This KB article explains a behavior observed in active/standby LACP setups, where interfaces conncred to the sandby device may switch to "Fast" mode even when configured for "Slow" mode.
Symptoms
• The interface connected to the standby peer unexpectedly switches to "Fast" mode.
• During a failover event, the interface that was previously in the "Collecting Distributing" state transitions to "Slow" mode and viceversa.
Topology
In this scenario, we have a two-member EX4650 VC, with each member connected to one of the upstream firewall clusters consisting of two devices. One firewall operates in active mode, while the other remains in standby. These connections form an LACP bundle on interface ae0. The LACP bundle is configured to operate in "Slow" mode on both ends—the EX4650 VC and the firewall cluster—as shown below:
root@user# run show lacp interfaces ae0
Aggregated interface: ae0
LACP state: Role Exp Def Dist Col Syn Aggr Timeout Activity
et-0/0/1 Actor No No Yes Yes Yes Yes Slow Active
et-0/0/1 Partner No No Yes Yes Yes Yes Slow Active
et-1/0/1 Actor No No Yes Yes Yes Yes Slow Active
et-1/0/1 Partner No No Yes Yes Yes Yes Slow Active
LACP protocol: Receive State Transmit State Mux State
et-0/0/1 Current Slow periodic Collecting distributing
et-1/0/1 Current Slow periodic Collecting distributing
When the firewall begins operating in active/standby mode, it is expected that the interface connected to the active device will enter the "Collecting Distributing" state, while the interface connected to the standby device will appear "Detached", as it does not forward traffic toward the VC. However, upon inspecting the standby interface, you may observe that it unexpectedly switches to "Fast" mode without any manual intervention:
et-0/0/1 Actor No Yes No No No Yes Slow Active
et-0/0/1 Partner No Yes No No No Yes Fast Passive
et-0/0/1 Defaulted Fast periodic Detached
This behavior is completely expected. The behavior—where one side shows “Fast” and the other is set to “Slow”—is a result of how LACP negotiation operates and how Juniper displays the state, which reflects what each side advertises. By default, Juniper uses a “Fast” transmission rate for LACP. If LACP BPDUs are not being exchanged with the peer, Junos cannot determine the peer’s actual LACP mode. In such cases, Junos defaults to displaying “Fast” as a placeholder, since it has no visibility into the peer’s advertised capabilities.
If LACP BPDUs are not being received from the firewall, we have no way to determine whether it is operating in “Slow” mode. As a result, during a failover, the interface that transitions from a "Detached" state to "Collecting Distributing" will be able to negotiate the correct transmission rate with us and the one that goes to "Detached" switches to the default “Fast” mode. This behavior occurs due to the lack of bidirectional exchange of LACP BPDUs between the two devices. In summary, this is expected behavior given the current setup.