FortiGate Administration Guide - FirewallShop.com

Router Dynamic
OSPF
Defining an OSPF AS
OSPF-enabled routers generate link-state advertisements and send them to their
neighbors whenever the status of a neighbor changes or a new neighbor comes
online. As long as the OSPF network is stable, link-state advertisements between
OSPF neighbors do not occur. A Link-State Advertisement (LSA) identifies the
interfaces of all OSPF-enabled routers in an area, and provides information that
enables OSPF-enabled routers to select the shortest path to a destination. All
LSA exchanges between OSPF-enabled routers are authenticated.
The FortiGate unit maintains a database of link-state information based on the
advertisements that it receives from OSPF-enabled routers. To calculate the best
route (shortest path) to a destination, the FortiGate unit applies the Shortest Path
First (SPF) algorithm to the accumulated link-state information. OSPF uses
relative cost as a basic metric for choosing the best route. Cost imposes a penalty
on the outgoing direction of a FortiGate interface. The cost of a route is calculated
by adding together all of the costs associated with the outgoing interfaces along
the path to a destination. The lowest overall cost indicates the best route.
The FortiGate unit updates its routing table dynamically based on the results of
the SPF calculation to ensure that an OSPF packet will be routed using the
shortest path to its destination. Depending on the network topology, the entries in
the FortiGate routing table may include:
• the addresses of networks in the local OSPF area (to which packets are sent
directly)
• routes to OSPF area border routers (to which packets destined for another
area are sent)
• if the network contains OSPF areas and non-OSPF domains, routes to AS
boundary routers, which reside on the OSPF network backbone and are
configured to forward packets to destinations outside the OSPF AS
The number of routes that a FortiGate unit can learn through OSPF depends on
the network topology. A single FortiGate unit can support tens of thousands of
routes if the OSPF network is configured properly.
Defining an OSPF AS, involves:
• Defining the characteristics of one or more OSPF areas.
• Creating associations between the OSPF areas that you defined and the local
networks to include in the OSPF AS.
• If required, adjusting the settings of OSPF-enabled interfaces.
For more information about how to perform these tasks using the web-based
manager, follow the procedure given below.
To define an OSPF AS
1 Go to Router > Dynamic > OSPF.
2 Under Areas, select Create New.
3 Define the characteristics of one or more OSPF areas. See “Defining OSPF
areas” on page 253.
4 Under Networks, select Create New.
5 Create associations between the OSPF areas that you defined and the local
networks to include in the OSPF AS. See “Specifying OSPF networks” on
page 254.
FortiGate Version 3.0 MR5 Administration Guide
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