Configuring Internet Routing

Articles » Configuring Internet Routing

The configuration of the gateway routers follow the principles explained in the Small Site Multihoming article. IP addressing and NAT is configured on both gateway routers, as shown in the next listing (only GW-A configuration is included in most examples).

IP addressing, DHCP and NAT configuration
hostname GW-A
!
ip cef
!
ip dhcp pool LAN
   network 192.168.0.0 255.255.255.0
   default-router 192.168.0.1
!
ip dhcp excluded-addresses 192.168.0.1 192.168.0.10
ip dhcp excluded-addresses 192.168.0.128 192.168.0.255
!
interface FastEthernet0/0
 description *** Inside LAN interface ***
 ip address 192.168.0.1 255.255.255.0
 ip nat inside
!
interface Serial0/0/0
 description *** Link to ISP A ***
 ip address 172.16.1.1 255.255.255.252
 ip nat outside
!
ip nat inside source route-map ISP_A interface Serial0/0/0 overload
!
route-map ISP_A permit 10
 match interface Serial0/0/0
  • To increase the overall reliability, the DHCP server is running on both gateway routers. Use the ip dhcp excluded-addresses configuration commands to ensure the routers always allocate addresses from non-overlapping pools.
  • The router configurations contain serial interfaces that should be replaced with Ethernet interfaces in modern deployments.

To implement reliable static routes on both gateway routers, you have to configure:

  • An IP SLA object to track end-to-end connectivity to an IP address that is “far enough” (at least within the core of the ISP network, tracking a server of an upstream ISP is even better).
  • A track object that monitors the state of the IP SLA object.
  • A default route that is inserted in the IP routing table based on the state of the track object.
  • Local policy routing to ensure that the IP SLA measurements always use the Internet interface (otherwise a gateway router with failed upstream link might use the default path provided by the other gateway router for its SLA measurements).

The relevant parts of GW-A configuration are included in the next list (the detailed description of the configuration and monitoring commands related to reliable static routing is available in the Small Site Multihoming article).

The only major difference between GW-A and GW-B is the default route configuration, where you would use a high administrative on the backup router (GW-B in our example) to make the default route floating; if Internet connectivity on GW-A is operational, the default route received through the routing protocol should override the static default route.

Static routes pointing to Ethernet interfaces should always have a next hop (more details). If you use this article as a blueprint for deployment with Ethernet uplinks, please add relevant next hops to the ip route commands.
Basic multihomed default routing setup
hostname GW-A
!
ip sla 15
 icmp-echo 172.29.0.1 source-interface Serial0/0/0
 timeout 200
 frequency 10
!
ip sla schedule 15 life forever start-time now
!
track 17 rtr 15 reachability
 delay down 10 up 20
!
ip local policy route-map LocalPolicy
!
ip access-list extended PingISP_A
 permit icmp host 172.16.1.1 host 172.29.0.1
!
route-map LocalPolicy permit 10
 match ip address PingISP_A
 set interface Serial0/0/0
!
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 10 name ISP_A track 17

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Next: Configuring Intra-Site Routing

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