Broadcast domains within the Cisco Nexus Fabric Manager consist of a 1-to-1 mapping of VLANs to VXLAN VNI segments. When creating a broadcast domain, you may assign a VLAN or it is assigned by the Cisco Nexus Fabric Manager. The VXLAN VNI portion of the mapping is always assigned by the Cisco Nexus Fabric Manager and is a fixed offset from the VLAN ID of 20,000 by default.
The act of creating a broadcast domain within the Cisco Nexus Fabric Manager does not result initially in any configuration being created or pushed to the fabric switches. An active member perimeter interface must be added to the broadcast domain for it to become active, and for the Cisco Nexus Fabric Manager to push the configuration to the leaf switches.
In all cases, when a perimeter interface is assigned one or more broadcast domains, the perimeter interface is put into VLAN trunking mode. This is done via the
switchport mode trunk
CLI command that is present on the switch interface. The VLAN ID assigned to the broadcast domain is added as part of the allowed trunked VLANs via the
switchport trunk allowed vlan
CLI command. By default, all VLANs associated with broadcast domains that are applied to perimeter interfaces, are added in VLAN trunking mode and are tagged VLANs.
To assign a broadcast domain to a perimeter interface so that the associated VLAN is not tagged, you must first add the interface to the broadcast domain and then set the untagged broadcast domain of the interface to the same broadcast domain. This sets the native VLAN on the interface to be the requested VLAN. However, the interface remains in a trunking configuration.