Ospf; Ospfv2 - Siemens SCALANCE XM-400 Configuration Manual

Technical basics
4.8 Routing function
4.8.3

OSPF

4.8.3.1

OSPFv2

Dynamic routing with OSPFv2
OSPF (Open Shortest Path First) is a cost-based routing protocol. To calculate the shortest
and most cost-effective route, the Short Path First algorithm by Dijkstra is used. OSPF was
developed by the IETF (Internet Engineering Task Force).
You configure OSPFv2 in "Layer 3 (IPv4) > OSPFv2 (Page 380)".
OSPFv2 divides an autonomous system (AS) into different areas.
Areas in OSPF
The following areas exist:
● Backbone
The backbone area is area 0.0.0.0. All other areas are connected to this area. The backbone
area is connected either directly or via virtual connections with other areas.
All routing information is available in the backbone area. As a result, the backbone area is
responsible for forwarding information between different areas.
● Stub Area
This area contains the routes within its area within the autonomous system and the standard
route out of the autonomous system. The destinations outside this autonomous system are
assigned to the standard route.
● Totally Stubby Area
This area knows only the routes within its area and the standard route out of the area.
● Not So Stubby Area (NSSA)
This area can forward (redistribute) packets from other autonomous systems into the areas
of its own autonomous system. The packets are further distributed by the NSSA router.
Routers of OSPF
OSPF distinguishes the following router types:
● Internal router (IR)
All OSPF interfaces of the router are assigned to the same area.
● Area Border Router (ABR)
The OSPF interfaces of the router are assigned to different areas. One OSPF interface is
assigned to the backbone area. Where possible, routes are grouped together.
● Backbone Router (BR)
At least one of the OSPF interfaces is assigned to the backbone area.
● Autonomous System Border Router (ASBR)
One interface of the router is connected to a different AS, for example an AS that uses the
routing protocol RIP.
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SCALANCE XM-400/XR-500 Web Based Management (WBM)
Configuration Manual, 05/2017, C79000-G8976-C248-12