D-Link DFL-260 Product Manual - Page 173

4.5.1. Dynamic Routing Chapter 4. Routing allows B's routing table information to be automatically shared with A. In the same way, OSPF allows firewall B to automatically become aware that network X is attached to firewall A. Under OSPF, this exchange of routing information is completely automatic. OSPF Provides Route Redundancy If we now take the above scenario and add a third NetDefend Firewall called C then we have a situation where all three firewalls are aware, through OSPF, of what networks are attached to the other firewalls. This is illustrated below. Figure 4.9. OSPF Providing Route Redundancy In addition, we now have route redundancy between any two of the firewalls. For example, if the direct link between A and C fails then OSPF allows both firewalls to know immediately that there is an alternate route between them via firewall B. For instance, traffic from network X which is destined for network Z will be routed automatically through firewall B. From the administrators point of view, only the routes for directly connected networks need to be configured on each firewall. OSPF automatically provides the required routing information to find networks connected to other firewalls, even if traffic needs to transit several other firewalls to reach its destination. Tip: Ring topologies always provide alternate routes When designing the topology of a network that implements OSPF, arranging NetDefend Firewalls in a circular ring means that any firewall always has two possible routes to any other. Should any one inter-firewall connection fail, an alternative path always exists. A Look at Routing Metrics In discussing dynamic routing and OSPF further, an understanding of Routing Metrics can be useful and a brief explanation is given here. 173