Configuring Cisco for SingNet Bizlink

Situation

You switched to SingNet Bizlink static IP plan and were given a Cisco 877. You have your own router and/or firewall in your current setup and all you need is an ADSL modem to replace your current CPE. This way, you can keep everything else in place and simply do an IP change.

Problem

You have searched for configuration samples for the Cisco and tried in vain to get it to work as a transparent bridge but failed. You need to use the Cisco because that is the only device you have that can connect you to the ADSL service.

Resolution

Go the routing way by creating another routing subnet between the Cisco and your existing router.

So the topology will look like this



Configurations

1. Set Cisco ATM interface as required with your public WAN IP eg 1.1.1.0/30

interface ATM0
no ip address
no atm ilmi-keepalive
dsl operating-mode auto
!
interface ATM0.1 point-to-point
ip address 1.1.1.2 255.255.255.252
ip nat outside
pvc 8/35
encapsulation aal5snap
!

2. Set the VLAN interface with a small subnet private ip that is not part of your own private IP eg 10.255.255.252/30

interface VLAN1
ip address 10.255.255.253 255.255.255.252
ip nat inside

3. Set your own router WAN interface within the same range as the Cisco VLAN interface, in this case 10.255.255.254 255.255.255.252

4. Set your own router LAN interface with the public LAN IP assigned by SingTel eg 2.2.2.0/28

5. Add a route on the Cisco for all traffic to point to SingTel side and another route to your public LAN network behind your own router/firewall.

ip route 0.0.0.0 0.0.0.0 1.1.1.1
ip route 2.2.2.0 255.255.255.240 10.255.255.254

6. Do a static NAT for your own router/firewall using the public WAN IP. This way traffic from the Internet will see the public WAN IP as your current router/firewall, as if the Cisco does no exist.

ip nat inside source static 10.255.255.254 1.1.1.2

Conclusions

Advantages
- You keep everything as it is and only need to change IP address.
- As good as configuring the Cisco as a bridge.

Disadvantages
- You MAY run into IP routing problem if you have private networks using the same range as the private IP assigned to the VLAN1 interface. However, by using the smallest network you can, this is quite unlikely.
- You have additional route when you do a traceroute from the inside.