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Community Member

Applying an IP address to an Ethernet subinterface?

Per other threads in this forum, I'm trying to use subinterfaces on a 2621's Fast Ethernet ports instead of using multiple secondary IP addresses on the physical port. I can create the subinterfaces, but IOS won't let me assign an IP address to them. The message I'm getting is being truncated, but starts off with:

"Configuring IP routing on a LAN subinterface is only allowed if that subinterface is already configured as part of an IEEE 802.10, IEEE 802.1Q, or I." which point the message is cut off. I'm trying to use the "ip address" interface configuration command to apply NAT-inside addresses to these subinterfaces. Any assistance will be greatly appreciated.


Community Member

Re: Applying an IP address to an Ethernet subinterface?

You have to configure your encapsulation type (ISL, 802.1Q) on the subinterface BEFORE you configure a IP address.


interface FastEthernet0/0.1

description VLAN 1

encapsulation isl 1

ip address

interface FastEthernet0/0.2

description VLAN 2

encapsulation isl 2

ip address yyy.yyy.yyy.yyy

interface FastEthernet0/0.99

description VLAN 99

encapsulation dot1Q 99

ip address zzz.zzz.zzz.zzz

interface FastEthernet0/0.100

description VLAN 100

encapsulation dot1Q 100

ip address nnn.nnn.nnn.nnn

Community Member

Re: Applying an IP address to an Ethernet subinterface?

And do not forget you need at least the ip plus feature set software to support trunking.

Community Member

Re: Applying an IP address to an Ethernet subinterface?

I'm not sure I can do this.

This 2621's FE port connects directly to a Cisco Aironet 350 wireless access point. That's the only thing to which it connects. The sole function of this FE port is to provide connectivity to the AP.

The AP serves several small workgroups, each of which has a non-Cisco bridge-style client device - essentially a box with an antenna on one side and an Ethernet port on the other. Each workgroup uses a different 172.xx.0.0 subnet... for the first group, for the second group, and so on. The total number of actual devices is small so the single AP can easily support them, but it's important to keep them on separate subnets.

I've been simply adding each workgroup's gateway IP as a secondary IP address on the 2621's FE port, and that's been working perfectly. But I believe the max number of secondary IP's per physical port is six. The 2600 supports far more than six subinterfaces, so this provides for expansion beyond six subnets.

As I understand it, 802.1Q is really just an encapsulation protocol. It requires a corresponding device on the "other end" of the wire. That's not the situation here - I'm not running VLAN's, I just need to logically access more than six subnets from a single FE port.

Any recommendations gratefully accepted. Thanks!

Community Member

Re: Applying an IP address to an Ethernet subinterface?

Looks like your stuck with secondary addresses

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