OSPF Design Guide | Open shortest path first CCNP Concepts


What is OSPF (open shortest path first)?

OSPF is define in RFC 2328 according to this Open Shortest Path First (OSPF) is internet routing protocol.  OSPF is classified as an Interior Gateway Protocol (IGP).  This means that it distributes routing information between routers belonging to a single Autonomous System.
OSPF is stands for Open shortest Path first. In Routing world by path selection there are two types of protocols.

OSPF is a link state routing protocol, OSPF use SPF algorithm to find the best path. There are two versions for OSPF one is version 2 and other is version 3 which supports IPV6. OSPF is the most widely-used interior gateway protocol (IGP) in large enterprise networks. OSPF use the trigger update, mean whenever any change in network occur it converges on a new loop-free routing structure within seconds. OSPF use Dijkstra's algorithm to calculate shortest path.

.. There are three tables in OSPF.
1.       Neighbor table
2.       Topology table
3.       OSPF Routing table

What is OSPF cost?
OSPF metric is known as cost and OSPF Cost is the overhead required to send packets across a certain interface.
OSPF use the following formula for calculating cost for a link.
OSPF Cost=100 / (bandwidth in MB)
For more detail visit OSPF Cost.

Routers on the same network segment are called neighbors. Two routers connected with each other become neighbors if they have the same area-id, subnet, authentication and hello/dead intervals.
Neighbor process starts with hello packets. Hello packets are sent periodically out of each interface using IP multicast. Routers become neighbors as soon as they see themselves listed in the neighbor's Hello packet. Read more OSPFneighbor relationship


OSPF Network Types:

OSPF networks types is a big topic. There are following OSPF network Types.
1.       Broadcast/Multi-access Network
2.       OSPF Point to point
3.       OSPF Non-broadcast Multiaccess (NBMA)     
NBMA have five mode of working.
                                i.            Non-Broadcast (RFC Standard)
                              ii.            Point to Multipoint (RFC Standard)
                            iii.            Broadcast (Cisco Proprietary)
                             iv.            Point to point
                               v.            Point to point, Non Broadcast



Summarization in OSPF:
Summarization in OSPF is implemented on main routers like ABR & ASBR and also summary configuration is different for ABR and ASBR in OSPF.
On ABR you can use command
ABR#area 1 range 11.1.0.0 255.255.0.0
And on ASBR you need to use
ASBR# summary address 10.1.0.0 0.0.0.255  


LSAs are building block of OSPF. There are total 11 types of LSA.
1.       LSA Type-1| Router LSA from one network
2.       LSA Type-2| Network LSA from more network (DR Generated)
3.       LSA Type-3| Summary LSA (ABR summary Route)
4.       LSA Type-4| Summary LSA (just IP address of ASBR)
5.       LSA Type-5| External LSA (ASBR summary Route)
6.       LSA type-6 (not supported by Cisco)
7.       LSA type-7
Read more OSPF LSA Types


In OSPF there is a basic requirement while designing a network with multiple areas that every area is directly connected to backbone area 0. Virtual link comes when some area e.g. area-2 in figure is not directly connected to area 0. There is area-1 in between area-2 and backbone area-0, so in this case communication between area-0 and area-2 is not possible. To make communication between these areas you need to create virtual-links.



OSPF support different type of areas.
OSPF area Types
*      Standard Area
*      Backbone Area
*      OSPF Stub area
*      OSPF Totally stubby Area
*      OSPF Not So stubby area

OSPF DR\BDR election:
To avoid the broadcast in network, in shared Ethernet segment DR are elected. DR send multicast to 224.0.0.4.
By default DR election base on router priority, router with high priority become DR  

How to configure OSPF on router
You can enable/configure OSPF on a router by following two methods.
·         Enable OSPF using network command in router process mode
·         Enable OSPF using IP OSPF command in interface mode
For more detail configure OSPF on router


OSPF Lab Examples:
Learn about Cloud Computing and Active Directory

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