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Community Member

Single Interface Routing

Hi all,

     I am a recent student to Cisco products and I have purchased some (what I thought was good) lab equipment to learn with on a budget. What I have is a 2948G switch and a 2620 router. My issue is this: the router has only one fastEthernet port. Is it possible to use VLANs and VLAN Interfaces on the router and switch to somehow emulate a second interface to connect to a WAN or subnet?

Your response is very much appreciated.

Thanks!

Jake

Everyone's tags (4)
11 REPLIES
Cisco Employee

Single Interface Routing

Did you try creating a sub-interface?

Not sure how much it would be helpful but you could def give it a try....

-Vijay

Community Member

Re: Single Interface Routing

I did though it threw an error when I ran fa0/0.2 and tried to set an ip:

% 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 ISL vLAN.

Forgive me, I am still young into the profession. I know this has to do with encapsulation or something of that nature.

Cisco Employee

Re: Single Interface Routing

you have to set up the encapsulation techinique first.

try configuring this command on the interface

encapsulation dot1q

-Vijay

Community Member

Re: Single Interface Routing

Thanks Vijay, that appears to have worked. Though on the side of a true connection, I would then add vlan 2 to the same port on the switch and another port only on vlan 2 for the external link correct?

Cisco Employee

Re: Single Interface Routing

Are you trying to perform inter-vlan routing?

-Vijay

Community Member

Re: Single Interface Routing

Essentially, though I may be wrong as I still need to pick up on the terminology.

Bascially I have an internet connection and a connection to a switch from the router. I will have the physical internet ethernet connection to the switch on a port on VLAN2. VLAN2 would only contain that port (internet) and the port of the router (which would also be on VLAN1 for it's network and routing).

Cisco Employee

Re: Single Interface Routing

sorry jacob can you send your netowrk diagram along with what you are trying to do? I could not much on your topology.

-Vijay

Community Member

Re: Single Interface Routing

I need to route internet provided over ethernet to a subnet through my router through my switch. The problem arises that I only have one physical fastEthernet port on the router. I hope this helps.

Community Member

Re: Single Interface Routing

Vijay I'm still having issues creating my vlans with intervlan operation. I would love to get some internet conenction going to test some security hardware. Thanks.

Bronze

Single Interface Routing

What you're talking about is called a "router on a stick" and is quite common.

The router acts as the layer 3 point for multiple subnets, but uses a single physical connection.

The switch acts as the layer 2.

Some tips:

1. Ensure your switch port connecting to the router is a trunk port.

2. Create separate dot1q sub-interfaces for each subnet you want to use.

3. Create vlans on your switch to match your sub-interfaces.  (vlan2 = fa0/0.2 vlan 5 = fa0/0.5)

3. Put your WAN-connected switch port on its own vlan.

4. Put all your other switch ports on the appropriate vlan for its use.

5. The router should handle everything else since it sees all sub-interfaces as connected.

Ven

Ven Taylor
Community Member

Single Interface Routing

Thanks Ven! This sounds exactly what I have been trying to do. I will try it when I get back to my lab.

Thanks for the pointer on the terminology too.

Jacob

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