Open shortest path first or OSPF.
OSPF is an industry standard powerful routing 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 vendor interoperability.
You can mix Cisco routers with Nortel Routers, HP routers and others and be assured that routes will
be exchanged between the different vendors.
Shortest path first or SBF, also known as the Dijkstra's algorithm, named after the developer who
published the algorithm in 1959, is used by OSPF to determine the shortest path or best route to a
destination.
So what are we going to cover is firstly an overview of OSPF.
Then 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 loopback and how they determine the router ID.
You can look at quite a few configurations and setups in OSPF, including multiple area OSPF and authentication.
OSPF once again is a link state routing protocol and will flood link state advertisements or listeners
throughout the network or area describing firstly the links attached to a router.
In other words, the radio 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 done?
That description would include, for example, the IP address of the interface, the subnet mask, the
type 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 link states forms the topological database or link state database.
Radha has formed relationships by sending out hello messages using either multicast address 224005 or
uni costs.
LSA is once again flooded throughout the network or area and the link state database is created from
these link state advertisements or Alice's OSPF once again uses the shortest path first algorithm or
Dijkstra's 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 Aussie model various protocols reside.
OSPF resides directly on top of IP.
OSPF does not use TCP or UDP for transporting of updates and information.
It resides directly on top of IP.
OSPF is referenced in the IP header with protocol ID 89, TCP is referenced with Protocol ID six and
UDP Protocol ID 17, and thus some would debate that OSPF resides at layer four.
For this course, however, we're going to say that OSPF resides at layer three directly on top of IP,
also at layer three.
For a lot of us in the real world, this isn't a major concern, but maybe from a theoretical point
of view.
Yes.
A Wireshark capture.
Showing various OSPF packets.
So as an example, this first one is from a router with an IP address of ten 111 going to the well known
multicast address for OSPF 224005.
So that's the address used by OSPF for hellos on, for instance, an Ethernet segment.
And you can see in the output here at layer two.
We've got the sauce Mac address of the rudder.
Sending information to an IP version for multicast address.
In this case, the well known multicast address.
For OSPF.
At Layer two, the ether type is zero 800 in hexadecimal.
In other words, referencing IP.
At least three.
We've got the source IP address once again of ten one one, one and destination of 224005.
You can see this is an IP version for packet.
And you can see here the protocol ID is OSPF in hexadecimal referenced as 0x59.
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 layer three, OSPF is referenced with protocol ID 89.
The source once again is ten 111.
Destination is the multicast address for OSPF residing directly on top of IP is OSPF.
So notice we have Ethernet IP OSPF.
OSPF does not use TCP or UDP as a transport.
It uses IP.
You can see the OSPF header.
It's version two of OSPF.
You can see that this is a hello packet.
In other words, type one.
You can see it's four areas, zero OSPF areas can be referenced either as single decimal numbers like
zero or in dotted IP decimal notation as in 0.0.0.0.
It's like saying tomato or tomato.
It's the same thing.
You can see the source is ten 111.
You can see it's a hello packet.
His information like the network mosque, the hello interval.
The root of priority, the root a date interval who the designated rata is, who the back of designated
rata is, who the active neighbor is, and so forth.
We're going to cover a lot of this information in the upcoming slides.
The important thing to notice once again.
Is that OSPF resides directly on top of IP.
If we look at the next message.
We're out at ten 112 is sending a database description to router ten 111.
You'll see once again that there is no TCP or UDP.
OSPF is referenced with protocol ID 89 in an IP version for header.
OSPF for this course resides at layer three.
But as mentioned, there's a bit of debate about exactly where it resides.
Arata sends links that advertisements.
To advertise the state of a link immediately when the state changes or periodically.
By default, every 30 minutes the databases will be synchronized with the state of the links.
OSPF routers from neighbor relationships and the list of neighbors is stored in the adjacency table
or OSPF neighbor table.
So topping the come on show IP OSPF neighbors will show you adjacent neighbors.
OSPF uses multicast addresses 2 to 4 005 and 224006, which are link local multi costs.
In other words, those multi costs cannot hop across a router.
The riders have to be directly connected.
OSPF will stall all routes learnt in the OSPF topology database or link state database.
The link state database contains all routers and attached links in the area or network.
All OSPF routers in the same area share the same database.
Best routes are then put in to the writing table, also known as the forwarding table.
There are various packet types used in OSPF.
The first type is a hello packet, which is used firstly to dynamically discover neighbours and secondly
to form neighbour relationships and maintain those neighbour relationships.
There are some default intervals for hellos 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 as serial links
and may non broadcast multi axis environments such as frame relay.
OSPF has what's called a dead interval or dead timer, which is four times the interval by default.
If you change the hollow interval, OSPF will automatically change the debt interval to a value four
times the hello interval on a specific interface.
So how those are used to discover neighbors?
And if a hello is not received within the date interval, the neighbor relationship is torn down because
it's deemed that the neighbor is no longer available.
The second type of packet is what's called a database description.
Written is d, d, 0dd, which is used to exchange brief versions of each linked state.
Advertisement.
OSPF is a link state routing protocol routers exchange information about the state of links through
link state advertisements.
When an initial relationship is formed between two OSPF 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 full
LSA information 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 link 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 that acknowledgments acknowledge or confirm receipt of a link state update message.