Time for us to see what could be so positive about something that has such a nasty name round poisoning
Well here's an example of a distance vector protocol walks for an hour or three connected to ten when
one's erst last 24 is advertising Toreadors one into that that network is one hop away.
Hobs not a very scientific metric.
But that's the only metric that distance vector protocol's understand.
It's called just a hop and it's just like what it sounds one router to another is one hop to the next
router to ops etc. so you can see another reason we might not use that we'll revisit that later.
But Round two is getting an advertisement and sending it out another Interphase and it's going to Router
one.
So Router one at this point is hearing about 10 1 1 0 or Slish 24 from two different sources and it's
going to look at that and say OK that's the exact same route.
But the metric is different it seems to be faster if I go through a router three as opposed through
router 2.
So we love this situation because we'll take all the redundancy we can get if Router one loses one path
to 10 1 1 0 slice 24 it has another one you know we kind of like that so you know everything would be
fine here.
But until that route goes away.
And I think you'll agree with me that when it becomes unavailable Let's take another look at 10 1 1
0 slice 24 for let's say that network goes down.
Well when it becomes unavailable we want all the readers to know about it and to know it as quickly
as possible right.
Me makes perfect sense it's not a trick question.
We want that convergence we wanted to know about it and say hey I don't know.
Rather three needs to tell him hey I don't know where this network is anymore.
So what.
Many people including myself many years ago thought is that ralliers three would just stop advertising
it well there is a problem there because if rounders 3 simply stops advertising it we're going to run
into an issue and I must show you what it is here.
And just as a quick reminder what I'm showing you next is without Raut poisoning and what what happened
is is that router 3 says OK 10 1 Windsor's left 24 is unavailable.
I'll stop advertising it.
The problem is that doesn't do anything about the entry in round or two's routing table.
So router is just going to keep telling everybody and in this case that includes router 1 Hey I know
we're 10 1 1 0 size 24 isn't it.
Two hops away even though the network itself is now unavailable.
So it's not enough for a router 3 to stop advertising it it's got to have a way to tell everybody else
hey that route designed around anymore that goes what happens next in or walks for without relm poisoning.
Is it Reller three's going to get an advertisement from round one and say hey that route is now available
via Rauner what you know put it back in my routing table start advertising it again.
And what you end up with is this mouse router one sends down an update to router three eventually says
hey I know we're 10 1 1 0 is it's three hops away.
And if that network is down on router 3 it's not going to be in the Round Table router 3 looks and says
hey OK I'll just put that in my rolling table and start advertising it.
And 10 loans aerosolize 24 hours for hops away.
So you can see again the trouble we end up with here and this is how all routing loops form if this
were in the center of a larger network.
So we end up with a real problem if we didn't have Raut poisoning but fortunately it's on by default.
We do have it.
So what happens in the case of poisoning.
Well Reller 3 will continue to advertise.
The last metric excuse me the last route but it will do so with a poisoned metric and this is another
funny thing about replicate Ripp only understands hop count and it understands that a hop count of 16
indicates and unreachable route.
So that is the metric router 3 advertizes with the route.
So this is what we have in our network instead because Raut poisoning works.
This is what happens router 3 does a stop advertising on router 3 continues to advertise it but with
a poisoned metric that's where the name comes from.
And it's telling routers one into hey you know it's 600 hops away all routers one and to get that update
and say hey you know I'm not going to I'm going to take that route out of my table and I will stop advertising
it.
So as a result of that poisoned update Reller 2 will no longer advertise the route up to Reller one
or vice versa.
So that's what I Raut poisoning can be a force for good.
It sounds evil but it is indeed a force for good.
Now let's go back and talk about that hot Cal for a second because if this is the first time you've
seen it you might have thought that sounds pretty wacky.
Well neither version of group is very scientific when it comes to determining the metric of a network
segment because again the hop count is the only thing it understands.
Now you and I could look at a gig Ethernet link in a 56 K link and say I think the gig or Netlink is
a little bit faster.
But the problem is really up or any distance vector protocol considers these two links to be the same
speed.
It's one hop.
It's the same metric to rip rip can't differentiate.
Now you could go in and do some tweaking and make it understand but we really don't want to be in that
business because we have other protocols.
We'll learn about later that we'll see.
OK I see what is going on here with the speeds and ologist but distance vector protocols by default
are not going to adjust they're just going to use that hop count.
Now with that full routing update every time all the time is the way I used to remember.
Now if your network is literally changing every minute you should not be watching this video you should
be at work.
And I kid mostly.
But again you know we want a nice stable network I'm sure we all have one little things happen from
time to time or we wouldn't have jobs but we do not need an update especially a full rounding up day
every 30 seconds by default because these unnecessary routing updates first they take a band wealth.
But first a router has got to put those updates together.
Got to send that goes across the line router and then the end gets it has to unpack it says hey it's
like reading yesterday's newspaper you already know everything.
So you just throw it away.
And so they're all thrown away.
But at the same time everything we do on a router or switch has a little bit of a cost.
And we don't want that cost of having a full update every 30 seconds because you're looking at multiple
packages because it's not like if you have 200 Ripp routes to advertise they're not all going to fit
in one packet so you end up with multiple packets on top of that.
Believe it's 25 per packet.
So we're getting into the nuts and bolts there.
But if you had that it would be 8 Ripp updates eight reep update packets just to contain every round
in that routing table.
Not an efficient way to go about business so we really don't like that now AIG or RPE sends a up update
only when there has been an actual change to report and even then the update contains only the changes
not the entire AIG crappy writing table.
That's a pretty good enhancement you see where that is coming from now right.
That's just one of the enhancements OSPF handles things a little bit differently.
But take my word for it's much more efficient than distance vector protocols handle.
But before I'm accused of reversion to schaming version 2 was quite an improvement over version 1 and
we know Cisco exams early any exam when you have two versions of something you should know the differences
between the two.
And here are four that I think you should know version to support subnet masking variable length subnet
mask if you will VLA sound more on that topic coming in the next section.
For now just remember version 2 supports it Version 1 does not version 2 multicast it's updates as is
a good IP address to memorize frankly 2:24 0 0 9.
I'm sure you so many debugs in the next couple of labs that that address will be you know you'll see
it when you close your eyes go to sleep at night.
It's a lot more efficient for routing protocol to multicast than to broadcast to 255 for each octet.
But that's what version 1 does it.
It broadcasts were version 2 multicast its updates.
As I mentioned previously just a quick reminder version to offer update authentication version 1 does
not in version 2 allows the network administrator to configure route summarization manually.
Version 1 doesn't allow that.
And we will see in a lab why that can be so very important.
Speaking of labs This is where we're going to start with Ripp and it should look awfully familiar we've
got our 172 twelth 123 0 slish 24 network running on our serial interfaces on all three routers.
Our service provider cloud or frame relay cloud we're not configure and that because that's our service
provider and we have a loopback interface on routers 2 and 3 there isn't one on one yet.
And I would go ahead and write this out because again we're not going to continually come back to the
screen and look at the drawing and I've got a couple of different con figues for a couple of different
labs and we will be subtracting and adding loopback interfaces that serial cloud the serial connections
will always be there.
Unless I tell you otherwise of course.
So going jot this down take a break and want to see the beginning of the next video.
We will jump straight into a rip.
See there