Hi,
are you talking about a router or switch interface?
1) On a router (sub)interface there is a general secondary IP problem: Ethernet broadcasts sent from one subnet will bother other subnets devices CPUs, traffic sent within one subnet can be captured on another subnet device. There are some routing limits, too - secondary IP participates in OSPF only if primary does, e.g.
2) On a switch int VLAN1 - it's possible to configure secondary IP address on some devices (Cat3500XL, e.g.), but it doesn't work, it's just a remain in IOS.
Regards,
Milan
