Router OSPF Overview

					             Router OSPF Overview
Router OSPF-Part1
Router OSPF is the command used to enable OSPF routing on a Cisco router. But
before we get into the command syntax I want to give you a brief summary of some
important OSPF details for the ICND2 and CCNA Exam.

Dynamic Routing Protocol
OSPF is a Dynamic Routing Protocol that can be used across many different brands of
routers and not just Cisco. For a little refresher, a dynamic routing protocol is able to
adapt quickly to networking changes in case a link breaks or a router ceases to
function. It will be able to find an alternate route automatically so your whole
network doesn’t come crashing down as opposed to static routes where the network
administrator would have to update all the new routes manually.

Interior Gateway Protocol
There are interior gateway protocols and exterior gateway protocols. An interior
gateway protocol works with a single group of networks and an exterior gateway
protocol connects different groups of networks together. OSPF is an interior gateway
protocol.

Interior gateway protocols have three different types of ways to determine the best
route between routers. There are distance vector, link-state, and a mixture of the two.
OSPF uses link-state.

Many different routing protocols can be configured on a Cisco router at a time. In
order for the routers to determine which routing protocol should take priority they
use administrative distance.

The lower the administrative distance the better. A directly connected route has an
AD of 0 and a statically configure route has an AD of 1. OSPF has an AD of 110. As a
comparison to other routing protocols EIGRP has an AD of 90 and RIP has an AD of
120. This means that if you have OSPF and RIP configured in a network, the routers
will choose the routes created by OSPF over RIP.

Link-State
The main difference between link-state and distance vector routing protocols is that
link state will only send updates when there is a topology change. Distance vector
however send updates at regular intervals, such as every 30 seconds. As a
comparison OSPF sends its topology table out every 30 minutes. Which is quite a bit
longer time to wait to here about updates. However, OSPF makes up for its
infrequent topology exchanges by using link-state advertisements which are tiny

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updates that get sent out immediately after a change is detected.

So, the benefit of link-state is that it can use less networking resources with fewer
updates and can converge much faster. Convergence means that all the routers in the
network have finished discussing the changes that have occurred and are no longer
sending updates back and forth.

Shortest Path First
OSPF uses what is called the SPF algorithm to calculate the shortest paths between
routers. Even though each OSPF router has the exact same topology, they each will
have a different point of view because the SPF algorithm gets calculated on each
router individually.

Areas
OSPF is able to scale well with large networks because it uses areas to break up an
autonomous system so that it can minimize all the routing updates that occur during
convergence. This is necessary because if you have a network with 50 different
routers and there are frequent changes, the SPF algorithm will have to be run every
time. And with many routers this can take a lot of time. If you split the 50 router
network into 5 different areas the topology tables are only propagated within each
area individually. By splitting it up this way the SPF algorithm can run much faster.

OSPF Routers have different roles depending on their location in an area. There are
Area Border Routers, Autonomous System Boundary Routers, Internal Routers, and
Backbone Routers.

More to come…
This article was meant to be a brief introduction to OSPF and I plan on talking more
in depth about different areas of OSPF such as LSA’s, SPF, areas, and configuration.

If you have any specific OSPF questions you would like to have answered please
comment below and let me know. I hope your ICND2 and CCNA preparation is
coming a long great. Keep up the hard work!


Router OSPF-Part2
This is Part 2 of the Router OSPF Serieswhich will cover topics about OSPF
Areasand OSPF LSA’s. Before continuing on I recommend reading Part 1 of the
Router OSPF Series which gives an introduction on the topic.

For the ICND2 and CCNA exam you will only need to configure a single area OSPF
implementation, but you will still be required to know many different things about a
full multi-area OSPF network.


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OSPF is a link-state routing protocol that stores a complete topology map of the
network it is in. In order for OSPF to know where the shortest routes are to each
network it has to calculate it off of the topology map using the Dijkstra algorithm
every time there is a change in the network. This process can take up a lot of CPU
cycles and slow down the routing process if there are many routers. In order for OSPF
to be able to scale well in larger network situations with a lot of routers it uses Areas
to logically break up the network.

OSPF Router Types
Backbone Router – Every OSPF implementation must have a backbone area. The
backbone area will always be area 0. A Backbone router is any router that has an
interface connected to area 0.
Area Border Router (ABR) – Are routers that have on interface in area 0 and another
interface in a different area. ABR’s sit between the two areas. All ABR’s are backbone
routers because they have an interface connected to area 0.
Autonomous System Boundary Router (ASBR) – Share information with other
routers that are not running OSPF.
Internal Router – Are routers that are not area border routers. They simply are
routers running OSPF, but are not passing information between areas.

Here is an example of a network with multiple OSPF areas so that you can visually
see the different OSPF router roles amongst the areas.




Types of OSPF Areas
Backbone Area – This area is also known as the standard area. Every OSPF
implementation has to have one. Backbone areas accept all types of LSA’s.
Stub Area – Will only receive summary LSA’s. Routing LSA’s are NOT allowed.
Totally Stubby Area – Absolutely no LSA’s are allowed.
Not So Stubby Area (NSSA) – This is a stub area that has a ASBR router that receives
information from another routing protocol other than OSPF and passes it into the
OSPF network.


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OSPF LSA Types
There are seven different types of Link-State advertisements that OSPF routers pass
around to check up on each other. When you type “show ip route” on a Cisco router
to see the routing table, ospf entries will show up with the letter “o” in front. There
are also many different variations that will show up in the routing table depending on
the LSA type.
LSA Type 1: “O” — Intra-Area — Passed around inside an area
LSA Type 2: “O 1A — Inter-area — Pass through an area
LSA Type 3: Summary LSA by an ABR
LSA Type 4: Summary LSA to an ASBR
LSA Type 5: “O E1″ or “O E2″ — From an ASBR about external links
LSA Type 6: This is a Groupe Membership Link LSA that is sent out by multicast OSPF
Routers
LSA Type 7: “O N1″ or “O N2″ — These are NSSA external routes from an ASBR

Well, I Think that is quite a bit of OSPF information for today. Tomorrow I’m actually
going to show you how to configure OSPF since we still haven’t gone over that yet.

More OSPF Tips:
OSPF Areas Types, OSPF Router Types & OSPF Route Types
How to Fix OSPF Split Area with GRE Tunnel?




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