Separating a test network w/ 1720 router

Unanswered Question
May 31st, 2007
User Badges:

I've got what may seem like an obvious problem, but I'm trying to create a test network separate from my production environment. I have a spare 1720 router with a WIC-1DSU-T1 card. I have configured the FE0 interface for the new test network, and the S0 interface for the production network. I'm having trouble getting the serial interface to work. It is in alarm state. Is that because I need "something" on the other end to attach S0 to? Currently, it's just attached to a switch in the production environment. Any help is appreciated.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
mohammedmahmoud Thu, 05/31/2007 - 15:11
User Badges:
  • Green, 3000 points or more


What kind of switches is it attached to, and please do post your configuration.


Mohammed Mahmoud.

bjornarsb Fri, 06/01/2007 - 01:00
User Badges:
  • Bronze, 100 points or more


What type of port are you using at the switch ?

Use a wic-1T on your 1720 instead.



pstebner1 Fri, 06/01/2007 - 06:56
User Badges:

You cannot attach a switch to the WIC-1SU-T1 interface. You would have to attach it to a serial interface on another router using a T1 crossover (cross 1,2 to 4,5)



dbuller Fri, 06/01/2007 - 08:59
User Badges:

Thank you all for your responses. The problem is that I have only one router, not two. Is there any configuration I could do with the existing router to separate the networks using only that one router? I do happen to have a WIC-1T if that helps.

My router config follows:

version 12.2

no parser cache

no service single-slot-reload-enable

service timestamps debug uptime

service timestamps log uptime

service password-encryption


hostname IT_Test_Router


logging rate-limit console 10 except errors

enable secret xxx


memory-size iomem 20

ip subnet-zero

ip name-server


no ip dhcp-client network-discovery




interface FastEthernet0

speed auto



interface Serial0

ip address

encapsulation ppp


ip classless

ip route

ip route FastEthernet0

no ip http server



line con 0

line aux 0

line vty 0 4


no scheduler allocate


pstebner1 Fri, 06/01/2007 - 10:11
User Badges:

Tha router wn't help you without another ethernet interface, unles I am misunderstanding what you are trying to do.

What kind of switch(es) do you have? Are any layer 3? Alternately, you could create a seperate VLAN on your existing switch for your test network, trunk your uplink to the router and create subinterfaces on the FE port of the router that the switch is uplinked to, depending on your goal. Can you maybe post a diagram of what you are trying to accomplish?


dbuller Fri, 06/01/2007 - 12:56
User Badges:

I have attached a very simple diagram of what I'm trying to do.

Yes, we have some layer 3 switches and layer 2 switches. I'm basically just trying to figure out how I can use the router to separate out the test network. My PIX firewall doesn't have any extra ports, or else I'd just hang it off of that.

It might come down to me just having to purchase another router, which I'm okay with, I just thought I should be able to do this with just one router.

sundar.palaniappan Fri, 06/01/2007 - 15:24
User Badges:
  • Green, 3000 points or more

Though you don't have a second interface to physically separate the two networks you may be able to logically separate the networks using router-on-a-stick feature. Configure two VLANs on the switch, set the switchport connected to the router as a trunk and on the router configure dot1q/isl sub-interface(s) to route traffic for the two VLANs. On your switch assign the ports to which the production or test PCs connected to the appropriate VLANs.

Here's a couple of links that can help you with the configuration.

Use the Cisco Feature Navigator tool see if your router and the IOS support router-on-a-stick feature.



dbuller Tue, 06/05/2007 - 09:43
User Badges:

Thank you Sundar and Pstebner1. I will try this configuration and see if it works. Wish me luck!

ryancolson Sun, 06/17/2007 - 07:52
User Badges:

My understanding is that the 1720 does not support trunking. I believe you would need a 1721 for that.

pstebner1 Sun, 06/17/2007 - 19:41
User Badges:


Good catch. The 1720 does not support sub-interfaces, so therefore no trunks.



This Discussion