OSPF Theory

OSPF Concepts, Area Design and Terms

Hello Everyone this is Open Shortest Path First(OSPF) Routing protocol Post:

Before moving ahead I request you to please see my Route summarization post here  , as it is utterly important to understand route summarization first. There is no point in applying Routing protocols like
ospf and eigrp without applying route summarization. so please first understand route summarization.

Lets first get familiar about ospf and its terms. OSPF is a link state routing protocol. Unlike distance vector routing protocols like RIP, link state routing protocol only sends changes to its neighbor routers. ( Remember in
RIP routing,router broadcast all their routing information saying " Hello I know about so and so routes" at certain intervals rather than forming neighbor relationship with other routers) . Link state routing protocols first establish neighbor relationship with its neighbors
and then only send changes as updates.

please refer to the below mentioned figures you can click to enlarge the figure

AREA: An area in ospf is a group of all routers that all have the same routing information. Cisco recommends this area to have no
more than 50 routers. Inside of Area, there are
1> Internal Routers: Routers that only knows about the specific area. eg in the figure the routers in area 0 knows all about the area 0.
All routers in an area have the same topology table(Roadmap) i.e all of the routers in area 0 knows everything
about the area, but every router within an area may have different routing table.
2> Autonomous Border Routers(ABR): Routers that sits between routers in the same autonomous system. ABR is responsible for
routing between different areas of the same autonomous system i.e ABR is the one that is able summarize.
3>Autonomous System Boundary Router(ASBR): is the one that connects different autonomous system. This ASBR can also summarize the route and advertise to different autonomous system

All Area Must connect to Area 0 ( backbone router). when designing ospf, it has to be a hierarchical designs ie have to group similar
subnets in similar areas.

OSPF Neighbor Relationship
A router running ospf forms neighbor relationship with other router by sending HELLO Messages on the chosen interface
1> once every 10 sec on broadcast/p-2-p networks
2>once every 30 sec on NBMA networks ( we will later discuss NBMA networks)

Hello contains all of the below information:
1>Router I.D : its like the name of the name of the router
2>Hello and Dead timers* : How often the these hello messages should be exchanged and the time after which if no hello messages
is received the neighbor is considered dead
3>Network Mask*: Network mask the router advertises
4>Area ID*:
5>Neighbors*:
6>Route Priority:
7>DR/BDR IP address:
8>Authentication Password*:

The * sign mean that these fields must match between two routers to talk with ospf language.

ok now we are ready to delve into ospf configuration. So In my next post we will configure a simple multi-area ospf and later move into the advanced topics. so stay on

Thank you
Ciscohowtos