Free CCNA | OSPF Introduction – Day 44 | 200-125 | Cisco Training

Cisco Certified Network Associate day 44
welcome back everyone. I am Imran Rafai your trainer for the entire series today we
start with OSPF and this with EIGRP is one of the most sought-after
topics in this entire series so without wasting much time let’s get
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that’s imran.rafai @ nwking o-r-g so what are we going to look at in
this video today we’re going to look at 2.4 now 2.4 configure
verify and troubleshoots single and multi area
OSPF version two for IP version 4 excluding authentication filtering
manual summarization redistribution stub, virtual link and LSA. so this is
not going to dwell too much into OSPF it would introduce you to OSPF to show how
much go SPF and that’s about it but since this is the configure topic I’m
going to take it in two videos at least two maybe three in this first video it’s
going to be theoretical I’m going to show you and explain to you the theory
of OSPF and over how old’s paid works in the next video we would get into the
configuration mode so we will start back tracer and we will configure SPF right
so what are we going to look at in this video we’re going to look at three
things the first one what is OSPF second we will look at OSP F the process
and how is P of works and then the third thing we will discuss is OSPF areas what
is DF like discussed in the last video
Oh SPF is a link state routing protocol compared to rip which is a distance
vector routing protocol now a link state routing protocol the link between the
networks of rather the link between routers I important and routing
decisions are made depending on the speed of the link right so if you have a
higher bandwidth link that link is gonna get more priority than a lower bandwidth
link which could be much shut up so if it’s distance vector like in ripped like
we discussed last time if it is one hopper it would choose that a material
over that is the slowest bandwidth connection that’s between them but in
OSPF even if it has to hop two routers reach to a certain network it would
choose that if that network order the drought is using high speed connections
we look at it in a little while in this video but for now you understand link
state routing protocol looks at the link state now OSPF as a routing protocol was
developed in the year 1988 now it was an open standard that means every
manufacturer could technically use OSPF and OSPF was used by a lot of providers
and that is why it is one of the most used routing protocol or rather IGP
integrated routing protocols OSPF also had and now the widow SBA version 2 that
suppose only IP version 4 righted to use it’s put only IP version 4 so at about
the same time they develop IP ospf v3 for ipv6 right so i perversion 6 is
supported and OSPF we 3 so though it was a like released in 1989 it took a while
for it to start being used so it was by 2008 when it released few more technic
capabilities to our OS pv3 and they released in 2008 right so major to image
RFC’s that went through to get OSPF to widespread use were released in 1989 and
then those another you see in 2008 why OSPF right we discuss a little bit last
video OSPF is much faster right the convergence is much much faster than
trip most of the time it’s less than one second and I will tell you how to
increase the convergence speed right when we discuss a little while later but
just know SPF is much much faster than direct OSPF it is an AI GP who is
because then it is classless now if you remember a rib is class full so even if
you had like a CIDR notation of let’s say Class A IP address if you have if
you’re doing Class A IP address slash 24 it wouldn’t accent that it would
consider if you type in IP address of t4 it would just consider it
as a network because it considers the whole network so it’s
class full at least version 1 Baidu SPF is classless
it is also God authentication so you could have if two routers need to do
share or spiff information we could we could set up an authentication where it
only if it authenticated password it wouldn’t become neighbors and only then
will it start sharing information right so authentication is also supported and
like we discussed it is open standard and so a lot of manufacturers use or SPF
now on a broad level if at all you look at it from like a high level of needles
eagle eye view OSPF basically works by sharing any as a nothing bar link-state
advertisements right link-state advertisements now what does the LSA
have the LSA basically an LSA is generated by a router right
that’s called LS a type 1 so we’re not worried about the illicit types but just
know LS a is generated by the router it’s called a router LS a now LS a
contains a lot of information primary is the router ID the router ID off the
router that generates the LSA it contains the network that it knows right
then it talks about the cost so there are few information that you need now
that’s all in LLC now else is they keep sharing Alyssa so the router 3 would
send an ELISA to router 5 write a 5 would send an LSA to router 3 right
these Ellis’s are nothing but data structures like we discuss and they stay
in something called as a link state database LSD be right so they collect
all these Alyssa’s that they receive and they put it into a database right so
ideally what happens is once they form neighbor not before they start sharing
LSA is they need to form neighbor so neighbors form by sharing hello mrs.
just now we look at all that in a while but just know what happens is once you
get a link state database formed now router 3 and I’m not a find will start
comparing unless a database right so it would say ok it would send a message
call database description right DB D this is a database description packet it
sends that packet to route 5 and they both in fact both of them send each
other than a DB d packet once they get the DVD what DB details is it just gives
them an overview or an index of what LS is that they have now if they compare
now around 305 they compare router 3 would this receive LS DVD from road 5
and say ok I have 3 4 & 9 so just send me five and seven right so it would rot
off router 3 would send an LS our link state request LS are 2.5 to send 5 & 7
similarly router 5 would say I know the same these three information so just
give me 1 & 2 so it would send an LS our 4 1 & 2 now once links taped request
comes what it does is it sends an LSU packet back right
LSU link state update so if our three records LS are our five would send an
LSU link state update right so it does this is how they update each other and
once that’s done after they do whatever exchange that they need to do you have a
state where oh this like in this case both these devices but if you have 10
routers all the 10 routers would have the exact same LSD be right this is how
OSPF starts calculating its route so once it has an LSD B takes it has all
information that it needs so it knows the entire network right it knows the
entire network of OSPF devices and then once it once you have all the
information and all the links that’s connected between these devices OSPF
uses something called Dijkstra’s shortest path first algorithm it’s very
old algorithm now it uses that algorithm to generate the routing table right so
very important stage is to have this LSD be synchronize across all devices in the
network right so like I said once this happens every device in this network
like for instance in this case there are eight routers and each of those routers
exchange LS a link state request and listed update and before that of course
they have to form neighbor ship we will talk about neighbor ship and how this
process of request and update happens but just for now understand that each of
these devices at least if it’s like a point-to-point network right if it’s a
point-to-point network each of these devices would share updates with each
other and they would have the same LSD B so if all the LSD B states are updated
if everybody has the same LSD B take it start each of them like for instance
router a can have the Dijkstra’s SPF algorithm run
and generator routing tables with genders routing table for itself that
routing table will not be the same for maybe four out of three nor three would
have its own routing table using the same database but it is calculating the
routing table from his perspective right from route three’s perspective how to
reach each of those device similarly router a would do from their perspective
right so to do that SPF algorithm they need to have the LSD be updated right so
how does high speed work like we discussed OSPF needs to do three things
first it needs to form a neighbor second it needs to update LSD be right build
and update the LSD be and third is to build a routing protocol now technically
if you see as an administrator this is the only step that we really need to
fine-tune right so we could do some control maybe like manually we can
configure the route ID all right oh we could manually change the timers or
maybe if you if you’re configuring the OSPF networks you don’t give the wild
cards now in this case OSP we will look at it how to configure the device
configure in the next video but OSPF uses wildcard mask so if your wildcard
masks not matching if you subnets are not matching if your authentications are
not matching neighbor ship will not form so one of the major things in OSP of
troubleshooting that you would be doing is to check why neighbor ship is not
forming so if neighbor ship is not forming you need to know what are the
parameters that need to match for the neighbor ship to form right so that’s
where you should concentrate building LSD be you are not really doing anything
as an administrator once the neighbors have formed the LSD begets updates by
itself routing table gets updated by itself right this is all done by the
OSPF device right you as an administrator is going to be bothered
about neighbor ship if neighbor ship is not forming this something that is wrong
and you need to troubleshoot so let’s take an example let’s take two devices
and for simplification I have given a router ID of 1 1 1 1 and this 2 2 2 2 2
right let’s connect a link between the two devices once a link comes up it says
ok the link is up right and let’s assume that OSPF is configured on both these
devices so I’ve just gone there and manually type the wisp of command and
then route OSPF and then I’ve given all that don’t worry we will do the
configuration but like I said in the next video but let’s assume OSPF
configuration part is done and I’ve connected the two devices immediately as
soon as link comes up router a would send a low packet right it sends a hello
packet and in the hello packet of course it’ll list all the information saying
throughout ID it’ll tell whatever networks let’s say whatever network it
has here it’ll list that and it will say that I’ve not seen any router so
basically it has not got any hello packet before this so it’s saying
I am not seen any anybody in this link right I – I haven’t seen anybody in this
link but I am 1.1.1 right it tells itself it will excuse itself and router
B receives that and when he receives a hello packet router B says ok I can see
that there is always a potential neighbour candidate in this link right
so I’m gonna do go into something called as that in its state right he is going
to be in its tape and he receives that packet now where does it come this hello
packet does not come to router B it is it’s not like a it is not unicast
communication it is not even a broadcast communication hello packet go into a
multicast IP address so it sends to 2 to 4 dot 0 dot 0 5 that’s the multicast IP
address of SPF this is we’re not able to know what happens is like when you talk
about multicast now a lot of people ask this question what what’s the subnet
mask format because there’s no subnet mask full multicast multicast is like a
radio signal right you have a radio signal and you know
t 1 point 0 FM 91 point 0 is the radio station what do you do you go to your
radio right oh whatever let’s assume this is the old radio so
you have a radio at home and you tune your device to 91 point 0 right and then
you receive a signal so similarly multicast is like that router ace sends
2224 time into 2 4 dot 0 dot 0 dot 5 this multicast address it sends router B
since it’s also configured for SPF it listens to this multicast address when
it listens it hears that Rob a has sent a hello packet right when it receives
that hello packet like I said it goes into in its state and it sends back yeah
but before we do that these are the things when you when you look at hello
packets for if for neighbors to form like if they have to form neighbor ship
the hello and dead interval needs to match so if at all you’re gone into
router a and you’re configured the hello packet hello a timer to something else
other than the default value is 10 if you configure to 30 or 50 or whatever
number and if not beep has not matched that value they will not form right so
this is critical you need to match this value authentication should match so if
both of them are doing authentication it should be the same authentication tag
that they’re doing and of course the authentication password also should
match ad ID of course that should match if one is doing in area 0 and area 1
that would be difficult what is area now wait for few slides we will talk about
area in a while but just know that area ID should match the perfect length
should also match that means if you’re doing if one side you’re saying it is
slash 24 the other side also should be slash 24 right so here another thing you
need to be critical which we will discuss in the next video
it is not subnet mask they use wildcard mask right and you know what wildcard
mask is so that’s something we using and of course de birria flag should also
match so these are the factors that should match for a neighbor to form
right so once this happens router to router B sends a reply hello back at
this it sends a look packet but the only thing is it says that it has already
seen 1.1.11 in this medium right so in this interface on this medium it has
seen 1.1.1 that’s another ways fifth candidate that is there on this and I am
2.2 not too so when rotten rotten le sees that and
says ok I see that he sending me a hello but I also see that he’s seen me already
that means I am already in two-way right I have sent a hello I received the hello
now when this happens throughout a what it does it sends its own hello but this
time it says I have already seen you and I’ve already seen myself as and I send
you saw me and you said you are there and I’ve sent right basically it says
I’ve seen one or one two one and two dotted or two and it says I’m 1.1 to one
so now rod BC is fine now I have sent a hello packet he seen me and he sent me a
hello back that means I’m also into it so both of them going to do it now this
is a very critical stage because in this case it is a point point because it’s a
serial connection but what if we use let’s say let’s say we have a switch
right and we use a shared medium right if there’s a shared medium it will not
most of the device here will not go beyond the stage everybody will be in
this stage among these four devices in a shared medium one of them would be a
designated router dr and another router would be a backup designated router now
what happens is each of these devices would form a full connection of what is
for connection we will see in a while but it
we’ll do form full connection only with the dr and bbl with the other non PDA
device or basically they say they are other device it would stay at this so
this particular router in this case let’s call it C and B C and D will only
stay at two-way it will not go beyond two-way it would always if you go to C
and look at IP ish show IP ospf neighbor it will say that it’s in full neighbor
ship with dr and media but with the other router it is in a two-way state
right so two-way state is very critical because that’s where it will be and at
this point basically what it said is when when they’ve good to another thing
that is critical like for if you’re doing troubleshooting you need to know
that if they’ve reached two-way that means al all these parameter in your
hello packet have matched so there’s no problem everything is mash as they are
ready to be neighbors all right so this is the 2a state so once to a state
happens they send a packet now we discuss this it’s called a database
description packet router a would send a database description packet and the
state would be in X dot exchange start what is database description it is like
an index sheet right on your if your if you read a book there’s an index sheet
it gives you everything that’s there in that book right so similarly a database
description is more like an index for the link state database of that device
right so router B also does the same thing it sends it database description
and it goes to an exchange state right one is an exchange start the other one
is an exchange so both of them are basically doing is sending a database
description back to each other once that happens we discussed this links link
state request link state update and link state acknowledgement right this is
acknowledgement ALS a but LS acknowledgement basically so depending
on how many LS is that they have to exchange it would send a link state
Qwest then the other device would send their link state update back and the
first device would send a link state acknowledgement saying I have received
it right and this goes on for as many times as they want daily exchange once
all their leases are exchanged they go into a state known as full right and
also before once this exchange is happening they would stay in a loading
state so if you see loading that means that Ellis’s are still being exchanged
the LS database has not been updated right so once all that exchange has
happened they go into a full adjacency state called as full so they once you
see they’re in full that means they are in full adjusts NC State like we
discussed this full state would be achieved in a shared medium now this is
point to point so there’s no other device point a point is generally
between two devices if it’s a shared medium like an Ethernet only full
legislation would happen only with dr and PDR the of the device which is not a
BTR deer would only have two-way neighbor ship right so how is dr and BDR
elected now that’s an important question so let’s say in in a shared medium okay
how do you how how does the dr election apple they are elections basically first
thing they do is they look at OSPF priority right
second thing is drought ready okay so if OSPF priority is highest was of as
highest that will become the dr if ospa priority matches like if it’s a tie
then they would look at highest router ID who is got the router IP highest
router ID they would become the dr similarly another BDR right a backup
designated router is elected and they would be a backup designated router now
if the dr were to die for faith for some reason immediately
the BDR would become TR and another PBR would be elected right this is very
critical now I didn’t make a slide for that because it just slipped my mind and
I just realized when I was doing and now I don’t want to pause this and create a
new slide and come back and just delay the whole video so I hope you understood
the process BDR and dr if there’s any confusion maybe in my next video i will
try to make a proper slide and explain the process so what are the packets we
discussed we discussed hello now hello is critical they need to match a lot of
parameters that need to match before neighbor ship is formed then we have
database descriptor packet we have link state request link state update link
state acknowledgement right and you know where these packets are used before we
go ahead to the next topic we need to talk a little bit about OSPF cost now
what is always be of cost we spoke about link state I’ll link state or routing
protocols it takes into consideration of link States link bandwidth and all that
factor we discussed right so there’s need to be a formula the formula OSPF
uses to calculate cost is a reference bandwidth by default reference bandwidth
is 100 Mbps divided by the link bandwidth right so for instance if it’s
a serial connection the speed is 1.544 mbps if you do the calculation you get a
cost of 64 Ethernet is a 10 Mbps line right if it is if you do the calculation
again hundred by 10 you get a cost of 10 and then fast ethernet is hundred Mbps
so if you do 100 by 100 you get a cost of 1 right now
what about Gigabit Ethernet if let’s say if if you have Gigabit Ethernet you have
thousand Mbps right but according to this calculation it will always be 1
right it will always be 1 anything more than fast internet will by default be
one cause there’s no fractions or there’s no point decimal points right so
if you want to if in your network you have
gigabit internet you need to go change this default value there is a provision
in OSPF configurations you can quote change the default value if you make
this as the default reference bandwidth is thousand then of course the
calculation will change gigabit will be one faster than it will be ten Ethernet
would be hundred and whatever that serial will be something whatever it is
right now once let’s say after we did so we did this we we form neighbor we built
LSD B now we need to build a routing table now how do we build a routing
table let’s assume after you’ve got the LSD link state database is ready
available everything is done now each of these router will have to run dice
Dijkstra’s shortest path first algorithm right so Rob okay now we’re looking at
this whole slide is going to be from drought ace perspective right router is
our hero and we are generating the routing table for router a right now
let’s say if it has to go to row two one what is the cost of course you know the
cost is 10 right and if you have to go to now let’s say let’s say okay to make
it easy let’s say we are trying to go from router a to router B we’re sort of
a road B’s here okay we’re trying to go from router a rotor B now let’s try to
get all the intermediate values so we will check all the intermediate value
for all the devices right so from robber a to reach Arad one the cost is 10 okay
router a to router to the cost is 100 because this is hundred right we are
good here now from rod a to go to rod three the cost is 11 how is it 11 10
plus 1 is 11 right 10 plus 1 is 11 and to go to router for the cost if you
do 10 plus hundred its 110 if you want good this way okay if you go this way
it’s 110 if you go this way also it is hundred then perfect so there’s nothing
changing now for router six if you go one hundred plus one it’s hundred one
that’s also fine right next to go to route of five it is ten plus 1 xi xi
plus one hundred one hundred and eleven which is fine to go to are seven now you
have two routes you can go like that or you can go like this so go like this 10
110 210 so this will be 200 and then if I go like this one hundred one hundred
and one hundred two hundred one this would be two hundred one between 210 and
201 201 is lesson that is why 201 is chosen right next to reach router be the
total cost you can go in two ways you can go like that or in fact more than
two it can go like that you can go like that or you can go like that if I’m
going like this the cost is I think 121 if you are going this way 10 100 what is
that 210 210 and 220 so this will be 220 if
you’re going like this 110 110 plus 100 210 this would be 210 and if you’re
going like this one hundred hundred one 201 201 or what am I saying
100 200 200 200 11 right here 211 correct so between this 121 is is the
lowest and that’s why the cost of going you
going to rod B is 121 right so this is how routing OSPF does it so it does in a
routing table it says for router 1 or basically doesn’t say router when it
sells routing I’m just this is a very simple simplified version of how a speed
works but basically it would give the route network and how to go to the
network so in this case to go to router 1 it says 10 and whatever if this is f 0
/ 1 and this is f 0 / 2 it would say okay to go to rod 1 go on f 0 / 1 and
this is the call right so you would have all that information in the routing
table using calculations similar to what we did here all right now we need to
discuss one new topic it’s about areas right so normally if
you configure no SPF and if it’s all of them are in a single area right I mean
just think about it it’s in 1 OC of area what happens is if there is any update
like for instance if there is a link there’s one link here and that link
terminates this device would basically start sending LS is out saying oh this
link is failed this link is fail and everybody would have to be updated now
if you are in an area where you have thousand rotors all the thousand dollars
will have to be updated because it’s in the same area also another way LS is
let’s say one of the routers one of the neighbors failed again the same thing
all the devices in that area will have to be affected because they
will have to do LSA update ones LSA update comes it’s not only there let’s
say update that happens once an LSA changes the topology basically changes
if that happens SPF will have to do so the recalculation
of entire routing table has to happen again right so this is a this is a big
process so if you have a lot of device thousand devices one area one look at
the number look at the size of your memory
to store this LSA right the LSD be is going to be huge right because it has to
keep a lot of information for so many devices and it has a recalculate so
every time a change happens in any part of the network you need to do a
recalculation OSPF algorithm as run I mean SPF algorithm as run and then and
it’s a big big big process so whether link changes or if there is
not data other than that Ellis’s have a timer of 30 minutes every 30 minutes to
let’s say around 300 and then let’s say this LSA every 30 minutes it has to
update itself so if the LSA generation timer is more than 30 minutes that
particular LSU will update it but ok with the only differences since Ellis’s
are generated at different times at different devices this particular event
is not going to like it’s not going to be a bottleneck where every device is
generating illus at the same time it could be at different times but again
this is something that will happen every 30 minutes for each lesson
now that’s also once at LSA updates again recalculation and there’s a lot of
problem one way to solve this problem or rather this is the way to solve the
problem is by dividing your network into areas now as an administrator you have
to plan your OSPF or rather your network diagram or network the way you connect a
network such that you always your area 0 is your main area right this is where
your connection to the outside forecast rapid right this is where because as a
rule if you creating new areas one of the main rule is that all the areas
should have won a be our area border router right you would have one area
border router what is the Navy important router area power router is a router
which has one network or one interface in one area and the other interface in
area 0 every area like I said has to connect to area 0 so one
to face in that particular area and the other interface in area zero right so
that’s what a br is a Lia border router now these all the networks that’s part
of one area it stays there but one of the router like I said should be in area
zero similarly all these routers are part of area one and one interface of
this ABR will be in area zero and area zero is what does connect to everything
else that’s that’s one of the critical stuff now what happens now
now let’s see this particular link goes down or rather if there’s one link
connector that goes down this sends an a let’s say this LSA is affected to only
area one no area one is the only area that has to bother about that
like area two or area zero should not even really bother about updates from
area one because that doesn’t put into them because your ABR what it does it it
summarizes right if you remember super super super netting it summarizes right
so our router fine does not it’s because it summarizes route three and other area
devices need not know anything about the changes that’s happening in area but
they will route of five will handle that situation so ABR will handle the
situation at the gate right so this is one of the reasons why we need to do
areas and if it’s confusing now don’t worry when we start doing configuration
or VoIP if in our next video and we take some examples you will have a much much
better idea of what OSPF area is right this video was basically to give you a
theoretical view of what OSPF is and how will always be work and when we come
back to the next video you would have a much much better understanding of OSPF
so thank you so much please join me back in the next video
and that we will take this is proof journey forward since you very very soon
bye bye

48 Replies to “Free CCNA | OSPF Introduction – Day 44 | 200-125 | Cisco Training”

  1. I didn't even watch the video but I just hit the like button caus I know its going to be awesome as always😍😍😍

  2. This video and this series are still valid. If anything changes, we will update this series accordingly. All the best.

    The full series can be found on this playlist:

    #FreeCCNA #CiscoCCNA #ImranRafai

  3. Again thanks a lot from me, on behalf of your fans for uploading such wonderful vedios , and wishing, it will contionue to the next within a very little interval…………………

  4. Imran please explain from where you got the 10, the 100 when doing the cost of OSPF between routers : i am trying to get the CCNA certificate though i have no IT background you make me learn and like the network: the calcuation seems easy but then when it comes to 10. 100 and then you are calculating i got stack as from where you got that numbers

    thank you

  5. multicast subnet is 24

  6. It's good to be here watching such educative and exciting tutorial video…. I must command you for this initiatives. Please keep on and update me for more and i will keep watching. Networking is my dream….

  7. even i disliked the video because of too much speaking and repeating things ,(though u know its a video and we can roll it back ,) i still assume that you do give the best ccna preparation course on youtube ,till now on what i have seen . thank you

  8. Hi Sir , i have question . What if Border router get Down ? It exist any setup for alternate border router? If not , it will affect entire area…

  9. Can we use normal subnet mask instead of wildmask to form neighborship,and what consequences will happen if I do this

  10. Thanks alot these videos are awesome waiting for other informative videos as well thumbs up for ur effort πŸ™‚

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