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OSPFv2 is a compatible upgrade to OSPF. It involves enhancements to protocol
message authentication, and the addition of a point-to-multipoint interface which allows
OSPF to run over non-broadcast networks, as well as support for overlapping area
ranges.
Area Configuration
– OSPF routers exchange information with other routers in their area
to determine the shortest path to every destination. Each router in a common area
should therefore have an identical map of their local network topology. At the top level,
the largest area is known as an Autonomous System, and contains all the routers in
your network. However, for large networks you should organize your OSPF routers into
smaller contiguous areas to reduce the amount of routing information that has to be
exchanged and to simplify network management.
When designing an OSPF network architecture, first create a backbone area to which
all other areas are adjacent. Note that when you enable OSPF for any IP interface on
the switch, it is assigned to the backbone by default (Area 0.0.0.0). As a general rule, no
area should contain more than 50 routers. To create a new area, designate an Area ID
that will be used by all of the other routers in this area, specify the area type as Normal,
Stub, or NSSA (chapter 2,3 “Configuring Global Settings for OSPF”), and then assign
the ID to an interface (chapter 2,3 “
Configuring OSPF
”). A Stub does not accept or send
external routing information. Instead, it uses a single default route for destinations
outside the area. Stubs further minimize the amount of routing data that has to be stored
or exchanged with other areas. An NSSA (Not-So-Stubby Area) is similar to a Stub,
except that it can import external route information into its area. Note that if there are
not external routes into your network, then there are no advantages to configuring a
Stub or NSSA.
Neighbors
– Neighboring OSPF routers within a common area are found using Hello
messages. These messages also list the other routers from which the originator has
received hello messages. When a router finds its address in the hello messages
received from another router, both routers initiate communications as neighbors.
Only after these routers successfully exchange and synchronize their routing tables, will
they be considered fully adjacent (chapter 2 “
Displaying the Interface Table
” or chapter
3 “
Displaying the DVMRP Neighbor Table”
). Routing information is only exchanged
between adjacent neighbors.
Designated Router
– A Designated Router (DR) and Backup Designated Router (BDR)
are selected by the OSPF protocol for each area. The Designated Router exchanges
routing information with all other routers in its area, and then floods Link State
Advertisements (LSAs) to each router, allowing them to update their database. This
eliminates the need for each router to exchange information with every other router in its
area. The OSPF protocol selects the DR and BDR based on the router with the highest
