Customer found LACP interface went down between two QFX5100 switches after upgrading Junos version to 21.4R3.S3.

Upon checking, all necessary LACP configuration remains the same that of before Junos upgrade. Yet there are no output of command "show lacp interface" on the QFX5100 switch. "show interface terse" shows the ae interface is down.

We found "link-speed" is configured as 80G in customer's ae interface configuration though.

Configuration knob “link-speed” for interface ae0 on two QFX5100 switches needs to be specified identical as the child port. In other words, if customer is using 2x 40G et-* ports as child ports of ae0, “link-speed” needs to be specified as 40G instead of 80G.

Interface ae0 comes up after adjusting the “link-speed”.

Publish KB

As to why link-speed 80g was honored in previous version and ae0 was up, here it is:

>>> On previous Junos versions such as 21.2R3-S2.9, the configuration below, link-speed mismatch between ae (80G) and its member ports (et-*, 40G), will be honored because of a bug.

set interfaces ae0 aggregated-ether-options link-speed 80g

>>> From Junos 21.4R3 onwards, the bug was fixed. In other words, “link-speed” of ae interfaces and its child/member ports must match otherwise the ae interface will be down. For example, in our case you are using et-0/0/52 and et-0/0/53, both 40G ports, as member interfaces of ae0. We must set link-speed of ae0 to 40G to bring it up.