Ospf overview, What are ospf areas and other ospf, Topology features – Dell POWEREDGE M1000E User Manual
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Configuring OSPF and OSPFv3
OSPF Overview
OSPF is an Interior Gateway Protocol (IGP) that performs dynamic routing
within a network. PowerConnect M6220, M6348, M8024, and M8024-k
switches support two dynamic routing protocols: OSPF and Routing
Information Protocol (RIP).
Unlike RIP, OSPF is a link-state protocol. Larger networks typically use the
OSPF protocol instead of RIP.
What Are OSPF Areas and Other OSPF Topology Features?
The top level of the hierarchy of an OSPF network is known as an OSPF
domain. The domain can be divided into areas. Routers within an area must
share detailed information on the topology of their area, but require less
detailed information about the topology of other areas. Segregating a network
into areas enables limiting the amount of route information communicated
throughout the network.
Areas are identified by a numeric ID in IP address format n.n.n.n (note,
however, that these are not used as actual IP addresses). For simplicity, the
area can be configured and referred to in normal integer notation. For
example, Area 20 is identified as 0.0.0.20 and Area 256 as 0.0.1.0. The area
identified as 0.0.0.0 is referred to as Area 0 and is considered the OSPF
backbone. All other OSPF areas in the network must connect to Area 0
directly or through a virtual link. The backbone area is responsible for
distributing routing information between non-backbone areas.
A virtual link can be used to connect an area to Area 0 when a direct link is
not possible. A virtual link traverses an area between the remote area and Area
0.
A stub area is an area that does not accept external LSAs (LSAs generated by
redistributing routes) that were learned from a protocol other than OSPF or
were statically configured. These routes typically send traffic outside the AS.
Therefore, routes from a stub area to locations outside the AS use the default
gateway. A virtual link cannot be configured across a stub area. A Not So
Stubby Area can import limited external routes only from a connected ASBR.