Found this article from Cisco Forum. Credits to original author.

Network Address Translation is a very common feature used to address some issues and also to meet some networks’ requirements such as, overlapped networks and Internet links.

In this small document we will discuss a business requirement example, and the main idea behind this example is to demonstrate how to implement and configure NATign with dual homed Internet edge Router  in conjunction with other Cisco IOS advanced features (Policy Based routing PBR and IPSLA ).

Also we will see how all of the above mentioned features work together and how IP SLA will work like a gear to this implementation in term of controlling the exit path of the traffic by controlling the default route in the routing table and PBR decision.

Company has bought a second Internet connection with 1 Mbps in addition to the existing one with 512 Kbps.

  • the requirement is to load share the traffic over those two links
  • web traffic and telnet traffic must use the the new ISP link ISP2  and all other traffic must go thorough the old ISP link ISP1
  • in the case of any of the above links gose down all the traffic should use the remaining link

this example has been configured in a lab environment and al the private ip addresses used in this document just for the purpose of this example


Proposed solution:

  • According to the above requirements we will use Policy Based routing feature to control LAN traffic going to the Internet and which path to use.
  • all traffic from the LAN subnet destined to tcp 23, 80 and 443 must be routed to ISP 2  link with next hop
  • all other traffic will go though ISP 2 with next hop of
  • as we do not have any subnet or ip ranges to use it over the Internet we have to use NATing with overload option to use the Internet interface IP address of each ISP link. For example traffic going through ISP 1 will be seen by ISP one and the Internet as it is from if it is through ISP 2 will be seen as it is from
  • In the case of one of the links go down we need all the traffic to use the other remaining link. This will be archived here by using IP SLA with ICMP echo that will be sent to each of the ISP next hop IP addresses in our example and
  • the ICMP echo will be sent every 1 second with time out of 500 msec
  •  if the icmp reply not heard from any of those next hops within 1 second that link will be considered down and the default route in the Internet router pointing to that hop will be withdrawn from the routing table and the PBR descion will be changed based on that as well

interface FastEthernet1/0
description LAN interface
ip address
ip nat inside
ip policy route-map PBR    —- this is for policy based routing

interface FastEthernet1/1
description To ISP 1
ip address
ip nat outside
interface FastEthernet2/0
description To ISP 2
ip address
ip nat outside

  • as we can see above the inside interface was configured as inside NAT interface also a policy based routing with a name of PBR applied to that interface, the configurations of this PBR will be described later
  • both of the Internet ISP links configured as outside NAT interfaces

IP SLA configurations:
ip sla 1
timeout 500
frequency 1
ip sla schedule 1 life forever start-time now

ip sla 2
timeout 500
frequency 1
ip sla schedule 2 life forever start-time now

  •  as we can see P sla 1 will sends icp echo to ISP 1 ip address every 1 second and IP sla 2 will send it to ISP 2

 track 10 rtr 1 reachability
delay down 1 up 1
track 20 rtr 2 reachability
delay down 1 up 1

  • if ip sla 1 did not get icmp replay within 1 second track 10 will be considered as down ( from ISP 1)
  • track 20 same for ISP 2

ip route track 10
ip route track 20

 we have two default routes each one point to one of the ISP’s IP address, also each static default route is associated with the corresponding IP SLA track created above

in this case if ISP 1 link is down the first default route will disappear from  the routing table ( we will see this through some verifications command later in his document).

access-list 10 permit
access-list 100 permit tcp any eq telnet
access-list 100 permit tcp any eq www
access-list 100 permit tcp any eq 443
access-list 101 permit ip any any

these ACLs will be used with PBR and NATing
route-map PBR permit 10
match ip address 100
set ip next-hop verify-availability 1 track 20
route-map PBR permit 30
match ip address 101
set ip next-hop verify-availability 2 track 10

  • we can see from the above route-map called PBR that we have several checks to our traffic coming from the LAN interface towards the Internet

first check is the ACL level

if the traffic soured from our LAN subnet and going to any destination using tcp 23, 80 or 443 then this traffic will be match with ACL 100

if any thing else then will be match with ACL 101

In case of telnet traffic tcp 23, this will be match by ACL 100 and route-map sequence 10

but in this sequence we have another check before we send the traffic to the next hope, we need to make sure this next hope is up and reachable this is done by the IP SLA /track 20 created above if this track is up then the traffic will be route thorough ISP2 with a next hop

if this track 20 is down then the default static route entry points to ISP2 will be withdrawn from the routing table and traffic matched by ACL 100 under the sequence number of 10 of the route-map will be routed according to the normal routing table which is through ISP1 ( because at this stage we have only one default static route left  points to ISP1).  Any other traffic has not matched by ACL 100 will use the route map sequence 30 with the same concept described above

Now we can see how IP SLA controlling the routing table and the  PBR choice !!!

route-map ISP2 permit 10
match ip address 10
match interface FastEthernet2/0
route-map ISP1 permit 10
match ip address 10
match interface FastEthernet1/1

those two Route maps will be used by the NAT command

Please note that we have in each of the route-maps match interface this interface representing the exit interface of that nat

this command is important if we do not use it the router always will use the first nating statement and all our traffic will be sourced in our example from !!

we will see that later in this document the effect of removing the match interface from the route-map

ip nat inside source route-map ISP1 interface FastEthernet1/1 overload
ip nat inside source route-map ISP2 interface FastEthernet2/0 overload

this is simply our nating commands each with is corresponding interface and route-map


for the verifications purposes we will use a loopback interface created on both ISP routers in our example to represent an destination in the Internet

which is 100100.100.100/32

show ip route
Routing entry for, supernet
Known via “static”, distance 1, metric 0, candidate default path
Routing Descriptor Blocks:
      Route metric is 0, traffic share count is 1
      Route metric is 0, traffic share count is 1

we have two default route in our routing table which means both ISP routers IP addresses are reachable by SLA icmp echo

show route-map PBR
route-map PBR, permit, sequence 10
Match clauses:
ip address (access-lists): 100
Set clauses:
ip next-hop verify-availability 1 track 20 [up]
  Policy routing matches: 24 packets, 1446 bytes
route-map PBR, permit, sequence 30
Match clauses:
ip address (access-lists): 101
Set clauses:
ip next-hop verify-availability 2 track 10  [up]
  Policy routing matches: 60 packets, 6840 bytes

both SLA traks 10 and 20 in UP state shown in the route maps show command

now lets ping from the an internal host in subnet and we enable debug of NATing on the Internet edge router to see the translated traffic


*Dec 19 20:24:44.103: NAT*: s=>, d= [80]
*Dec 19 20:24:44.371: NAT*: s=, d=> [80]

this is showing us that icmp traffic translated to ->,

this means that icmp traffic has been match with ACL 101 and because track 10 is up traffic sent to then translated using NAT

this is the PBR debug result for the above ping

*Dec 19 20:25:12.247: IP: s= (FastEthernet1/0), d=, len
100, FIB policy match
*Dec 19 20:25:12.251: IP: s= (FastEthernet1/0), d=, g=19, len 100, FIB policy routed
*Dec 19 20:25:12.259: NAT*: s=>, d= [81]
*Dec 19 20:25:12.623: NAT*: s=, d=> [81]

Now lets see the result when we do a telnet session from the internal network:


*Dec 19 20:26:00.375: IP: s= (FastEthernet1/0), d=, len
44, FIB policy match
*Dec 19 20:26:00.375: IP: s= (FastEthernet1/0), d=, g=17, len 44, FIB policy routed
*Dec 19 20:26:00.383: NAT*: s=>, d= [57504]    — the traffic used link —–
*Dec 19 20:26:01.159: NAT*: s=, d=> [25782]

lets shut down ISP1 link to simulated a link down and see how IP SLA will work in this situation:


*Dec 19 20:27:54.139: %TRACKING-5-STATE: 10 rtr 1 reachability Up->Down
*Dec 19 20:27:57.895: NAT*: s=>, d= [82]
*Dec 19 20:27:58.099: NAT*: s=, d=> [82]

now our ICMP traffic match by ACL 101 is using the link of ISP2 with as the source IP.

we can see bellow that interface connected to ISP 1 is still up, but because the next hop not reachable via ICMP,  IP SLA removed the default route that uses ISP1 next hop from the routing table

interfaces up/up but default route to ISP1 disappeared because of SAL track 10

FastEthernet1/0          YES NVRAM  up                    up

FastEthernet1/1       YES NVRAM  up                    up

FastEthernet2/0        YES manual up                    up

show ip route
Routing entry for, supernet
Known via “static”, distance 1, metric 0, candidate default path
Routing Descriptor Blocks:
      Route metric is 0, traffic share count is 1

lets bring it back to up now

*Dec 19 20:31:29.143: %TRACKING-5-STATE: 10 rtr 1 reachability Down->Up

Routing entry for, supernet
Known via “static”, distance 1, metric 0, candidate default path
Routing Descriptor Blocks:
      Route metric is 0, traffic share count is 1
      Route metric is 0, traffic share count is 1


*Dec 19 20:32:15.559: NAT*: s=>, d= [183]
*Dec 19 20:32:16.071: NAT*: s=, d=> [183]

Now lets remove the match interface command from each of the NAT route-maps and see the result

(config)#route-map ISP1
(config-route-map)#no ma
(config-route-map)#no match in
(config-route-map)#no match interface fa1/1
(config-route-map)#route-map ISP2
(config-route-map)#no ma
(config-route-map)#no match int fa2/0

#clear ip nat translation *

then we do ping and telnet we will see al the traffic will be translated to regardless which exit the traffic is using !!!


*Dec 19 20:33:47.615: NAT*: s=>, d= [184]
*Dec 19 20:33:48.067: NAT*: s=, d=> [184]

*Dec 19 20:34:51.675: NAT*: i: tcp (, 21603) -> (, 23) [
*Dec 19 20:34:51.679: NAT*: i: tcp (, 21603) -> (, 23) [
*Dec 19 20:34:51.683: NAT*: s=>, d= [64704]
*Dec 19 20:34:51.847: NAT*: o: tcp (, 23) -> (, 21603)
*Dec 19 20:34:51.847: NAT*: s=, d=> [52374]
*Dec 19 20:34:52.123: NAT*: i: tcp (, 21603) -> (, 23) [

lets put match interface back  to the nat route-maps

*Dec 19 20:36:23.379: NAT*: i: icmp (, 16) -> (, 16) [18
*Dec 19 20:36:23.383: NAT*: i: icmp (, 16) -> (, 16) [18
*Dec 19 20:36:23.387: NAT*: s=>, d= [185]
*Dec 19 20:36:23.827: NAT*: o: icmp (, 16) -> (, 16) [
*Dec 19 20:36:23.827: NAT*: s=, d=> [185]


*Dec 19 20:36:52.099: NAT*: i: tcp (, 16305) -> (, 23) [
*Dec 19 20:36:52.099: NAT*: i: tcp (, 16305) -> (, 23) [
*Dec 19 20:36:52.103: NAT*: s=>, d= [46655]
*Dec 19 20:36:52.259: NAT*: o: tcp (, 23) -> (, 16305)
*Dec 19 20:36:52.259: NAT*: s=, d=> [41145]
*Dec 19 20:36:52.355: NAT*: i: tcp (, 16305) -> (, 23) [
*Dec 19 20:36:52.359: NAT*: s=>, d= [46656]
*Dec 19 20:36:52.375: NAT*: i: tcp (, 16305) -> (, 23) [

to conclude the above configuration example, by using NAT with other Cisco IOS features in particular IP SLA the network will be more automated and reliable, we can track the next hop reachability and we may use other advanced features of IP sla such as link jitter, in the case that we have VOIP traffic. Also by using PBR functionalities we were able to classify our traffic and send it based on the requirements over the two links to avoid congesting one link and leave the other link as passive/back up only.

Thank you
Marwan Alshawi