CCNA Exploration: Routing Protocols and Concepts Chapter 3 Case Study
Objectives:
•
•
•
•
Describe the function of routing protocols.
Describe the function of metrics.
Illustrate the how hop count is used to chose the best route.
Illustrate the function of Administrative Distance.
Intro:
Yada Inc. has called you again; Even though their network still works (based on the configuration and
setup done by you during your last visit), the complexity of static routing is not worth it anymore and they
need your help during the migration from static to dynamic routing. The major goal is to have full layer 3
connectivity within Yada network.
Topology:
© 2009 Cisco Learning Institute
CCNA Exploration: Routing Protocols and Concepts Chapter 3 Case Study
The Scenario:
You get to Yada Inc. office, check the routers and notice they still have the static routes left by you during
your last visit. Because Yada’s network is operational and cannot be stopped, the migration must be as
smooth as possible. The alternate link idea must also be kept after the dynamic routing implementation is
done.
The Ethernet link between R1 and R4 is used by network 1 to reach the internet. The path over the serial
links (R1, R2, R3, R4 and Internet) must only be used by network 1 to reach Internet if the main link fails.
Problem 1 – The Migration
Due the simplicity of the network and the low resources installed on the routers, you elect RIPv1 as the
routing protocol and because network 1 uses the R1’s fa0/1 link to reach internet, you decided to
configure R1 and R4 first.
You know RIPv1 configuration is pretty straight-forward and all what it takes is to add the router’s direct
connected interfaces which will be part of the RIPv1 process. You document the used commands as
listed below:
On R1:
R1(config)# router rip
R1(config-router)# network 192.168.1.0
R1(config-router)# network 192.168.2.0
R1(config-router)# network 192.168.3.0
R1(config-router)#end
On R4:
R4(config)# router rip
R4(config-router)# network 192.168.2.0
R4(config-router)# network 192.168.4.0
R4(config-router)# network 192.168.6.0
R4(config-router)#end
After issue the commands above on R1 and R4 respectively you double check their routing tables and
notice no changes.
© 2009 Cisco Learning Institute
CCNA Exploration: Routing Protocols and Concepts Chapter 3 Case Study
Question 1:
Is this the expected behavior? Why there are no changes on the routing tables?
Answer: Yes, the behavior is correct and expected. R1’s and R4’s routing tables were populated with
static routes but the very same routes are being exchanged by RIPv1. RIPv1 routes have administrative
distance 120 while static routes have administrative distance 1. Since static routes have lower AD, they
are considered better paths than the ones learned via RIPv1 and thus, kept on the routing table.
Moving forward, you configure RIPv1 on R2 and R3. Once more you document the commands as follows:
On R2:
R2(config)# router rip
R2(config-router)# network 192.168.3.0
R2(config-router)# network 192.168.5.0
R2(config-router)#end
On R3:
R3(config)# router rip
R3(config-router)# network 192.168.5.0
R3(config-router)# network 192.168.6.0
R3(config-router)#end
A quick check on R2 and R3 routing tables reveals no changes once more.
After all 4 routers have been configured to use RIP, it is time remove the static routes previously
configured on R1, R2, R3 (refer to Exploration 2 – Chapter 2 Case Study for more information). The
commands are documented bellow:
On R1:
R1(config)#
R1(config)#
R1(config)#
R1(config)#
R1(config)#
no
no
no
no
no
ip
ip
ip
ip
ip
route
route
route
route
route
192.168.4.0 255.255.255.0 192.168.2.2
192.168.5.0 255.255.255.0 192.168.3.2
192.168.6.0 255.255.255.0 192.168.3.2
0.0.0.0 0.0.0.0 192.168.2.2 50
0.0.0.0 0.0.0.0 192.168.3.2 60
On R2:
R2(config)# no ip route 192.168.1.0 255.255.255.0 192.168.3.1
R2(config)# no ip route 192.168.6.0 255.255.255.0 192.168.5.2
On R3:
R3(config)# no ip route 192.168.1.0 255.255.255.0 192.168.5.1
R3(config)# no ip route 0.0.0.0 0.0.0.0 192.168.6.2
© 2009 Cisco Learning Institute
CCNA Exploration: Routing Protocols and Concepts Chapter 3 Case Study
On R4:
R4(config)# no ip route 192.168.1.0 255.255.255.0 192.168.2.1 50
R4(config)# no ip route 192.168.1.0 255.255.255.0 192.168.6.1 60
You check the routing tables of the internal devices. By communicating via RIPv1, all of them have routes
to each other ensuring fully connectivity. You issue a few pings from PC1 towards PC2 and from PC1 to
the other internal network devices and watch them be successful.
To check whether or not outside devices are reachable, you issue a few pings to random external
addresses but they all fail.
Question 2:
Why are the pings failing?
Answer: Even though all routers are correctly configured to RIPv1 and R4 has a default route to Internet,
such route was never added to R4’s RIPv1 process. Because of this, R4 does not advertise a default
route via RIPv1 to the other Yada’s network devices. No device but R4 has default route to the Internet
configured.
In order to solve the problem, R4’s RIPv1 needs to advertise its static routes to other RIPv1 speaker
routers within Yada’s network. As long as R4 has a default route to the internet and a redistribute
command is properly configured, Cisco’s implementation of RIPv1 will advertise it such route to the other
routers as a default route. You create the static default route on R4 and configure its RIPv1 to advertise it.
The commands are listed below:
R4(config)# ip route 0.0.0.0 0.0.0.0 70.63.129.1
R4(config)# router rip
R4(config-router)# redistribute static
R4(config-router)# end
A quick check on R1’s routing table reveals the new route:
R1#
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 192.168.2.2 to network 0.0.0.0
C
C
192.168.1.0 is directly connected, FastEthernet0/0
192.168.2.0 is directly connected, FastEthernet0/1
© 2009 Cisco Learning Institute
CCNA Exploration: Routing Protocols and Concepts Chapter 3 Case Study
C
192.168.3.0 is directly connected, Serial0/0
R
192.168.4.0 [120/1] via 192.168.2.2, 00:00:01, FastEthernet0/1
R
192.168.5.0 [120/1] via 192.168.3.2, 00:00:01, Serial0/0
R
192.168.6.0 [120/1] via 192.168.2.2, 00:00:01, FastEthernet0/1
R* 0.0.0.0/0 [120/1] via 192.168.2.2, 00:00:14, FastEthernet0/1
R1#
Question 3:
Is R1 the only router within Yada’s network to learn the default route?
Answer: No. Because the other routers are also properly running RIPv1, all the other routers learn the
new default route as well.
The alternate path:
Yada network is now running a routing protocol (it has dynamic routing implemented). Any changes on
the topology will force the protocol to re-adjust routes on the routers’ routing tables. Because new routes
will be advertised and learnt automatically between Yada’s internal routers, the alternate route to Internet
via serial links will be automatically placed in R1’s routing if the main link via R1’s FastEthernet0/1 fails.
Question 4:
Why does RIPv1 consider R1’s fa0/1 link the main link (optimal path) and R1’s serial0/0 link an alternate
path (sub-optimal path) for packets coming from network 1? In other words, what makes R1’s fa0/1 link
better than the R1’s serial0/0, from RIPv1 standpoint?
Answer: RIPv1 uses hop count to calculate the best route. RIPv1 knows that:
1. R4 is the router announcing the path to Internet
2. R1 is only 1 hop away from R4 via fa0/1 link
3. R4 is 3 hops via R1’s serial0/0.
Base on hop count, R1’s fa0/1 is the best path to the Internet and therefore, placed on R1’s routing table.
If R1’s fa0/1 fails, R1’s se0/0 automatically becomes the best exit point and is placed in R1’s routing
table.
As a final test, you manually bring fa0/1 down and watch RIPv1 adjusting R1’s routing table to adapt itself
to the new situation. It now shows serial0/0 as the default route.
© 2009 Cisco Learning Institute