L3 Sub Interfacing on a HWIC-4ESW

Answered Question
Feb 12th, 2009

Is it possible to create multiple L3 subinterfaces on a port on a HWIC-4ESW? The HWIC is in an 1841 running IOS 12.4.

I have this problem too.
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Correct Answer by Paolo Bevilacqua about 7 years 11 months ago

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Leo Laohoo Thu, 02/12/2009 - 20:44

I don't think this is possible. Cisco EtherSwitch HWICs are 10/100BASE-T Layer 2 Ethernet switches with Layer 3 routing capability. (Layer 3 routing is forwarded to the host and is not actually performed at the switch.) Traffic between different VLANs on a switch is routed through the router platform.


dnorthrup Thu, 02/12/2009 - 20:57

Thanks. I found the same document but could find no reference to L3 here or in any other resource. Obviously I was hoping to find some method to do this.

Richard Burts Fri, 02/13/2009 - 08:35


Perhaps I am not clear about what you are trying to do on the Etherswitch in the 1841. But it seems to me that you can get what I think you are trying to do by doing this:

- configure several VLANs in the switch.

- assign switch ports to different VLANs.

-configure VLAN interfaces, assign IP addresses.

You can not do multiple layer 3 subinterfaces in the Etherswitch but multiple VLAN interfaces gets you essentially the same result.



dnorthrup Fri, 02/13/2009 - 09:18

Hi Rick. Thanks for getting back to me. When I picked the project up the 4-port switch card had already been purchased with the intent to carry multiple subnets off of one port. This configuration is still needed so the plan is to swap out for a HWIC-1FE card.

Paolo Bevilacqua Fri, 02/13/2009 - 09:53

You should be able to do that with the HWIC-4ESW. For example:

interface fa x/y/z

switchport mode trunk

interface vlan10

ip address z.z.z.z

interface vlan 15

ip address k.k.k.k

This way you will have the two subnets 802.1q tagged as 10 and 15 out the referenced port.

Or, same VLANs as above but:

interface x/y/0

switchport mode access

switchport access vlan 10

interface x/y/1

switchport mode access

switchport access vlan 15

You will have untagged packets for the two subnets, separately, on the 1st and 2nd port of the module.


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