The text identifier of the numbered interface, associated with the
specified virtual router, whose IPv6 address is used as the source
address when transmitting IPv6 packets on unnumbered remote access user
links. Types/formats/examples for this string include:
Loopback
loopback <id>
'loopback 0'
ATM Virtual Circuit
atm <slot>/<port>.<distinguisher>
'atm 3/1.100'
Ethernet
{ fastEthernet | gigabitEthernet } <slot>/<port>
'fastEthernet 3/0'
'gigabitEthernet 3/0'
Ethernet VLAN
{ fastEthernet | gigabitEthernet } <slot>/<port>:<vlanID>
'fastEthernet 3/0:1000'
'gigabitEthernet 3/0:1000'
Channelized Serial
serial <slot>/<port>:<channelSpecifier>[/<channelSpecifier>]*
'serial 3/0:4' (T1/E1)
'serial 3/0:2/4' (T3/E3)
'serial 3/0:2/1/1/4' (OC3/OC12 - channelized DS3)
'serial 3/0:2/1/1/1/4' (OC3/OC12 - virtual tributaries)
Other formats may be supported over time.
An empty string indicates the referenced interface is unspecified, e.g.,
when this IPv6 interface is numbered.