So router 1 can ping the loopback of router 3
but notice the ping was sent from 10.1.1.1 to router 3.
With ping you can specify the source interface of a ping
so I could say loopback 0 or specify the source IP address.
So let’s specify loopback 0
in other words, pings are being sent from the loopback of router 1 to router 3
but the pings are failing because router 3 is receiving a ping
from an IP address of 1.1.1.1
sh run int loopback0
this router has this IP address configured on loopback0
and we are telling the router to ping the loopback of router 3
using the IP address as configured on loopback 0.
We can see that in the debug on router3
does router 3 have that route in its routing table?
The answer is no, so debug ip packet do the ping again
what we can see is that it’s unroutable.
Router 3 doesn’t know how to get back to 1.1.1.1
so we need to configure the loopback of router 1 on router 3
and this is the problem
how many routes do you need to configure
on this routers to allow for full communication?
A lot of static routes need to be configured
to allow full connectivity between these routers.
So sh run | include route on router 3
shows me that I have 1 static route configured.
So I need to configure another static route 1.1.1.1
and that’s available from router 3 points of view via 10.1.2.1
from router 3 points of view to get to this loopback
it needs to forward traffic via this IP address.
However, the ping is still failing, we can see a message on router 3
saying that it receives a message from 10.1.2.1
stating that this address is unreachable.
So in other words
router 1 is sending traffic from this loopback interface
to this loopback interface
and it gets successfully to router 3.
Router 3 tries to route it back to router 2
but router 2 is saying host unreachable.
In other words, router 2 doesn’t have a route to 1.1.1.1
which as we can see in the routing table isn’t there.
So we have to configure a static route
on router 2 to the loopback of router 1 via 10.1.1.1
does the ping work now? Yes, it does.
so ping succeeds from the loopback of router 1
to the loopback of router 3 because router 2 now knows
about the loopback of router 3 as well as the loopback of router 1.
Are we finished?
show ip route, so router 1 kind of looks right
let’s do a show run, show run | include route
Router 1 has 3 static routes in the routing table.
1 for this network, 1 for this network and 1 for this network
so that looks good. Router 2 show ip route
router 2 only has a static route to the loopback of router 1
and loopback of router 3 which is correct for the network shown in the diagram.
What about router 3? show ip route
it has a static route to the loopback of router 1
it has a static route to 10.1.1.0
but it doesn’t have a static route to the loopback of router 2
I'll just turn off debugging
and then try and ping the loopback of router 2
what you’ll notice is the pings fail.
So what I need to do is configure a route
to the loopback of router 2 via the next hop 10.1.2.1
so router 3 can now ping the loopback of router 2
it can ping the loopback of router 1 that’s good
router 1 can ping the loopback of router 2
and the loopback of router 3 I'll turn debugging off on router 2
it can ping the loopback of router 1 and the loopback of router 3.
now I did add some extra networks
show ip route on router 1
shows me that I added 10.1.10.0 as well as 1.1.1.2
so can router 2 ping 1.1.1.2 and the answer is no
in the routing table of router 2
I only have a static route to 1.1.1.1
so I would need to add another route for 1.1.1.2
so I could type IP route 1.1.1.2/32 mask 10.1.1.1
and now I’ll be able to ping 1.1.1.2
now do I have to use explicit mask as per
the subnets configured on interfaces and the answer is no.
I could as an example remove this static route
and change that to something like this
so in other words, I’m going to match 10.1.1.1.0
and anything in that range is going to be forwarded to 10.1.2.1
so ping 1.1.1.1 that works and 1.1.1.2 also works
you need to be careful however doing that
because this doesn’t just match those 2 networks
it matches anything in the range 1.1.1.0
but in this case based on our topology, I can do that
router 1 also has this additional network that I added
so I need to configure that network as well on router 2
as well as I'll just copy this to make it easier, as well as router 3
and you need to be careful doing copying and pasting like I’ve just done
because that should be 2.1 on this router.
So ping 10.1.10.1 ping works ping 10.1.10.1
which is the loopback to IP address configured on router 1.
So here’s the problem, we configured a very small network
but we configured 3 static routes on router 1
we've configured 4 static routes on router 2
and we've configured 4 static routes on router 3
that’s 11 routes just to get this network working
it would have been told if I hadn’t use a summary route on router 3
that's a lot of work for very small network
and that’s 1 of the problems with static routes, it’s a lot of work
to configure this routes and you the person that has to configure them.
Dynamic routing protocols allow routes to be automatically learned
and adjusted so the work is shifted from your shoulders to the routers.
The routing protocols do the heavy lifting
rather than you having to do all this manual configuration.