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This vlog is a continuation of a previous vlog discussing administrative distance or the believability of

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a routing protocol. In this example, we've got four routers, router 1, router 2, router 3 and router 4.

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Router 2 in this topology is only running OSPF.

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If we can see that by using the show IP protocols command, it has a relationship with both router 1 and

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router 4. Router 3 in this topology

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is running rip and EIGRP. Now in the real world, you typically don't enable a whole bunch of routing 

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protocols on your router.

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we're doing it here to explain administrative distances. So show IP protocol on router 1

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router

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1 is running rip, EIGRP as well as OSPF. Router 4 is doing the same, so on router 1 show IP route

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the route of interest in this topologies quadruple four

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that's a loopback on router 4.

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We can see in the routing table of router 1 that the route appears in the routing table and the protocol

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selected was EIGRP.

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That's because EIGRP has an administrative distance of 90 which we can see here.

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OSPF has an administrative distance of 110.

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The loopback of router 2 has been learnt via OSPF because we are not running EIGRP on router 2 the

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lower the administrative distance, the more believable the routing protocol.

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So this route was selected because EIGRP has a lower admin distance. Show

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IP OSPF database shows us that the route is available in the OSPF database, but EIGRP was selected because it

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has the lowest administrative distance.

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So at the moment, show run section EIGRP shows us that EIGRP is enabled on router 1.

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So if we remove EIGRP from router 1 and then look at the routing table what you'll notice is that

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the route has been added from OSPF.

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The next hop is via 10.1.1.2 if we scroll up previously

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the next hop was 10.1.1.2.2 which is router 3.

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But because EIGRP is no longer running on router 1, the route is now available via router 2 and was

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learnt through OSPF.

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So as an example, if we trace to that route now notice that it goes via router 2 we're still running

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rip on this router.

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So rip is running on router 3, router 1, and router 4 but the rip route is not selected for quadruple

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four because of RIP has a higher administrative distance when compared to OSPF.

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So the loopback of router 3 is advertised via rip to router 1

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and because OSPF is not advertising that route, the rip route is put into the routing table, but because

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OSPF is advertising this route and it has a lower administrative distance

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when compared to rip the OSPF route is used, not the rip route.

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So to confirm that and try and prove it to you, I'm going to remove EIGRP from router 3.

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So router 3

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is running rip, it's not running EIGRP rip is the only routing protocol running on router 3.

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So on router 1, we can see the loopback of router 3 learnt via rip but we don't see the loopback

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of router 4 because OSPF takes precedence.

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So let's remove OSPF, show IP route

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We haven't learnt about the route through rip yet, so let's do some investigation to make sure things are

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working show IP route on router 3

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notice the route has been learnt via rip.

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On router 1 it's now available

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I was a bit impatient, rip takes longer to advertise routes. So now ping

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quadruple 4, trace quadruple 4 it's going via router 3 and that's because the only routing

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protocol that's running on router 1 is rip.

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So the routing table is populated with rip routes, the router won't be able to ping the loopback of router 2 because

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rip is not running on router 2, So it need to enable rip on router 2 if we wanted full connectivity

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in this network

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but for now, I'm not going to do that.

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I want to prove to you that OSPF takes precedence.

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So notice the rip route is in the routing table with an administrative distance of 120

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hop count is 2 it's available via 10.1 .2.2.

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It's been in the routing table for 19 seconds and it's available via gigabit 01.

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Let's enable OSPF network and I'll enable OSPF on all interfaces, show IP OSPF neighbor we have a neighbor

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relationship with router 2 show IP route notice the routing table has been updated

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OSPF takes precedence over rip.

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So the path used to get to quadruple 4 is via router 2 if we enable EIGRP on this router

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traffic will still go via router 2 because we need to enable EIGRP on router 3. So let's do that

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router EIGRP, enable EIGRP in all interfaces.

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neighbour relationship has been established. So show IP EIGRP neighbour, neighbour relationship is available

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show IP route.

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The EIGRP route has now been put into the routing table and if we trace to that loopback, it's going

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via router 3.

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So administrative distances work as follows:

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The lowest number wins, 

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different routing protocols have different administrative distances set by default.

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You can change them.

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RIP is 120, OSPF is a 110, EIGRP internal routes is 90

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but I could, as an example, add a route to the routing table statically at a static route to force

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the traffic to go via router 2.

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Now when we look at the routing table, notice the static route has replaced EIGRP because the static

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route to a next-hop IP address has an admin distance of 1.

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There's nothing more believable than you, as in the router believes you more than it believes a algorithm

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such as EIGRP or OSPF

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and that's because you know what you're doing and the router believes you more than it believes are

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the routers.

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So once again

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trace is via router 2, show run pipe include route we've got the static route forcing the traffic 

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via

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router 2.

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I'll remove the static route, show IP route EIGRP is the most believable now.

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Traffic is going via router 3, remove EIGRP

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OSPF route takes precedence now traffic is going via

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router 2. So that is a demonstration of administrative distance or the believability factor of a route.

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for the CCNA exam, you need to know admin distances. in a separate video

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I'll show you an example with BGP.

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I hope you've enjoyed the video.

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Please like it and please subscribe to my YouTube channel.

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I wish you all the very best.