7.21.4 BUG (SUP-215681)
After upgrading my RB5009 from RouterOS 7.20.8 long-term to RouterOS 7.21.4 long-term, the WAN Ethernet link on ether8 started to flap repeatedly (82 times in 2 hrs) when connected directly to a Vodafone DOCSIS 3.1 cable modem operating in bridge mode.
The same physical setup was stable on RouterOS 7.20.8. The issue started immediately after the upgrade to 7.21.4.
Update (2026-05-6): The RB5009 ether8 WAN link flapping after upgrading to RouterOS 7.21.4 appears to be resolved by setting all Ethernet interfaces to the new RB5009 default l2mtu=1596.
Update (2026-05-10): The RB5009 ether8 WAN link was stable for several days, but after the last router reboot the link flapping returned without any configuration changes. Case re-oppened.
Update (2026-05-18): Upgrading RB5009 to RouterOS 7.22.3 resolved the ether8 WAN link flapping.