Open shortest path first or OSPF OSPF is an industry standard powerful writing protocol used in many
networking environments in the world today the word open means that it's an open standard.
This is a great advantage to using OSPF because of Vendy interoperability.
You can make Cisco routers with Nortel routers HP routers and others and be assured that rats will be
exchanged between the different vendors.
Showed us both first or SBF also known as the digesters algorithm named after the developer who published
the algorithm in 1959 is used by SPF to determine the shortest path or best route to a destination.
So are we going to cover these.
Firstly an overview of OSPF.
We look at the features of OSPF.
I'd like to show you how to set up a network using a single OSPF area.
I'm going to show the importance of packs and how they determine the Rodda ID.
We look at quite a few configurations and set ups in OSPF including multiple area OSPF and authentication
OSPF once again is a link state routing protocol and will flood links that advertizements or elysées
throughout the network or area.
Describing Firstly the links attached to a router.
In other words the router interfaces and the state of those links the state meaning a description of
an interface and its relationship to neighboring routers is the interface up or is the interface down.
That description would include for example the IP address of the interface the subnet mask the top of
network to which it is connected and the routers that are connected to that network and so forth.
The collection of all of these links states forms the top alogical database all links state database
routing form relationships by sending out Hello messages using either multicast address 2 2 4 0 0 5
0 unique costs Elisei is once again a flooded throughout the network area and the link state database
is created from these links to advertisements or elysées.
Always be if once again uses the shortest path first algorithm or digesters algorithm to determine the
best path to each destination.
Now there's a bit of debate about this.
Engineers will often discuss where in the model various protocols reside OSPF resides directly on top
of IP OSPF does not use TCAP or UDP for transporting of updates and information it resides directly
on top of IP always is referenced in the IP header with protocol ID 89 TZP reference with protocol IDs
6 and UDP protocol ID 17 and thus some would debate that OSPF resides at Layer 4.
For this cause however we are going to say that OSPF resides a layer 3 directly on top of IP.
Also at least three for a lot of us in the real world.
This isn't a major concern.
That may be from a theoretical point of view.
Yes why is Shaw capture showing various OSPF packets.
Says an example the first one is from Iraq with an IP address of 10 1 1 1 Going to the well-known multicast
address for OSPF 2 2 4 0 0 5.
So that's the address used by always be a follows on for instance an Ethernet segment and you can see
in the output here at Layer 2.
We've got the source MAC address of the Rodda sending information to an IP version 4 multicast address.
In this case the well-known multicast address V.F. Atlay to the ether type is 0 800 in hexadecimal.
In other words referencing IP
at least three we've got the source IP address once again of 10 1 1 1 and this a nation of 2 2 4 0 0
you can see this as an IP version for packet and you can see here the protocol ID is OSPF in hexadecimal
referenced as 0 x 5 9 using a Windows calculator.
We can set it to use hexadecimal set the value to 59 and then change it to decimal and you can see the
protocol ID is 89.
So at least three OSPF is referenced with protocol ID 89 the source once again is 10 1 on 1 destination
is the multi-course address for OSPF residing directly on top of IP is OSPF So notice we have ethernet
IP OSPF OSPF does not use TCAP or UDP as a transport.
It uses IP.
You can see the OSPF header version 2 of OSPF you can see that this is a hollow packet.
In other words type 1 you can see it's for areas 0 0 SBF areas can be referenced either a single decimal
numbers like 0 or in dotted IP decimal notation as in 0.0 0.0.
It's like saying tomorrow or tomato.
It's the same thing.
You can see the sources 10 1 1 1 you can see it's a hollow packet is information like that network mosque
the hollow interval the route of priority the route a date interval to the designated route is to the
backup designator Audi's who the active neighbors and so forth we're going to cover a lot of this information
in the upcoming sleights the important thing to notice once again is that OSPF resides directly on top
of IP.
If you look at the next message where rodded 10 1 1 2 is sending a database description to route it
10 1 1 1 you'll see once again that there is no TZP UDP OSPF is referenced with particle ID.
89 in an IP version 4 header always be it for this course resides at least three.
But as mentioned there's a bit of debate about exactly where it resides.
ARADAS sends links that advertizements to advertise the state of the link immediately when the state
changes or periodically by default every 30 minutes.
The databases will be synchronized with the state of the links always be of routers or neighbor relationships
and the list of neighbors is stored in the adjacency table or OSPF neighbor table so topping that come
on show the OSPF neighbors will show you adjacent neighbors always uses multicast addresses 2 2 4 0
0 5 and 2 2 4 0 0 6 which are link local multicast.
In other words those multicast cannot hop across a router.
The writers have to be directly connected always fearful store all routes learnt in the OSPF typology
database or link state database.
The links state database contains all routers and attach links in the area or network will always be
of routers in the same area.
Share the same database.
Best routes are then put into the writing table also known as the forwarding table.
There are various packet types used in OSPF.
The first type is a hollow packet which is used firstly to dynamically discover neighbors and secondly
to form neighbor relationships and maintain those neighbor relationships.
There are some default intervals for halos on broadcast multimedia segments such as Ethernet.
The interval is 10 seconds.
The second default interval is 30 seconds which is used on non-broadcast segments such a serial links
an MBA made non-broadcast multi-axis environments such as Frame Relay always has what's called a data
interval or dead timer which is four times the interval by default.
If you change the hollow interval I was fearful automatically change the date interval to value 4 times
the hello interval on a specific interface so hello are used to discover neighbors and if a hello is
not received within the data interval the neighbor relationship is torn down because it's deemed that
the labor is no longer available.
The second top of packet is what's called a database description written as DD or dbd which is used
to exchange brief versions of each link state advertisement OSPF as a link state routing protocol routers
exchange information about the state of links to link state advertisements.
When an initial relationship is formed between two SPF routers they will exchange database descriptions
giving each other an overview of what the database contains.
If parts of the database are missing on one router it will send a link state request requesting fool
alla same formation mation from the neighboring router the neighboring router will send what's called
a link State Update which is a packet that contains links state advertisements and as mentioned is typically
sent in response to a linked state request.
This contains detailed information about the link state database rather than just an overview of it
which was contained in the database description.
Links state acknowledgements acknowledge or confirm receipt of the link state update message.