Adopted from Kaur & Kumar, 2015)
supports different types of areas depending on the requirements of a network.
These areas are:
Normal area also referred to as regular area connects to the backbone area via
one or more area border routers. The link state advertisement (LSA) types that
are exchanged between a normal area and the backbone area are the Inter–Area–
Prefix LSAs and AS External LSAs. ASBRs are used in normal Areas.
Stub Area: In
order to reduce the amount of external routing information that is flooded in
an area, that area can be configured as a stub area. A stub Area also connects
to the autonomous systems backbone Area via one or more ABRs but does not allow
the use of internal ASBRs and flooding of AS External LSAs, since these LSAs
are normally flooded all over the autonomous system to disseminate external
route information. A stub area uses Inter–Area–Prefix LSA as a default route
for all routing information that needs to be forwarded via the backbone area to
the external autonomous system. For IPv6, prefix length of this LSA is set to
(NSSA): NSSA is like a stub area. However, in an NSSA, ASBR is used to
allow autonomous system external routes into an NSSA using redistribution.
The ASBR redistributes the external routes and then generates type 7 LSAs
that are flooded within the NSSA. In NSSA, Type 5 LSA is not allowed.
However, an ABR can be optionally configured to connect the NSSA to other
areas to convert type 7 LSAs to Type 5 LSAs and then floods these
converted LSAs all over the autonomous system (Cisco.com, 2016).
Designated Router (DR) and Backup Designated Router (BDR)
Different types of networks
present OSPF with a unique challenge to manage. A network could be
point–to–point or a multiple access network providing a shared medium for
multiple routers to communicate. In a multiple access network, if each router
floods the network with LSAs, the same information about a link state will be
forwarded from multiple sources, leading to a large amount of router CPU load
and bandwidth consumption. In a multi–access network, OSPF uses a single router
called designated router (DR) to control how LSAs are flooded. The purpose of
using the DR is to minimize the number of adjacencies formed so that all
topology tables on routers can be synchronized.
A backup designated router
(BDR) is a hot standby router for the DR in the same network type. The BDR
receives LSA packets and routing updates from OSPF adjacent routers but does
not flood the LSA updates. The BDR only works if the DR fails. Each router in a
multiple access network establishes adjacency with the DR and the BDR.