- [Instructor] OSPF has per-interface designations
that change the behavior of how OSPF operates
on that given interface.
Engineers nowadays will most often use ethernet connections,
which default to a type of broadcast.
On these broadcast networks, one of the routers
will be chosen to be the designated router, or DR,
and one will be the backup DR.
All routers on the segment will exchange their LSDB
only with the DR.
If for some reason the DR fails, then the backup DR
will assume the role of DR, and a new BDR will be elected.
The process of using a DR and BDR adds efficiency
to a busy OSPF network.
When routers want to send messages to the DR or BDR,
they do so via 224.0.0.6, which is the all SPF DR's
multicast address.
In these scenarios, there are three types of router.
DR, BDR, and everything else is considered a DR other.
While DR others will maintain neighborships with each other,
they will stop at the two way state,
as they will only form full adjacencies with a DR and BDR.
Recalling from earlier, the OSPF metric used is cost.
A cost is associated with each outgoing link
to reach a destination.
Take a look at this diagram.
Each router has an identical link state database,
but each will determine best path
from their individual perspective.
Take router one, for example.
While it has three paths to reach router four,
it will add up the cost for each path,
and choose the one with the lowest cost
to reach the destination.
Notice it only takes into account the outgoing cost
of each path along the way.
OSPF will often work right out of the box,
with very little tuning, especially on smaller networks.
Take, for example, this network,
that only maintains 10 routers.
There is no tuning done here, and it functions just fine.
What if this network grew to have a few hundred routers
with regular reconvergence?
With everything in a single area, the constant reconvergence
would eat additional memory and CPU on the routers,
and can even overrun smaller devices.
It's for this very reason that OSPF
has the concept of areas.
When multiple areas are used, each maintains
a separate link state database.
When an interface flaps in one area, it only causes
reconvergence in that area,
and doesn't spread to the others.
There are a few rules when it comes to configuring areas.
First, all routers in a single area should be contiguous.
All routers in that same area should touch.
Second, all router interfaces in the same subnet
should be configured for the same area.
Third, all non-backbone routers must traverse
the area zero, the backbone area, to reach one another.
Fourth, a router can be completely in a single,
non-backbone area.
These are known as internal routers.
And last, routers can also have interfaces
in non-backbone and backbone areas at the same time.
These are known as area border routers.
Areas have far more features than are listed here,
as it's outside the scope of the CCNA.
For a production network, I would encourage you greatly
to do some additional study on them.