Link-state tracking (LST) binds the link state of multiple interfaces, creating LST groups with upstream (to-be-followed) and downstream (to-follow) interfaces. LST monitors the state of upstream interfaces and automatically transfers the upstream state to the downstream interfaces. If all the upstream interfaces in a LST group are down, the downstream interfaces are administratively configured as down after approximately five seconds. If any upstream interface in a LST group is up, the downstream interfaces are not affected. The role of the LST group is to keep the downstream interfaces in the same state as the upstream interface.
An interface can be an aggregation of ports, multi-link trunks (MLT) or link aggregation groups (LAG). Interfaces can only belong to one LST group. You can configure LST using CLI or EDM. LST receives updates from Port Manager, MLT, and VLACP regarding the upstream state of ports and trunks in the group.
LST can detect a link failure of upstream interfaces and shutdown downstream interfaces, eliminating loss of traffic and allowing the source to reroute traffic. When a LST group disables a downstream interface, the interface can only be enabled by the LST group. You can recover the downstream interfaces that LST disabled by removing the interfaces from the LST group or by disabling the LST group. You can administratively enable or disable LST group downstream interfaces with shutdown commands. A LST group cannot enable ports that you administratively disabled.
Note
If you administratively enable an interface which LST disabled, only the administrative status of the interface changes. The interface remains disabled until the LST group enables the interface, or until you remove the interface from the LST group.
For MLTs, a last-link-down event triggers a down operational state and a first-link-up event triggers an up operational state. You cannot delete a MLT that is a member of a LST group.
For LAGs, a static LAG trunk ID associated with a LACP administrative key can be a member of a LST group. You can add LAGs to a LST group by specifying the LAG trunk ID. You cannot break the association between trunk ID, LACP key, and ports while the LAG trunk is in a LST group. For LACP, you cannot add ports with link-aggregation enabled to a LST group, or enable link-aggregation on interfaces already in a LST group.
You can add LST group interfaces configured with VLACP. For upstream interfaces with VLACP enabled, when the physical link is up with a VLACP partner, the operational state is up. Otherwise the operational state is down. If VLACP is enabled, the value of the VLACP have partner field and the link status correspond to the operational state of upstream interfaces. For upstream interfaces with VLACP disabled, the up and down operational states correspond directly with the physical link.
You can add LST group interfaces configured with SLPP Guard. When LST disables a port that is already disabled by SLPP Guard, the interface is unblocked by SLPP Guard and the blocking timer clears.
You can add LST group interfaces configured with BPDU Guard or MACsec. BPDU Guard and MACsec can enable or disable ports administratively. An interface is enabled if both LST and BPDU Guard or MACsec consider the port enabled. If BPDU Guard or MACsec disables the port, the port remains down and does not link up.