Ospf - Sonal

7/30/2019 Ospf - Sonal

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OSPF - Open Shortest Path First


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IP Routing

IP Routing is the process of delivering IP

Packets from one device to another, across an IP

network, using routers.

There are two techniques of performing IP routing.

1) Static Routing. 2) Dynamic Routing.


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Routing Techniques

STATIC ROUTING DYNAMIC ROUTING

Route table built by networkmanager.

Route table built dynamicallyby router.

Easy to troubleshoot. Requires in-depth knowledge of

the protocol to troubleshoot.

No capability of re-route. Automatic re-route.

Administrative effort requiredto maintain routing intelligence

No administrative effortrequired to maintain routing

intelligence.

Supported by almost all TCP/IPhosts.

Not all TCP/IP hosts supportdynamic routing.

Used in small networks. Used in medium & largenetworks.


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Routing Vs Routed Protocols

There are basically two type of protocols : Routed

Protocols and Routing Protocols.

Routed Protocols are the ones which takes/carries

the data (info.) over the network like TCP, UDPRTP etc.

Routing Protocols are responsible for determining

the path i.e they does routing i.e they finds the best

suitable path for packet to travel like RIP, EIGRP,

BGP, OSPF etc.


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Routing Protocol Classification

Routing protocol are classified into three types -

1) IGP 2) EGP 3)

HYBRID

Interior Gateway Protocol (IGP) used within in an

Autonomous System.

Eg :- OSPF, IS-IS, RIP Exterior Gateway Protocol (EGP) are used between

an Autonomous System.

Eg :- BGP

H brid Protocol have characteristics of both IGP as


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OSPF - Introduction

Interior routing protocol(IGP).

Works on layer 3 of OSI model.

Link state routing protocol.

Use IP as routed protocol, IP protocol no. 89

Open standard hence can be used in multi vendor

environment.

Converges large networks.

Intra-domain i.e. works within single autonomous

system (AS).


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Characteristics

Uses cost as metric.

Sends triggered & incremental updates.

Uses multicasts instead of broadcasts.

Converges very quickly.

Classless and allows summarization.

Conserves network bandwidth.

Supports VLSM.

AD value = 110.

Two-level hierarchy.


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Operations of OSPF

OSPF can work in different scenarios :-Single area.

Multiple areas.

Different network topologies of OSPF are :-

Broadcast multi-accessPoint-to-point

Point-to-multipoint

No broadcast multi-access (NBMA)

OSPF build & maintain following tables :-

Neighbor table

Topology database

Routing table


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Operations Cont.

Once the OSPF process is started on router , it startssearching neighbors.

Once the router has discovered all its neighbors it

will form neighbor table & start exchanging DBDs

& LSA.

Once its has collected all networks & routes , it will

be in FULL state and will form adjacency with its

neighbors. Now it will form topology database & will run SPF

algorithm to find shortest possible route or path to

reach each network.

Once it has best routes to reach each network , it will


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OSPF in Multi-access network type

Adjacency:- 2 OSPF neighbors are adjacent when they exchange DBD &LSAs.

OSPF works differently in Multi-access network, it selects DR & BDR to

reduce the no. of adjacencies within an area.

Election of DR & BDR :-

When an OSPF router becomes active and discovers its neighbors, it checksfor an active DR and BDR. If a DR and BDR exist, the router accepts them.

If there is no BDR, an election is held in which the router with the highest

priority becomes the BDR.

If more than one router has the same priority, the one with the numerically

highest Router ID wins.

If there is no active DR, the BDR is promoted to DR and a new election is

held for the BDR.


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Adjacencies without & with DR in

multi-access OSPF network


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OSPF Router Stages

Down :- This is the first OSPF neighbor state. It means that no information(hellos) has been received from this neighbor.

Attempt :- This state is only valid for manually configured neighbors in anNBMA environment. In Attempt state, the router sends unicast hello packetsevery poll interval to the neighbor from which hellos have not been receivedwithin the dead interval.

Init :- This state indicates that the router has received a hello packet from itsneighbor, but the receiving routers ID was not included in the hello packet.

2-Way :- This state indicates that bi-directional communication has beenestablished between two routers.

Exstart :- the router and its neighbor establish a master/slave relationship anddetermine the initial DD sequence number in preparation for the exchange ofDatabase Description packets..


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OSPF Router Stages Cont

ExchangeIn the exchange state, OSPF routers

exchange database descriptor (DBD) packets.

LoadingIn this state, the actual exchange of

link-state information occurs.

FullIn this state, routers are fully adjacent witheach other. All the router and network LSAs are

exchanged and the router databases are fully

synchronized.


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Router types

Internal router :- Routers whose interfaces belong to the same area. Theserouters have a single link-state database.

Backbone router :- Routers with at least one interface attached to the

Backbone Area. In some cases an Internal router whose all interfaces belong

to the Backbone Area is also a Backbone Router.

Area border router (ABR) :- Routers who connects one or more areas to theBackbone Area. For each of connected areas an ABR must maintain a

separate Link-State Database. Only summarized topology information is

propagated by an ABR to the Backbone Area.

Autonomous system boundary router (ASBR) :- An ASBR connects to

other routing domains. ASBRs are typically located in the backbone area.


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Router types


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Router types in multi-access network

Designated Router (DR) :- Router in multi-

access network which is responsible for all

adjacency within a single area.

Backup Designated Router (BDR) :- Routerbackup to DR within a single area.

DROTHER :- routers other than DR & BDR

within a single area.


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OSPF Areas types

Backbone Area :-It is the core of the OSPF network. All other areas must beconnected to it. Backbone area must be contiguous.

Regular Area :- Regular area have more than one connection to the

Backbone. Its primary function is to connect users & resources. By default

regular area doesn't allow traffic from another area to pass through it.

Stub Area :- A Stub Area is an area which has no information aboutnetworks external to the OSPF domain. For any external destination an

Internal Routers will use default route pointing to an ABR. Configuring an

area as a Stub Area is the way to improve the performance of routers by

reducing size of their Link State Databases.

Totally Stub Area :- Totally Stub Area is a modification of a Stub Area.Routers within Totally Stubby Area use a default route pointing to an ABR to

reach all destinations outside their area.


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OSPF Areas types


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Link State Database

A place to store the information of router in OSPF is called Link StateDatabase. Each router running link state protocol must have such a

database.

Exactly the same database content is a critical factor for routing

decision process.

Incomplete database information results in incomplete network viewand ultimately routing problems.


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LSA & LSA Flooding

Router's details are send across network in units calledLink State Advertisement (LSA).

Each router generates one LSA describing itself : its

links and neighbors.

An information distribution mechanism used by OSPF

is called flooding. The name comes from the similarity

to water behavior.


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LSA 1, LSA 2 & LSA 3

LSA 1 - Also called Router LSA. LSA 1 are generated by all OSPF routers.This is most fundamental LSA and it lists all interfaces of a router to otherrouters or other networks. The flooding of LSA Type 1 is limited only to

originating area.

LSA 2Also called Network LSA. LSA 2 are generated by DR on every

multi-access network (like Ethernet). LSA 2 lists all routers connected to thesame segment. The flooding of LSA Type 2 is also limited only to originating

area.

LSA 3Also called Network Summary LSA. It is originated by ABR.They are send only to a single area to advertise information to Internal

Routers about destinations outside that area. This information can be alsosummarized before sending to prevent sharing information about area

topology to other areas and save routers resources. Using LSA 3 an ABR also

advertise the destinations within its attached areas into the Backbone Area.


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LSA 1, LSA 2 & LSA 3


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LSA 4 & LSA 5

LSA 4 - Also called ASBR Summary LSA. To advertise an ASBR routersuse LSA type 4. These LSAs are send only between ABRs and announcewhich ABR in the network works as the ASBR. LSA 4 is not limited, like

LSA 1 and 2, to only one area and it can be replicated by all ABRs.

LSA 5Also called External LSA. LSA 5 are originated by ASBRs. They

advertise ether a destination external to the OSPF. LSA 5 can also advertiseDefault Route for the OSPF domain. These LSAs are flooded to all areas

(except stub areas) and are received by all Internal Routers.


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LSA 4


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LSA 5


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Summarization

Multiple areas are a great way to limit computationand propagation of routing updates; the hierarchicalapproach imposed by using multiple areas allows forintelligent summarization on ABRs and ASBRs.

Types of summarization in OSPF :-

Interred summarization :- Performed at the ABR,creates Type 3 LSAs. Type 4 LSAs advertise therouter IDs of ASBRs.

External summarization :- Performed at the ASBR,creates Type 5 LSAs.


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THANK YOU

Questions...?


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Ospf - Sonal

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