OSPF Quick Review 1695007552

OSPF Quick Review
OSPF Properties
• Summarize For “Open Shortest Path First”
• Open Standard Protocol
• Know By RFC 2328 & RFC 2740
• Protocol ID 89 in IP Header
• OSPF is a link-state routing protocol & Classless Protocol.
• OSPF uses Dijkstra's shortest path first (SPF) algorithm to calculate

the shortest path to a destination network.
• OSPF uses cost as the metric for calculating the shortest path. The cost
is based on the bandwidth of the link.
Cost = (10^8)/Bw
• Use IP Multicast address (224.0.0.5) for all Routers and use (224.0.0.6)
For DR&BDR
• Admin Distance = 110
• OSPF supports multiple paths to a destination network, which allows

for load balancing and redundancy.
• OSPF uses areas to divide a large network into smaller, more

manageable units.
• OSPF routers exchange information about the network using link-state

advertisements (LSAs). Each router sends out LSAs to inform other
routers of its links and their states.
OSPF Tables
1- Neighbor Table : R#Show ip ospf neighbor
This table lists all the OSPF routers that are directly connected to the
local router.
It includes information such as:-
• the neighbor's IP address

• router ID
• the state of the adjacency.
2- Topology Table: R#Show ip ospf database
This table contains the complete topology of the network. used to

calculate the shortest path to a destination network.
It includes information about:-
• all the routers, networks, and links in the network, along with
their associated costs
3- Routing Table: R#Show ip route
This table contains the best path to each destination network in the
network. It is derived from the topology table.
• the next hop router

• the cost of the path
• the outgoing interface.
4- Link-state database:-
This database contains all the LSAs that have been received from other
OSPF routers.
It includes information about: -
• Links.
• Routers.
• Networks in the Network.
• The link-state database is used to build the topology table.
5- Forwarding table:-
This table is used by the router to forward packets to their destination.

It is derived from the routing table.
• Destination network.
• The Next Hop Router.
• the Outgoing Interface.
The Important Advantages OF OSPF
• Fast convergence: SPF algorithm is very efficient and can converge
quickly, meaning that network changes can be detected and responded
to quickly.
• Scalability: used Hierarchical design. It supports areas, which can be

used to divide a large network into smaller. This helps to reduce the
size of the routing table and limit the propagation of routing
information.
• Load balancing: OSPF supports multiple paths to a destination

network.
• Security: Supports authentication
The Important disadvantages OF OSPF

• Resource requirements: OSPF requires more resources than some
other routing protocols, particularly in terms of CPU and memory. This
can be a disadvantage in smaller or less powerful networks. Can reduce
this by Using Different Types Of Areas.
OSPF Packet Types :-

1- Hello Packet
Discover Neighbors and Communicate With them
2- DBD … Database Description
check if the LSDB between the two routers is the same
3- LSR … Link-State Request
Requests specific link-state records from an OSPF neighbor
4- LSU … Link-State Update
Sends specific link-state records that were requested . This packet is
like an envelope with multiple LSAs in it.
5- LSAck … Link-State Acknowledgment
OSPF is a reliable protocol, so we have a packet to acknowledge the
others.
OSPF Routers Types
• internal routers
• backbone routers
• Area Border Router (ABR)
• autonomous System Border Router (ASBR)
internal router
This is a router that has all of its interfaces in the same area.
Backbone Router
This is a router which resides in backbone area.
ABR
This is a router that has interfaces attached to 2 or more areas.. ABR
connect the Areas Together.
ASBR
This a router that connects the OSPF network to another network that is
not OSPF.
OSPF LSAs Types
LSA Type-1: Router LSAs :-

The Router LSA is generated by each router for each area it is located. In
the link-state ID you will find the originating router’s ID.
LSA Type-2: Network LSAs :-

Network LSAs are generated by the DR. The link-state ID will be the
interface IP address of the DR.
LSA Type-3 : Summary LSAs :-

The summary LSA is created by the ABR and flooded into other areas.
"LSA Type-4: Summary LSAs "ASBR LSAs :-

Other routers need to know where to find the ASBR. This is why the ABR
will generate a summary ASBR LSA which will include the router ID of the
ASBR in the link-state ID field.
LSA Type-5 : External LSAs :-

also known as autonomous system external LSA: The external LSAs are
generated by the ASBR.
LSA Type 6 : Multicast OSPF LSAs

Not supported and not used.
LSA Type 7 : OSPF NOT SO STUBBY AREA (NSSA) EXTERNAL LSA

also known as not-so-stubby-area (NSSA) LSA: As you can see area 2 is a
NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type 5). To
overcome this issue we are generating type 7 LSAs instead.
OSPF Area Types
1. Normal Area (Standard Area)

2. area 0 (Backbone Area)
3. Stub Area
4. Totally stubby area
5. NSSA (Not-so-stubby area)
6. Tottaly NSSA
)Area 0) Backbone Area
Backbone Area (Area 0) is the central of OSPF Network and all the other Areas are
connected to thsese Backbone Area. It is similar to Standard Area but it is build as
Area 0.
Stub Area
Stub Area is the first specific Area. They are generally used to reduce LSA flooding,
the LSDB and routing table size. So, this type of Area is used for the places that is a
little sensitive to high traffic.
Router(Config)# Router Ospf 1
Router(Config)# area 1 Stub
Totally stubby area
Totally Stubby Area is one step more strict Area than Stub Area. This is a Cisco
Proprietary OSPF Area.
Router(Config)# Router Ospf 1
Router(Config)# area 2 Stub no-summary
)NSSA) Not-so-stubby area
Not So Stubby Area name is the most funny name in network world, I think : ) Funny
and useful. With OSPF NSSA, the External LSAs (Type 5) that is not accepted by Stub
Areas, accepted as Type 7
Router(Config)#Router Ospf 1
Router(Config)#area 3 nssa
Totally NSSA
Totally Not So Stubby Area is like Not So Stubby Area except one diffference. In
Totally NSSA, Summary LSAs(Type 3 and Type 4) are also not accepted. It get these
with only one default route.
Router(Config)#Router Ospf 1
Router(Config)#area 4 nssa no-summary
We use this types to reduce Routing Information and BW.

Router ID, DR and BDR Election in OSPF
Router ID
Each OSPF router selects a router ID (RID) that has to be unique on your
network. OSPF stores the topology of the network in its LSDB (Link State
Database) and each router is identified with its unique router ID.
OSPF uses the following criteria to select the router ID:

1. Manual configuration of the router ID.
R(Config)# router Ospf 1

R(Config-router)# router-id X.X.X.X
2. Highest IP address on a loopback interface.
3. Highest IP address on a non-loopback interface.
DR & BDR
DR = Designated Router
BDR = Backup Designated Router
• In an OSPF broadcast network, OSPF elects one router to serve as the designated
router (DR) and another router on the segment to act as the backup designated
router (BDR).
• This minimizes the amount of repetitive information that is forwarded on the
network.
• OSPF forwards all messages to the designated router.
• When a router is not the DR or BDR it’s called a DROTHER.
OSPF uses the following criteria to select the DR/BDR:

Highest OSPF Priority: -
Priority (By Default = 1) Can Change it by Command:-
R(Config)# ip ospf priority value
Highest Router-ID: -
If Priorities are Equals Will Elect The Highest Router ID

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OSPF Quick Review

• Open Standard Protocol

• Know By RFC 2328 & RFC 2740

• OSPF is a link-state routing protocol & Classless Protocol.

• OSPF uses Dijkstra's shortest path first (SPF) algorithm to calculate

• Admin Distance = 110

• OSPF supports multiple paths to a destination network, which allows

• OSPF uses areas to divide a large network into smaller, more

• OSPF routers exchange information about the network using link-state

1- Neighbor Table : R#Show ip ospf neighbor

It includes information such as:-

• the neighbor's IP address

2- Topology Table: R#Show ip ospf database

This table contains the complete topology of the network. used to

It includes information about:-

3- Routing Table: R#Show ip route

It includes information such as:-

• the next hop router

It includes information about: -

This table is used by the router to forward packets to their destination.

It includes information such as:-

• Scalability: used Hierarchical design. It supports areas, which can be

• Load balancing: OSPF supports multiple paths to a destination

• Security: Supports authentication

The Important disadvantages OF OSPF

OSPF Packet Types :-

LSA Type-1: Router LSAs :-

LSA Type-2: Network LSAs :-

LSA Type-3 : Summary LSAs :-

"LSA Type-4: Summary LSAs "ASBR LSAs :-

LSA Type-5 : External LSAs :-

LSA Type 6 : Multicast OSPF LSAs

LSA Type 7 : OSPF NOT SO STUBBY AREA (NSSA) EXTERNAL LSA

1. Normal Area (Standard Area)

We use this types to reduce Routing Information and BW.

OSPF uses the following criteria to select the router ID:

R(Config)# router Ospf 1

2. Highest IP address on a loopback interface.

3. Highest IP address on a non-loopback interface.

OSPF uses the following criteria to select the DR/BDR:

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