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The user is unable to assign an IP address to a Layer 3 (L3) sub-interface on a Catalyst 3750 or 3550 switch

Core issue

A routed port is a physical port that acts like a port on a router. It does not have to be connected to a router. A routed port is not associated with a particular VLAN, as is an access port. A routed port behaves like a regular router interface, except that it does not support VLAN sub-interfaces. Routed ports can be configured with an L3 routing protocol.

When trying to configure an IP address on a sub-interface of a switch, this message may be displayed:

% Configuring IP routing on a LAN subinterface is only allowed if that sub-interface is already configured as part of an IEEE 802.10, IEEE 802.1Q, or ISL vLAN.

There is no option to configure the encapsultation on the sub-interface.

Resolution

By default, the Catalyst 3750 or 3550 is a switch performing L2 functions and has L3 unicast routing disabled. You can enable this function by issuing the ip routing global command.

If you want to convert any port to a L3 port, issue the no switchport command.

Do not configure Ethernet sub-interfaces on a switch. Configure it as a trunk, and terminate the L3 IP interface on a Switched Virtual Interface (SVI).

All the features that are normally found on the Catalyst 3750 and 3550 are not available on the switch. That is, there are certain unsupported features on this platform. These features are not supported:

  • Configuring an IP address on a sub-interface of a switch

  • Configuring the encapsulation on the sub-interface

  • Network Address Translation (NAT)
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