Open Shortest Path First (OSPF)

How OSPF works on FortiGate units

When a FortiGate unit interface is connected to an OSPF area, that unit can participate in OSPF communications. FortiGate units use the OSPF Hello protocol to acquire neighbors in an area. A neighbor is any router that is directly connected to the same area as the FortiGate unit, and ideally is adjacent with a state of Full. After initial contact, the FortiGate unit exchanges Hello packets with its OSPF neighbors regularly to confirm that the neighbors can be reached.

The number of routes that a FortiGate unit can learn through OSPF depends on the network topology. A single unit can support tens of thousands of routes if the OSPF network is configured properly.

 

External routes

OSPF is an internal routing protocol. OSPF external routes are routes where the destination using a routing protocol other than OSPF. OSPF handles external routes by adjusting the cost of the route to include the cost of the other routing protocol. There are two methods of calculating this cost, used for OSPF E1 and OSPF E2.

OSPF external1 (E1)

In OSPF E1 the destination is outside of the OSPF domain. This requires a different metric to be used beyond the normal OSPF metrics. The new metric of a redistributed route is calculated by adding the external cost and the OSPF cost together.

 

OSPF external2 (E2)

OSPF E2 is the default external type when routes are redistributed outside of OSPF. With OSPF E2, the metric of the redistributed route is equivalent to the external cost only, expressed as an OSPF cost. Dropping the OSPF portion can be useful in a number of situations, on border routers that have no OSPF portion for example or where the OSPF routing cost is negligible compared to the external routing cost.

 

Comparing E1 and E2

The best way to understand OSPF E1 and E2 routes is to check routing tables on OSPF routers. If you look at the routes on an OSPF border router, the redistributed routes will have an associated cost that represents only the external route, as there is no OSPF cost to the route due to it already being on the edge of the OSPF domain. However, if you look at that same route on a different OSPF router inside the OSPF routing domain, it will have a higher associated cost – essentially the external cost plus the cost over the OSPF domain to that border router. The border router uses OSPF E2, where the internal OSPF router uses OSPF E1 for the same route.

 

Viewing external routes

When you are trying to determine the costs for routes in your network to predict how traffic will be routed, you need to see the external OSPF routes and their associated costs. On your FortiGate unit, you find this information through your CLI.


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