Cisco Router Dual WAN Uplinks with NAT

Dual WAN uplinks for resilience are a common request when configuring small business routers. I’ll work with the topology below and go through the configuration.

Screenshot 2020-02-13 at 20.56.40.png

The only devices I’ll be going over are R1 and PC1. In the real world, the rest would be out of your control anyway. I’ll include the GNS3 project file at the end of this post if you would like to play with it.

Configuration

The first thing we need to do is configure the interfaces of the two ISP connections.

interface GigabitEthernet0/0
  description ISP1
  ip address 100.1.1.2 255.255.255.252
  ip nat outside
!
interface GigabitEthernet1/0
  description ISP2
  ip address 200.2.2.2 255.255.255.252
  ip nat outside
!

And the LAN interface

interface GigabitEthernet6/0
  description LAN
  ip address 192.168.1.1 255.255.255.0
  ip nat inside
!

Next we’ll configure our routes via both ISP’s. To make the failover work we need to track some objects on the primary connection. This will make failover occur if the internet connection goes down. I would advise you track reachability of a couple of hosts to avoid failing over if somebody else is having an issue. I would also advise against tracking the next hop, as an issue within the ISP network wouldn’t cause failover to occur but may prevent you from reaching the internet. We do this using ip sla to two different known hosts (I used Google’s public DNS servers for this demo).

ip sla 100
icmp-echo 8.8.8.8 source-interface GigabitEthernet0/0
threshold 1000
timeout 1000
frequency 10
ip sla schedule 100 life forever start-time now
ip sla 101
icmp-echo 8.8.4.4 source-interface GigabitEthernet0/0
threshold 1000
timeout 1000
frequency 10
ip sla schedule 101 life forever start-time now

Next we create two track objects to monitor the ip sla’s for reachability.

track 100 ip sla 100 reachability
!
track 101 ip sla 101 reachability

Then a track object to track the first two objects. By using the boolean or option, the track will go down if all of the tracked objects go down but will not if only one goes down.

track 105 list boolean or
object 100
object 101

Now we will add our default routes via both ISP’s. We will use the track 105 object to determine if ISP1 is up and add it to the routing table. Otherwise we will add ISP2 with a metric of 10.

ip route 0.0.0.0 0.0.0.0 100.1.1.1 track 105
ip route 0.0.0.0 0.0.0.0 200.2.2.1 10

That should be the routing done, now we will need to configure NAT to allow the LAN clients to access the internet. We’ll create an access list to define the LAN traffic that should be translated.

ip access-list standard NAT-INSIDE
permit 192.168.1.0 0.0.0.255
!

And we’ll use some route-maps to match the LAN traffic on the outside interfaces for translation.

route-map RM-NAT-ISP2 permit 20
match ip address NAT-INSIDE
match interface GigabitEthernet1/0
!
route-map RM-NAT-ISP1 permit 10
match ip address NAT-INSIDE
match interface GigabitEthernet0/0
!

And finally, the NAT configuration commands.

ip nat inside source route-map RM-NAT-ISP1 interface GigabitEthernet0/0 overload
ip nat inside source route-map RM-NAT-ISP2 interface GigabitEthernet1/0 overload

Verification

Now we can do some testing. If we ping from PC1 to 8.8.8.8, the ping should succeed. You can also perform a traceroute from PC1 to 8.8.8.8 to verify the route the traffic flows.

PC1#ping 8.8.8.8
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 48/85/196 ms

PC1#traceroute 8.8.8.8
Type escape sequence to abort.
Tracing the route to 8.8.8.8
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.1.1 16 msec 48 msec 28 msec
2 100.1.1.1 104 msec 72 msec 44 msec
3 1.2.1.1 56 msec 60 msec 68 msec

You can use show ip nat translations on R1 to verify the NAT translations over ISP1.

R1#show ip nat translations
Pro Inside globalInside local Outside localOutside global
icmp 100.1.1.2:1025192.168.1.11:168.8.8.8:16 8.8.8.8:1025
udp 100.1.1.2:4501 192.168.1.11:49157 8.8.8.8:334378.8.8.8:33437
udp 100.1.1.2:4502 192.168.1.11:49158 8.8.8.8:334388.8.8.8:33438
udp 100.1.1.2:4503 192.168.1.11:49159 8.8.8.8:334398.8.8.8:33439
udp 100.1.1.2:4504 192.168.1.11:49160 8.8.8.8:334408.8.8.8:33440
udp 100.1.1.2:4505 192.168.1.11:49161 8.8.8.8:334418.8.8.8:33441
udp 100.1.1.2:4506 192.168.1.11:49162 8.8.8.8:334428.8.8.8:33442

And also, show ip route on R1 should show the next hop as ISP1.

S*    0.0.0.0/0 [1/0] via 100.1.1.1

Now, if we suspend a link connected to the ISP1 route, it doesn’t matter which one, our topology should failover.

Screenshot 2020-02-13 at 21.44.38.png

First thing you should notice is the track objects going down on R1.

*Feb 13 21:44:28.847: %TRACKING-5-STATE: 100 ip sla 100 reachability Up->Down
*Feb 13 21:44:28.851: %TRACKING-5-STATE: 101 ip sla 101 reachability Up->Down
*Feb 13 21:44:29.835: %TRACKING-5-STATE: 105 list boolean or Up->Down

And the default route on R1 should have changed to ISP2.

S*0.0.0.0/0 [10/0] via 200.2.2.1

And if we repeat the same ping and traceroute from PC1 to 8.8.8.8, they should still work fine, but the route should show ISP2 as the second hop instead of ISP1.

PC1#ping 8.8.8.8
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/56/84 ms

PC1#traceroute 8.8.8.8
Type escape sequence to abort.
Tracing the route to 8.8.8.8
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.1.1 12 msec 88 msec 24 msec
2 200.2.2.1 52 msec 32 msec 36 msec
3 1.2.2.1 64 msec 64 msec 36 msec

show ip nat translations on R1 should also show the NAT translations to ISP2 now.

R1#show ip nat translations
Pro Inside globalInside local Outside localOutside global
icmp 200.2.2.2:1024192.168.1.11:2 8.8.8.8:28.8.8.8:1024
udp 200.2.2.2:4501 192.168.1.11:49167 8.8.8.8:334378.8.8.8:33437
udp 200.2.2.2:4502 192.168.1.11:49168 8.8.8.8:334388.8.8.8:33438
udp 200.2.2.2:4503 192.168.1.11:49169 8.8.8.8:334398.8.8.8:33439
udp 200.2.2.2:4504 192.168.1.11:49170 8.8.8.8:334408.8.8.8:33440
udp 200.2.2.2:4505 192.168.1.11:49171 8.8.8.8:334418.8.8.8:33441
udp 200.2.2.2:4506 192.168.1.11:49172 8.8.8.8:334428.8.8.8:33442

Now if we resume the link to the ISP1 route, the track objects will come back up and everything should fail back.

*Feb 13 21:50:43.871: %TRACKING-5-STATE: 100 ip sla 100 reachability Down->Up
*Feb 13 21:50:43.871: %TRACKING-5-STATE: 101 ip sla 101 reachability Down->Up
*Feb 13 21:50:44.839: %TRACKING-5-STATE: 105 list boolean or Down->Up

Notes

There are a couple of things to note with this configuration:

  • There is zone-based firewall configuration in this demo. I highly recommend one if you aren’t using a dedicated firewall.
  • Inbound connections using port forwarding and the primary connection IP address will not failover if ISP1 fails.
  • The IOS image used in this GNS project is c7200-advipservicesk9-mz.152-4.S5.bin. You’ll have to provide that yourself.

Downloads

GNS3 Project – Dual WAN

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