it is explained on the same page your link refers to:
By default, the 10/100 ports and the dual-purpose ports on Cisco ME 3400-12CS and ME 3400-2CS switches are configured as UNIs, and the SFP-only module uplink ports are configured as NNIs. No ports are ENIs by default.
If the switch is running the metro base or metro access image, only four ports on the switch can be configured as NNIs at one time. If the switch is running the metro IP access image, there is no limit to the number of NNIs that can be configured on the switch. All ports on the switch can be configured as UNIs or ENIs.
The default state for a UNI or ENI is administratively down to prevent unauthorized users from gaining access to other ports as you configure the switch. Traffic is not switched between these ports, and all arriving traffic at UNIs or ENIs must leave on NNIs to prevent a user from gaining access to another user's private network. If it is appropriate for two or more UNIs or ENIs to exchange traffic within the switch, the UNIs and ENIs can be assigned to a community VLAN. SeeChapter 11 "Configuring VLANs," for instructions on how to configure community VLANs.
Note Even though the default state for a UNI or ENI is shutdown, entering the default interface interface-id command changes the port to the enabled state.
The default status for an NNI is administratively up to allow a service provider remote access to the switch during initial configuration.
A port can be reconfigured from UNI to NNI or ENI and the reverse. When a port is reconfigured as another interface type, it inherits all the characteristics of that interface type. When you reconfigure a UNI or ENI to be an NNI, you must enable the port before it becomes active.
Changing the port type from UNI to ENI does not affect the administrative state of the port. If the UNI status is shut down, it remains shut down when reconfigured as an ENI; if the port is in a no shutdown state, it remains in the no shutdown state. At any time, all ports on the Cisco ME switch are either UNI, NNI, or ENI.
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