This vlog is a continuation of a previous vlog discussing administrative distance or the believability of
a routing protocol. In this example, we've got four routers, router 1, router 2, router 3 and router 4.
Router 2 in this topology is only running OSPF.
If we can see that by using the show IP protocols command, it has a relationship with both router 1 and
router 4. Router 3 in this topology
is running rip and EIGRP. Now in the real world, you typically don't enable a whole bunch of routing
protocols on your router.
we're doing it here to explain administrative distances. So show IP protocol on router 1
router
1 is running rip, EIGRP as well as OSPF. Router 4 is doing the same, so on router 1 show IP route
the route of interest in this topologies quadruple four
that's a loopback on router 4.
We can see in the routing table of router 1 that the route appears in the routing table and the protocol
selected was EIGRP.
That's because EIGRP has an administrative distance of 90 which we can see here.
OSPF has an administrative distance of 110.
The loopback of router 2 has been learnt via OSPF because we are not running EIGRP on router 2 the
lower the administrative distance, the more believable the routing protocol.
So this route was selected because EIGRP has a lower admin distance. Show
IP OSPF database shows us that the route is available in the OSPF database, but EIGRP was selected because it
has the lowest administrative distance.
So at the moment, show run section EIGRP shows us that EIGRP is enabled on router 1.
So if we remove EIGRP from router 1 and then look at the routing table what you'll notice is that
the route has been added from OSPF.
The next hop is via 10.1.1.2 if we scroll up previously
the next hop was 10.1.1.2.2 which is router 3.
But because EIGRP is no longer running on router 1, the route is now available via router 2 and was
learnt through OSPF.
So as an example, if we trace to that route now notice that it goes via router 2 we're still running
rip on this router.
So rip is running on router 3, router 1, and router 4 but the rip route is not selected for quadruple
four because of RIP has a higher administrative distance when compared to OSPF.
So the loopback of router 3 is advertised via rip to router 1
and because OSPF is not advertising that route, the rip route is put into the routing table, but because
OSPF is advertising this route and it has a lower administrative distance
when compared to rip the OSPF route is used, not the rip route.
So to confirm that and try and prove it to you, I'm going to remove EIGRP from router 3.
So router 3
is running rip, it's not running EIGRP rip is the only routing protocol running on router 3.
So on router 1, we can see the loopback of router 3 learnt via rip but we don't see the loopback
of router 4 because OSPF takes precedence.
So let's remove OSPF, show IP route
We haven't learnt about the route through rip yet, so let's do some investigation to make sure things are
working show IP route on router 3
notice the route has been learnt via rip.
On router 1 it's now available
I was a bit impatient, rip takes longer to advertise routes. So now ping
quadruple 4, trace quadruple 4 it's going via router 3 and that's because the only routing
protocol that's running on router 1 is rip.
So the routing table is populated with rip routes, the router won't be able to ping the loopback of router 2 because
rip is not running on router 2, So it need to enable rip on router 2 if we wanted full connectivity
in this network
but for now, I'm not going to do that.
I want to prove to you that OSPF takes precedence.
So notice the rip route is in the routing table with an administrative distance of 120
hop count is 2 it's available via 10.1 .2.2.
It's been in the routing table for 19 seconds and it's available via gigabit 01.
Let's enable OSPF network and I'll enable OSPF on all interfaces, show IP OSPF neighbor we have a neighbor
relationship with router 2 show IP route notice the routing table has been updated
OSPF takes precedence over rip.
So the path used to get to quadruple 4 is via router 2 if we enable EIGRP on this router
traffic will still go via router 2 because we need to enable EIGRP on router 3. So let's do that
router EIGRP, enable EIGRP in all interfaces.
neighbour relationship has been established. So show IP EIGRP neighbour, neighbour relationship is available
show IP route.
The EIGRP route has now been put into the routing table and if we trace to that loopback, it's going
via router 3.
So administrative distances work as follows:
The lowest number wins,
different routing protocols have different administrative distances set by default.
You can change them.
RIP is 120, OSPF is a 110, EIGRP internal routes is 90
but I could, as an example, add a route to the routing table statically at a static route to force
the traffic to go via router 2.
Now when we look at the routing table, notice the static route has replaced EIGRP because the static
route to a next-hop IP address has an admin distance of 1.
There's nothing more believable than you, as in the router believes you more than it believes a algorithm
such as EIGRP or OSPF
and that's because you know what you're doing and the router believes you more than it believes are
the routers.
So once again
trace is via router 2, show run pipe include route we've got the static route forcing the traffic
via
router 2.
I'll remove the static route, show IP route EIGRP is the most believable now.
Traffic is going via router 3, remove EIGRP
OSPF route takes precedence now traffic is going via
router 2. So that is a demonstration of administrative distance or the believability factor of a route.
for the CCNA exam, you need to know admin distances. in a separate video
I'll show you an example with BGP.
I hope you've enjoyed the video.
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I wish you all the very best.