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8/8/2019 Ospf by Harsh
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OPEN SHORTEST PATH FIRST
OSPF
BY: - HARSH KUMAR
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Introduction to Open Shortest Path First (OSPF)
Open Shortest Path First (OSPF) is a
routing protocol which is used to allow
routers to dynamically learn routes from
other routers and to advertise routes to other
routers.
Advertisements containing routes are
referred to as Link State Advertisements(LSAs) in OSPF.
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Specifications
1. Keeps track of the state of all the various network connections (linkslinks) between itself and a network itis trying to send data to. This makes it a link link--state state
routing protocol.
2. Supports the use of classless IP address rangesand is very efficient.
3. Uses areas to organize a network into a hierarchalstructure; it summarizes route information toreduce the number of advertised routes andthereby reduce network load and uses adesignated router (elected via a process that is partof OSPF) to reduce the quantity and frequency of Link State Advertisements.
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Specifications
4. Does require the router to have a more powerful
processor and more memory than other routing
protocols.
5. Selects the best routes by finding the lowest cost paths to a destination. All router interfaces
(links) are given a cost. The cost of a route is
equal to the sum of all the costs configured on all
the outbound links between the router and thedestination network, plus the cost configured on
the interface that OSPF received the Link State
Advertisement on.
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The Shortest path to a destination is not necessarily the path
with the least number of hops
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OSPF cost
The cost (also called metric) of an interface inOSPF is an indication of the overhead required tosend packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidthindicates a lower cost. (There is more overhead³higher cost´ and time delays involved in crossinga 56k serial line than crossing a 10M Ethernet line.
The formula used to calculate the cost is:The formula used to calculate the cost is:
cost= 10000 0000/bandwith in bpscost= 10000 0000/bandwith in bps
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Example
For example, it will cost
108/107 = 10 to cross a 10M Ethernet line while it
will cost
108 /1544000 = 64 to cross a T1 line.
By default, the cost of an interface is calculated
based on the bandwidth; you can force the cost of an interface with the
ip ospf costip ospf cost <value><value> interface subconfigurationinterface subconfiguration
mode command.mode command.
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Link-State Routing Process
Each routers learns about its own directlyconnected networks
Hello Packets
Link State Packet (LSP)Flooding to all neighbors
Calculating SPF
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The Shortest path to a destination is not necessarily the path
with the least number of hops
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Neighbors
Routers that share a common segment become neighbors on that segment.
Neighbors are elected via the Hello
protocol.Hello packets are sent periodically out of
each interface using IP multicast.
Routers become neighbors as soon as theysee themselves listed in the neighbor's Hello packet. This way, a two waycommunication is guaranteed.
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Adjacencies
Adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyondthe simple Hello exchange and proceed into thedatabase exchange process.
In order to minimize the amount of informationexchange on a particular segment, OSPF elects onerouter to be a designated router (DR), and one router to be a backup designated router (BDR), on eachmulti-access segment.
The BDR is elected as a backup mechanism in casethe DR goes down. The idea behind this is that routershave a central point of contact for informationexchange. Instead of each router exchanging updateswith every other router on the segment, every router exchanges information with the DR and BDR. TheDR and BDR relay the information to everybody else.
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Example
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Router types
OSPF elects two or more routers to manage theLink State Advertisements:
Designated Router (DR)
It is the router to which all other routers within an areasend their Link State Advertisements. The DesignatedRouter will keep track of all link state updates andmake sure the LSAs are flooded to the rest of thenetwork using Reliable Multicast transport.
Backup Designated Router (BDR)The election process which determines the Designated
Router will also elect a Backup Designated Router (BDR). The BDR takes over from the DR when the DR fails.
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Router types
OSPF elects two or more routers to manage
the Link State Advertisements:
Designated Router (DR)
Backup Designated Router (BDR)
A priority value of zero indicates an
interface which is not to be elected as DR or
BDR. The state of the interface with priorityzero will be DR OTHER .
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Areas and Border R outers
As previously mentioned, OSPF uses flooding to exchangelink-state updates between routers. Any change in routinginformation is flooded to all routers in the network. Areasare introduced to put a boundary on the explosion of link-
state updates. Flooding and calculation of the Dijkstraalgorithm on a router is limited to changes within an area.
All routers within an area have the exact link-statedatabase.
Routers that belong to multiple areas, and connect these
areas to the backbone area are called area border routers(ABR). ABRs must therefore maintain informationdescribing the backbone areas and other attached areas.
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Enabling OSPF on the R outer
Enabling OSPF on the router involves the following twosteps in config mode:
1. Enabling an OSPF process using the router ospf <process-id> command.
2. Assigning areas to the interfaces using the network <network or IP address> <mask> <areaid> command.
The OSPF process-id is a numeric value local to the router.It does not have to match process-ids on other routers. It is
possible to run multiple OSPF processes on the samerouter, but is not recommended
The network command is a way of assigning an interfaceto a certain area.
The mask contains wild card bits where 0 is a match and 1is a "do not care" bit, e.g. 0.0.255.255 indicates a match inthe first two bytes of the network number.
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RTA#
interface Ethernet0
ip address 192.213.11.1 255.255.255.0
interface Ethernet1
ip address 192.213.12.2 255.255.255.0
interface Ethernet2ip address 128.213.1.1 255.255.255.0
router ospf 100
network 192.213.0.0 0.0.255.255 area 0.0.0.0
network 128.213.1.1 0.0.0.0 area 23
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THANK YOU