a loopback address is used for micro BFD session formation.

Solution:

To establish a micro BFD session, use a loopback address as shown below:

Example configuration on PTX1000

set interfaces ae103 mtu 9192
set interfaces ae103 aggregated-ether-options bfd-liveness-detection minimum-interval 100
set interfaces ae103 aggregated-ether-options bfd-liveness-detection multiplier 5
set interfaces ae103 aggregated-ether-options link-speed 100g
set interfaces ae103 aggregated-ether-options lacp active
set interfaces ae103 aggregated-ether-options lacp periodic fast
set interfaces ae103 unit 0 family inet address 172.17.103.1/31
set interfaces et-0/0/0:0 gigether-options 802.3ad ae103
set interfaces ae103 aggregated-ether-options bfd-liveness-detection neighbor 1.1.1.1   >>>> Loopback IP address of remote device
set interfaces ae103 aggregated-ether-options bfd-liveness-detection local-address 2.2.2.2  >> Loopback IP address of local PTX device device
 
jtac@ptx> show lacp interfaces

Aggregated interface: ae103
    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      et-0/0/0:0     Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      et-0/0/0:0   Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
    LACP protocol:        Receive State  Transmit State          Mux State
      et-0/0/0:0                Current   Fast periodic Collecting distributing
 
jtac@ptx> show bfd session
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
1.1.1.1                  Up        et-0/0/0:0     0.500     0.100        5

After Router Reboot

jtac@ptx> show lacp interfaces 

Aggregated interface: ae103
    LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
      et-0/0/0:0     Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
      et-0/0/0:0   Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
    LACP protocol:        Receive State  Transmit State          Mux State 
      et-0/0/0:0                Current   Fast periodic Collecting distributing
 
jtac@ptx> show bfd session 
                                                  Detect   Transmit
Address                  State     Interface      Time     Interval  Multiplier
1.1.1.1                  Up        et-0/0/0:0     0.500     0.100        5

Note: Changing the local aggregate interface IP address as the local address instead of the local loopback IP address would also produce seamless results as shown in the above output. But the best practice would be to configure micro BFD based on loopback addresses in PTX Series devices.

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