GNS3 - VitualBox Part 3: Adding Redundant Internet ASBR's to an OSPF Backbone Area

Adding three VirtualBox Debian Linux Gateways that provide multiple paths to the Internet. this article starts using the Backbone Topology configured in Part 2 of the series.

Introduction

The illustration above depicts what will be configured:  three Debian Linux Gateways (Philadelphia, Pittsburgh and Harrisburg).  These three gateways are connected to the OSPF Backbone Area (0.0.0.0) and bridged to the host laptop's wireless adapter; they receive a DHCP address as the default gateway.

Specific details of network addressing are presented in the figure above.

Adding the Debian Linux Gateways

The gateways are VirtualBox servers preconfigured for the network.  These are connected to the respective Cisco 7206 routers' OSPF Backbone Area Fast Ethernet adapter (Philadelphia, 172.16.253.1; Pittsburgh, 172.16.252.1 and Harrisburg, 172.16.251.1, respectively) over adapter eth1.  Adapter eth0 is bridged to the host laptop's wlan0 interface and receives a DHCP address (and default gateway) from a wireless router running DD-WRT; the bridged wlan0 interfaces are part of OSPF Area 192.168.0.0, corresponding to the DHCP range 192.168.1.x / 255.255.255.0.

Each gateway has IP Forwarding enabled and uses Quagga routing software.  The pertinent OSPF configuration provided by the /etc/quagga/quagga.conf file (for Philadelphia as an example) is:

router ospf
 ospf router-id 192.168.1.3
 network 172.16.253.0/24 area 0.0.0.0
 network 192.168.1.0/24 area 192.168.0.0
 area 0.0.0.0 range 172.16.0.0/12
 default-information originate always

The router-id identifies the router as part of the wlan0-bridged OSPF area 192.168.0.0.  The two network lines specify the networks in which OSPF is active and the area defines the OSPF Backbone configuration as a summarized network (172.16.0.0 - 172.16.31.255).

The statement "default-information originate always" instructs the OSPF routing process to advertise itself as a default gateway to exterior networks -- the gateways are Autonomous System Boundary Routers (ASBRs).

Checking the Configuration

Prior to booting the three Debian Linux Gateways, check the routing tables, OSPF neighbors and default gateways of the four Cisco 7206 routers.  As configured in Part 1 and Part 2 of the series, there was only one Debian Linux Gateway acting as an ASBR -- Coudersport.  Thus, the Philadelphia, Pittsburgh and Harrisburg routers' default gateways are the OC-3 interfaces connected to the Coudersport OC-3 interface; all Internet-bound traffic must route through the Coudersport Cisco 7206 and then the Coudersport Debian Linux Gateway.

Upon booting the three new Debian Linux Gateways, check their routing processes by telnetting to port 2604, the Quagga ospf daemon.  Each should see the host laptop and a local Cisco 7206 router as OSPF neighbors; additionally, they should see each other as neighbors with 192.168.1.x router-ids.

Once the new Debian Linux Gateways are working properly, check and compare (to the previous information) the routing tables, OSPF neighbors and default gateways on the Cisco 7206 routers.  At this point, the local Debian Linux Gateway will appear as a neighbor.  More importantly, the default gateway will now be that local Debian Linux Gateway (Philadelphia, 172.16.253.2; Pittsburgh 172.16.252.2 and Harrisburg 172.16.251.2, respectively).

There are now four ASBRs in the network -- the Debian Linux Gateways.  This provides several benefits:

1. Internet-bound traffic is now distributed through four gateways, local to each data center (Coudersport, Philadelphia, Pittsburgh and Harrisburg).  If one, two or three local ASBRs fail, traffic will route through another (as determined by the OSPF shortest-path algorithm).
2. Previously, all Internet-bound traffic routed through the single Coudersport ASBR.  Internet-bound traffic now uses a local gateway, not the OC-3 and backup T-1 leased lines.

The video below illustrates the process.