The Dual Homed Link uses two links with a number of VLANs split between
them from the common pool of vlans, in such a way that any vlan is not
associated with both of the redundant links at the same time to avoid formation
of loops and also VLANs are still connected to the core when one link fails.
STP is disabled on both the DHL links implicitly.
When linkA or linkB goes down or becomes up and both the ports are
are part of some vlan-map, then this
trap is sent to the Management Entity, with the DHL port information.