D-Link NetDefend DFL-1660, User Manual

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Instead of having to manually insert this routing information into the routing tables of A, OSPF
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
4.5.1. Dynamic Routing Chapter 4. Routing
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