Overview Of Ospf (Open Shortest Path First) - Dell PowerConnect B-RX Configuration Manual

Configuring OSPF Version 2 (IPv4)

Overview of OSPF (Open Shortest Path First)

OSPF is a link-state routing protocol. The protocol uses link-state advertisements (LSA) to update
neighboring routers regarding its interfaces and information on those interfaces. The router floods
these LSAs to all neighboring routers to update them regarding the interfaces. Each router
maintains an identical database that describes its area topology to help a router determine the
shortest path between it and any neighboring router.
The device supports the following types of LSAs, which are described in RFC 2328 and 3101:
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OSPF is built upon a hierarchy of network components. The highest level of the hierarchy is the
Autonomous System (AS). An autonomous system is defined as a number of networks, all of which
share the same routing and administration characteristics.
An AS can be divided into multiple areas as shown in
represents a collection of contiguous networks and hosts. Areas limit the area to which link-state
advertisements are broadcast, thereby limiting the amount of flooding that occurs within the
network. An area is represented in OSPF by either an IP address or a number.
You can further limit the broadcast area of flooding by defining an area range. The area range
allows you to assign an aggregate value to a range of IP addresses. This aggregate value becomes
the address that is advertised instead all of the individual addresses it represents being
advertised. You can assign up to 32 ranges in an OSPF area.
An OSPF router can be a member of multiple areas. Routers with membership in multiple areas are
known as Area Border Routers (ABRs). Each ABR maintains a separate topological database for
each area the router is in. Each topological database contains all of the LSA databases for each
router within a given area. The routers within the same area have identical topological databases.
The ABR is responsible for forwarding routing information or changes between its border areas.
An Autonomous System Boundary Router (ASBR) is a router that is running multiple protocols and
serves as a gateway to routers outside an area and those operating with different protocols. The
ASBR is able to import and translate different protocol routes into OSPF through a process known
as redistribution. For more details on redistribution and configuration examples, refer to
route redistribution"
BigIron RX Series Configuration Guide
53-1002253-01
Router link
Network link
Summary link
Autonomous system (AS) summary link
AS external link
Not-So-Stubby Area (NSSA) external link
on page 705.
Figure 101 on page 680. Each area
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