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This is one of multiple videos discussing multi area OSPF.

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This is both a troubleshooting as well as an explanation of video.

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We've been told in this topology that router one on the left is not able to ping router six on the right,

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router one is running IGP.

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So the command show IP protocols.

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Shows us that the only routing protocol enabled on this rudder is air GOP.

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En route of six show IP protocols shows us that OSPF is enabled on router six.

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We can also see that the router has a loopback address of 6.6.6.6, so quadruple six can write a one

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ping that address and the answer is no pings or failing.

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All disable IP domain, look up to speed things up and let's do a trace to that loopback after the moment

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of the trace is going nowhere.

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I'll do a debug IP packet to see what happens and I'll ping with only one ping to that address.

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So ping 6262626.

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Repeat one.

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And let's do that again.

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And I'll turn off debugging.

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Notice here.

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Source 1.1.1.1.

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Pinging 6262626.

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We're told that it's unreadable.

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These are the messages are air copy messages.

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The ping is unsuccessful.

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So show IP rot shows us that that network doesn't exist in the routing table.

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If we try and ping ten .1.5.1, that also fails and so does 10.15 to 2.

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So we can't ping this network or the loopback of router six.

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What about the loopback of router five?

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So we able to ping the loopback or out of five, but not the loopback of router six.

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And if we trace to ten .1.5.1, the trace fails immediately because the route doesn't exist in the routing

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table.

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Ten .1.4.0 does exist in the routing table, so we can ping 10.1 to 4 to one as well as ten .1.422.

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In other words, we can get to this network.

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Now again, we are running choppy on this router.

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So these routes have been redistributed from OSPF into IGP.

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Let's look at rather two and see if rather two can ping the loopback of router six.

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It's not able to do that, so show IP route.

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We are not seeing that loopback in the routing table.

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So let's have a look at out of five.

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Ken Rotter five paying the loot back of Rider six.

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The answer is it can so show IP route.

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It's able to ping the loopback or write a six.

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The loopback exists as an OSPF rat in the routing table of right of five.

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What about router four?

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Paying loopback over out of six that ping fails show IP route that two loopback doesn't exist in the

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routing table.

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So let's have a look at router of five by using the command show IP protocols and tell me if you can

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see the problem by looking at this output.

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What do you think's the problem when you look at this output?

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And to give you a hint, I'll do the same command on router four to show IP protocols on router four.

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There's the output for router four.

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He has the output four out of five.

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What's the problem with this configuration?

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Okay.

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So notice the following.

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It's routing for areas one and two.

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The strata has interfaces in multiple areas but doesn't have area zero configured.

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So show run section OSPF.

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Shows us that the router is configured for Area one and Area two, but is not configured for Area zero.

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Waterfall is configured for Area one.

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It's a requirement in OSPF that area zero be connected to other areas.

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In other words, this is configured as area one.

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This is configured as area two, but area two is not adjacent or bordering area zero.

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That's a problem.

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If we look at router three over here, show IP protocols, we can see that this router is configured

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as an area border router.

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It has interfaces in area zero and interfaces in area one.

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So show IP OSPF interface brief shows us that gigabit zero zero is configured in area zero gigabit zero

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one is configured in area one.

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So that looks good.

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But rather five isn't configured in that way.

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So show IP OSPF interface brief notice.

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It has interfaces in area one and area two, but not in area zero.

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That's why rather one can ping the loopback of router four and the loopback of router five, but not

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the loopback of router six.

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Because.

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OSPF is correctly configured with Area zero.

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And Area one over here.

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It works.

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But here we have a problem.

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We need to create what's called a virtual link.

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Between rather three and rather five.

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So we have to essentially pull or extend area zero from router three to right of five to make root of

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five and area border router.

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So we will be essentially creating a tunnel where Area Zero traverses across area one to router five.

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Area zero is going to be tunneled across area one.

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So that root of five becomes an area border rider, which means that Area two is adjacent to area zero.

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That is required in OSPF.

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All areas have to be adjacent to area zero when you have multiple OSPF areas.

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So show run section OSPF.

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We can see the configuration of router three.

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Now when you create your virtual links, you have to configure the other side using the router ID,

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not the IP address of the router but the router ID.

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So the command show ip ospf interface will show us the local routers router id.

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So the rata ID is 3 to 3 to 3 to 3.

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We can also use the command show ip ospf and we'll be able to see that router id.

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So notice router ids 3.32323.

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En route of five show IP OSPF.

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The right ID is 5.5 to 5.5.

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So back on route a three show run section OSPF.

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That's the configuration of OSPF on router three.

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And that's the configuration of rather of five.

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So.

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Rather three.

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Rather.

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OSPF one.

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Area one.

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This is the area that we are traversing.

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The Virtual link and the neighbors router ID is 5.5 to 5.5.

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Now there are options where you can set authentication and other options.

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I'm not going to do that.

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I'm simply going to create the virtual link.

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So Radha ospf one.

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Area.

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One virtual link and the right ID that we'll connect to on this side is 3.3 to 3.3.

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So show ip ospf neighbor.

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You can see that a neighbor relationship was established.

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Using Virtual Link zero.

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We can see that over here.

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No election takes place on a virtual link.

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Show ip ospf neighbor.

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On this side we can see something similar.

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A neighbor relationship has been formed from router three directly to router five through the OSPF virtual

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link.

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So logically it's as if these two routers rather three and router five are connected to one another.

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So Ken wrote a three ping the loopback of router six.

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Notice the router is in the routing table, so that looks better.

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And notice router three can ping the loopback of router six.

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And if we go back to router one, right, a one can now ping the loopback of router six and when we

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do a trace notice it goes to router two.

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Rather three out of four.

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We're out of five and rather six.

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This tunnel is not like a jerry tunnel.

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It's an OSPF mechanism.

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So traffic still goes via the physical links and you see every hop in the network which you wouldn't

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see if you were using Jerry.

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The virtual link is simply used for OSPF link advertisements.

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So in other words, to advertise routes.

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So we've now solved the problem.

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We've been able to configure the network to allow right of one to ping router six.

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Let's have a look at some more information on router three.

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So show IP route.

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We are learning about that network as an inter area network.

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It's coming from a separate area.

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So IP OSPF database.

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For Area zero notice we have rather two, rather three and root of five in area zero.

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Rather five is shown his DNA.

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The neighbor relationship will not age.

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So do not age this neighbor relationship because it's set through a virtual link.

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So we see rather link states or LSA type ones, net link states or LSA type tos.

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We see summary losses or LSA type threes.

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We see something similar for Area one and we see external routes in the OSPF database.

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So that was an example of how to configure OSPF virtual links to solve a problem where you've got an

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area separated by another area which is not area zero.

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In OSPF, all areas have to touch the backbone area.

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In other words, area zero.

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So here we are extending area zero across area one to meet the OSPF design criteria.

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I hope you enjoyed this video.

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If you did, please like it and please subscribe to my YouTube channel.

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I wish you all the very best.