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A previous blog I explained why you would use a loop interface and I gave you one reason.
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Let's look at another reason.
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Another reason why you want to use a look back is because writing protocols such as always use the loopback
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to determine the ideas of routers in the network.
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So these routers
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show IP interface.
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I've configured a loopback zero with this IP address and I've also configured the loop banks on each
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router Sarada one is 1 and 2 1 6 8 1 2 1 router Tuesday or 2 rotisseries or treat.
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Now why is that important.
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It's important for routing protocols.
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So when we enable writing protocols such as SPF it selects a router ID for itself.
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So I'm going to enable OSPF on all interfaces on the router and just simply put them in areas Zira.
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So show IP SPF interface brief SPF is enabled on all interfaces on the router show IP SPF interface.
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Notice show loopback is 0.
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Has this IP address configured in area 0 but the router ID is 1 9 2 1 6 8 1 to 2.
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In other words the Rodda ID selected is the highest IP address of any interface.
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And if they are loop backs the loopback override the physical interfaces.
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So the Rodda IDs are elected on the highest loopback address.
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In this case 1 9 2 1 6 8 1 to 2.
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Now that's important because in OSPF a router is identified by its Rodda ID it's kind of like your name.
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So my name is David.
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It would be terrible if my name changed everyday whenever there was a problem.
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And that's the issue with Louise.
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If you don't use a loopback interface and the Rotto I.D. was selected off a physical interface and that
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interface when down the road the idea would change but it's consistent.
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Notice on gigabit 0 1 the router ID is still 1 2 1 6 8 1 2 2.
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So let's have a look at the problem.
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Not a one.
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We've got two IP addresses configured on physical interfaces and then we've got these loopback interfaces.
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But what I'm going to do now is remove the loop back into faces of Route 1.
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So show IP interface brief do that again.
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We only have IP addresses on the physical interfaces.
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So now Rodda OSPF one network and I'll just enable all interfaces once again notice a neighbor relationship
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has established.
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We've actually got two.
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But notice the neighbor ID is 1 2 1 6 8 1 to 2 ways on this side the neighbor I.D. is 10 that one to
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two to one the highest IP address of a physical interface.
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This loopback doesn't exist on route one so on router to show IP who is P.F. neighbor hibernation ship
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is full and established tendered one to two to one right one show IP OSPF neighbor notice a difference.
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Neighbor ID is 1 and 2 1 6 8 1 2 IP address on the interface that the neighbor is using it is tendered
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one or two to two.
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And on this interface gigabit 00 extended one or two.
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So the same neighbor ID the loopback.
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And just to make sure that its not confusing at that here as well weve got the loopback configured on
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rodded too.
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In addition to to the to the to the ATI brought it to now when we look at the neighbor relationship
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with one neighbor ideas tend toward one or two to one that's this interface.
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Gigabit is one the IP addresses and the physical interfaces are those on road a one.
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So why is the wrong idea important.
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Well when an interface goes down and the road is reputed the Rada ID will change and in certain cases
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in a whisper if we specify the Rodda ID in commands one of those cases which you don't need to know
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if DNA is a virtual lynx in this command.
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We are specifying the router ID of the remote router a virtual link allows us to create a virtual transit
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across an area.
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In this example of backbone areas on the left area one is in the middle an area two is on the right
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area.
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Two is separated from the backbone area by area one but a virtual link allows us to tunnel areas zero
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across area one and full one of the requirements of always piaffe which states that all areas have to
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be adjacent to areas zero.
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Now the problem is you know typology at the moment and wrote a one show IP OSPF interface.
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Notice the right of ID is tendered one to two to one.
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So that's the route ID of the router and you can see that on both interfaces what I'm going to do however
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is shut.
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The zero 0 1 down so currently show IP OSPF interface shows that this is the route ID if the router
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rebooted the Rodda ID would change instead of rebooting the router.
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I'm going to type clear IP CPF process and I'm going to clear all processes
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so show IP OSPF interface.
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In this case the right ID didn't change.
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So what I'll do is save the configuration and reboot the Rodda.
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So why is that a problem.
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If the interface that you're using was selected by the Rodda at the Rodda ID and you configured that
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draw to ID and this command and the interface went down and the rotor rebooted this command would no
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longer work because the Rodda Id had changed.
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Noticed Cisco Telli the router Id usually the highest IP address on the box or the highest loopback
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address if one exists.
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The run ID is only calculated at boot time at any time when the process is restarted.
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So what's the recommendation.
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For OSPF specify a loopback address and then manually configure the Rodda ID.
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So the Rondo's reboot show IP OSPF interface.
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Notice the router ID has changed.
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It's now 10.0 wondered wondered what that means if you're using a virtual link as an example the virtual
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link would break and your network would break.
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OK that concludes this blog entry.
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Thanks Pedro for the question.
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Please feel free to send me questions and I'll do my best to answer all of them.
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I'll see you tomorrow.
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All the very best.
8800
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