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 XYZ.com 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
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 10.1.1.0/24 destined to tcp 23, 80 and 443 must be routed to ISP 2 link with next hop 172.16.1.2
all other traffic will go though ISP 2 with next hop of 192.168.1.2
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 192.168.1.1
if it is through ISP 2 will be seen as it is from 172.16.1.1
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 192.168.1.2 and 172.16.1.2
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1 second that link will be considered down and the default route in the Internet router pointing to that hop will be withdrawn from the routing table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
description LAN interface ip address 10.1.1.1 255.255.255.0 ip nat inside ip policy route-map PBR ---- this is for policy based routing
description To ISP 1 ip address 192.168.1.1 255.255.255.0 ip nat outside ! interface FastEthernet2/0
description To ISP 2 ip address 172.16.1.1 255.255.255.0 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 icmp-echo 192.168.1.2 timeout 500 frequency 1 ip sla schedule 1 life forever start-time now
ip sla 2 icmp-echo 172.16.1.2 timeout 500 frequency 1 ip sla schedule 2 life forever start-time now
as we can IP 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 0.0.0.0 0.0.0.0 192.168.1.2 track 10 ip route 0.0.0.0 0.0.0.0 172.16.1.2 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 10.1.1.0 0.0.0.255 access-list 100 permit tcp 10.1.1.0 0.0.0.255 any eq telnet access-list 100 permit tcp 10.1.1.0 0.0.0.255 any eq www access-list 100 permit tcp 10.1.1.0 0.0.0.255 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 172.16.1.2 1 track 20 ! route-map PBR permit 30 match ip address 101 set ip next-hop verify-availability 192.168.1.2 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 10.1.1.0/24 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 172.16.1.2, 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 172.16.1.2
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 192.168.1.1 !!
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 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "static", distance 1, metric 0, candidate default path Routing Descriptor Blocks: 192.168.1.2 Route metric is 0, traffic share count is 1 * 172.16.1.2 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 172.16.1.2 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 192.168.1.2 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 100.100.100.100 from the an internal host in subnet 10.1.1.0/24 and we enable debug of NATing on the Internet edge router to see the translated traffic
now our ICMP traffic match by ACL 101 is using the link of ISP2 with 172.16.1.1 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 10.1.1.1 YES NVRAM up up
FastEthernet1/1 192.168.1.1 YES NVRAM up up
FastEthernet2/0 172.16.1.1 YES manual up up
show ip route 0.0.0.0 Routing entry for 0.0.0.0/0, supernet Known via "static", distance 1, metric 0, candidate default path Routing Descriptor Blocks: * 172.16.1.2 Route metric is 0, traffic share count is 1
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 (config-route-map)#
#clear ip nat translation *
then we do ping and telnet we will see al the traffic will be translated to 192.168.1.1 regardless which exit the traffic is using !!!
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.
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