The border will become the D.R. if the D.R. files designated Rodders are chosen on a per segment basis.
So in this example, on this Ethernet segment or two may be chosen as a designated rider, but on this
segment between our four and or seven of form may be chosen as a designated rider on a per physical
segment basis.
A designated route is chosen.
So just because our four is not a designated rider on this segment does not mean that our four is not
a designated rider on this segment.
Every Ethernet segment requires the election and maintenance of a designated rider.
So from an update point of view, if this link goes down or one is updating its designated rider using
this multicast address 224006 or to the designated rider updates all of its neighbors on that segment.
So are three or six or five and or four get the update about the change of foal then floods that update
to or seven or seven will then flood that update to any neighbors that it may have not shown in this
diagram.
The same with or five or six or three and so forth.
The update will be flooded throughout the topology, just as in this example where all four receive
the update from our two and then sends it to our seven.
So just to sum up, designated riders or dealers are elected firstly based on the highest priority.
The range is from 0 to 55.
The default value is one zero disables the ability for this rider to become a designated rata or backup
designated rider.
If the priorities are the same, the rider with the highest rata ID will become the designated rider.
It's important to realize that preemption doesn't exist with designated riders.
So in this example, if the priority of router two was set to ten and the priority of right of five
was set to five and an election took place or two would become the designated Rada.
However, if r t went down, rather five would become the designated rider, and for instance, R three
would become the backup designated rider.
Let's assume that the priorities of the other riders are set to one, but rather three has the highest
rate of ID, so rather three becomes the designated rider.
When R2 comes back again, it will not become a designated rider or backup designated rider.
In other words, it's not preemptive because our five is already the designated rider and all three
is the backup designated rider or two will just accept that fact and will become known as a DH or other,
or draw the depending which term you prefer.
It will not try and preempt another election and try and become the designated rider.
All other riders, including our two, will become DE or others in a topology like this.
Let's talk about the shortest path first algorithm or SPF algorithm.
The SBF algorithm places each router at the root of a tree and calculates the shortest path to each
node using the Dijkstra's algorithm based on the cumulative cost that is required to reach that destination.
So as an example, if rather one wants to get to a network behind router two, it's going to determine
the best route based on the cost, which uses a Formula ten to the eight divided by bandwidth.
So as an example, this is at1 link router.
One wants to send traffic to router two.
Will it take this RT one link or will it use the path via router three using a ten meg and ten meg link?
Now rip using hop count would send the traffic directly to our t, but what would OSPF do?
OSPF by default uses a reference bandwidth of ten to the eight.
You can change this and you need to when you've got gig and ten gig links.
OSPF has been around for many years and in the early days there were no speeds such as gig, ten gig
and so forth.
So today, if you've got gig and ten gig links, you're going to want to change this reference bandwidth.
But for now, let's assume that we're using the default.
So the cost of a link is ten to the eight divided by the bandwidth in bits per second.
Now, one kilobits per second equals 1000 bits per second.
One megabits per second is 1 million bits per second.
Ten megabits per second is 10 million bits per second.
So the cost of a ten megabits per second link is ten to the eight divided by 10 million, which gives
you a cost of ten using the same formula for at1 link.
So for this link between R one and r t, the cost is ten to the eight divided by 154 for 000, which
gives you a cost of 64.
So the cost using this link would be 60 for the cost if the traffic was sent via r two would be as follows
ten to the eight.
But what about 10 million, which is ten?
But they are two lengths of ten, meg, so the total cost is 20.
So the cost via the PT one is 64, but the cost via the ten meg links is 20.
So OSPF is going to choose the link via R2 to send traffic to all three because the cost is lower.
Based on this formula ten to the eight divided by bandwidth.
So the default formula used for costing is ten to the eight divided by bandwidth.
Now you can change the reference bandwidth by using this command within the OSPF process or the cost
reference bandwidth and then specifying a value in megabits per second.
The default is 100.
In other words, the reference bandwidth is 100 megabits per second.
100 megabits per second has a cost in OSPF of one.
If you change this to 1000 as an example, a gigabit link would be seen as the reference bandwidth.
Or if you change it to 100,100 gig link would be used as a reference bandwidth.
You need to do this command on all routers where you have interfaces with bandwidths greater than fast
Ethernet.
You can also change the cost on an interface.
So rather than OSPF calculating the cost of the link based on the bandwidth, you can set the OSPF cost
by tapping the command IP OSPF cost and then specifying a value.
It's very important that the bandwidth statements on your router interfaces be configured correctly
because that's what OSPF deems the speed of the link to be.
Your copy does the same thing.
It uses bandwidth as part of the calculation when working out the shortest route to the destination.