The text identifier of the numbered interface, associated with the
specified virtual router, whose IP address is used as the source address
when transmitting IP 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 IP interface is numbered.