Welcome to the vlog.
In this video, we're going to look at why specific routes were chosen rather than others in a network
running Cisco routers.
So in this topology, I have four routers running within gns3. Let's have a look at router 1
and use the command show IP route to view the routing table. So various routes have been learnt.
the one I want to point out is this one. In the routing table of router 1
this network has been added by EIGRP.
D indicates EIGRP as the routing protocol that added to the route.
So we've got multiple routes but this route is the one of interest, so we should be able to ping
4.4.4.4? which we can.
That's the loopback of router 4.
To get to that network, we go via
10.1.2.2 and then 10.142, in other words, it's going via this link
and then this link to get to the back of for.
Let's have a look at which routing protocols are running on the router, show IP protocols shows us that
OSPF is enabled.
We are running OSPF on all interfaces on the router.
EIGRP has also enabled on this router
and so is rip.
So router 1 has three routing protocols enabled now typically you won't do that in the real world
but we're doing that here to prove a point.
So let's look at the OSPF database so show IP OSPF database in the OSPF database.
We have a runner ID for router 4.
In other words, that route exists in the OSPF database.
So why is OSPF not chosen for this route?
OSPF is being used for this route but it's not being used for this route.
Why is that? So
to help troubleshoot we can use the following commands
show IP OSPF interface brief.
OSPF is enabled on both interfaces of router 1, show IP OSPF neighbor, but it only has a neighbor
relationship to router 2. Show IP EIGRP neighbor, EIGRP only has a neighbor relationship to router 3.
Now, in this topology, I didn't enable EIGRP on router 2, router 2 only has OSPF enabled. On router
3 in this topology, we only have EIGRP and rip enabled but on router 2 it is learning about
the network 4.4.4.4 through OSPF from router 4 so router 2 is able to ping that loop
back.
But on router 1, the network is not shown in the routing table and that's because of administrative
distance.
EIGRP has an administrative distance of 90.
OSPF has an administrative distance of 110.
In other words, if a router such as router 1 learns about a network such as 4.4.4.
4 from both an OSPF router and an EIGRP router. The EIGRP router is deemed to be more reliable.
It's more believable.
The lower the administrative distance, the more believable a route.
In this example, OSPF has a higher admin distance when compared to EIGRP.
So to get to this network, EIGRP is believed because it has a lower administrative distance when
compared to OSPF, so for both the loopbacks of router 3 and router 4 EIGRP is used but OSPF is used for
the loopback of router 2
So let's concentrate on this network, EIGRP is used at the moment.
So on router 3
this is the configuration of EIGRP if I shut it down the link on router 3, so I'm gonna shut
down this port.
That means Rawda three will no longer advertise network photophobia at to write a one.
What happens now and what you'll notice is the entry is now replaced with an upfront.
and if we trace to quadruple four it's going via router 2.
If I no shut that a link on router 3 the links should come up and EIGRP should advertise
the route once again on router 1 the route is replaced with an EIGRP entry rather than an OSPF
entry because EIGRP has a lower admin distance than OSPF does.
So EIGRP is chosen rather than OSPF.
So just to prove the point again I'll shut the interface this is the interface on router 3.
Before I did that an EIGRP route was used, but now an OSPF route is used tracing to that loopback, it's
now going via OSPF.
No shut the link.
Do a trace again, still hasn't switched over yet.
Let's try again.
Notice it went from that to this, so a different path was taken.
So because both EIGRP and OSPF are advertising the same network, there's a conflict and only
one entry can be put into the routing table because EIGRP has a lower administrative distance than OSPF.
EIGRP by default has an admin distance of 90 for internal EIGRP routes,
OSPF has an admin distance of 110 and because of that EIGRP was chosen.
I'll cover other scenarios based on the topology and separate videos.
I hope you enjoyed the video.
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I want to wish you all the very best.