Lab #7

OSPF in a multi-area network

The purpose of this lab is to show how routing in complex topologies can be managed by decomposing them in areas.
First of all, we solve the network addressing problem, by subnetting a block of 128 IPv4 addresses in a suitable number of subnets.
Then, we create the network topology in GNS3 and configure the routers.
Finally, we analyze the routes established by OSPF.

Subnetting

The network topology we want to create in GNS3 is depicted in the following picture.

lab7-topology-sketch

lab7-topology-sketch

The network consists of 7 routers and 5 end systems. Routers are connected by means of 9 point-to-point links.
For the sake of OSPF routing, we partition the routers in three areas.
Area 0 consists of routers R4, R5 and R6.
Area 1 consists of routers R0 and R1.
Area 2 consists of routers R2 and R3.
As the picture above shows, network addressing requires 14 subnets.
We assume that a block of 128 contiguous IPv4 addresses is available: 192.168.24.0/25.
We perform subnetting with variable length mask with:

netmask /30 for 9 point to point links between routers;
netmask /28 for 5 links from routers to endpoints.

The following table shows how the available address range can be split into the required 14 subnets.
lab7-subnets

lab7-subnets

Notice that, after subnetting, a few small address ranges remain unused.

Preparation steps

Before this experiment is executed, it is required that you have previously configured GNS3 to make it able to instantiate a Docker container of a Linux-based router based on the FRR (Free Range Routing) software suite, as described in Lab #2.

Experiment steps

Recreate the following topology in GNS3. Choose the "GNS3 VM" server to instantiate all the devices of this lab.

lab7-topology

When the devices are still inactive, for each of the routers (R0, R1, R2, R3, R4, R5 and R6), right-click on the router icon and select the Configure option.
Then, click the "Advanced" tab and add /etc/frr to the Additional directories to make persistent.
Start all devices.
Open PC1 terminal and execute the command:
```
ip 192.168.24.66/28 192.168.24.65
save
```
Open PC2 terminal and execute the command:
```
ip 192.168.24.82/28 192.168.24.681
save
```
Open PC3 terminal and execute the command:
```
ip 192.168.24.98/28 192.168.24.97
save
```
Open PC4 terminal and execute the command:
```
ip 192.168.24.114/28 192.168.24.113
save
```
Open PC5 terminal and execute the command:
```
ip 192.168.24.50/28 192.168.24.49
save
```

Open R0 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R0
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.1/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.65/28
  ip ospf passive
!
interface eth1
  ip address 192.168.24.25/30
!
interface eth2
  ip address 192.168.24.18/30
!
router ospf
  ospf router-id 1.1.1.1
  network 192.168.24.16/30 area 1
  network 192.168.24.24/30 area 1
  network 192.168.24.64/28 area 1
!
line vty
!

Finally, stop and restart the R0 container to start the ospfd daemon.

Open R1 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R1
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.2/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.81/28
  ip ospf passive
!
interface eth1
  ip address 192.168.24.26/30
!
interface eth2
  ip address 192.168.24.22/30
!
router ospf
  ospf router-id 1.1.1.2
  network 192.168.24.20/30 area 1
  network 192.168.24.24/30 area 1
  network 192.168.24.80/28 area 1
!
line vty
!

Finally, stop and restart the R1 container to start the ospfd daemon.

Open R2 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R2
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.3/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.97/28
  ip ospf passive
!
interface eth1
  ip address 192.168.24.41/30
!
interface eth2
  ip address 192.168.24.34/30
!
router ospf
  ospf router-id 1.1.1.3
  network 192.168.24.32/30 area 2
  network 192.168.24.40/30 area 2
  network 192.168.24.96/28 area 2
!
line vty
!

Finally, stop and restart the R2 container to start the ospfd daemon.

Open R3 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R3
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.4/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.113/28
  ip ospf passive
!
interface eth1
  ip address 192.168.24.42/30
!
interface eth2
  ip address 192.168.24.38/30
!
router ospf
  ospf router-id 1.1.1.4
  network 192.168.24.36/30 area 2
  network 192.168.24.40/30 area 2
  network 192.168.24.112/28 area 2
!
line vty
!

Finally, stop and restart the R3 container to start the ospfd daemon.

Open R4 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R4
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.5/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.17/30
!
interface eth1
  ip address 192.168.24.21/30
!
interface eth2
  ip address 192.168.24.9/30
!
interface eth3
  ip address 192.168.24.2/30
!
router ospf
  ospf router-id 1.1.1.5
  network 192.168.24.0/30 area 0
  network 192.168.24.8/30 area 0
  network 192.168.24.16/30 area 1
  network 192.168.24.20/30 area 1
!
line vty
!

Finally, stop and restart the R4 container to start the ospfd daemon.

Open R5 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R5
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.6/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.33/30
!
interface eth1
  ip address 192.168.24.37/30
!
interface eth2
  ip address 192.168.24.10/30
!
interface eth3
  ip address 192.168.24.6/30
!
router ospf
  ospf router-id 1.1.1.6
  network 192.168.24.4/30 area 0
  network 192.168.24.8/30 area 0
  network 192.168.24.32/30 area 2
  network 192.168.24.36/30 area 2
!
line vty
!

Finally, stop and restart the R5 container to start the ospfd daemon.

Open R6 auxiliary console. At the root command prompt:

sed -i '/^ospfd/s/no/yes/' /etc/frr/daemons
echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf
chown frr:frr /etc/frr/vtysh.conf
touch /etc/frr/frr.conf
chown frr:frr /etc/frr/frr.conf
vi /etc/frr/frr.conf

Use the vi editor to edit the /etc/frr/frr.conf file as follows.

frr version 8.1_git
frr defaults traditional
hostname R6
no ipv6 forwarding
service integrated-vtysh-config
!
interface lo
 ip address 1.1.1.7/32
 ip ospf passive
!
interface eth0
  ip address 192.168.24.1/30
!
interface eth1
  ip address 192.168.24.5/30
!
interface eth2
  ip address 192.168.24.49/28
  ip ospf passive
!
router ospf
  ospf router-id 1.1.1.7
  network 192.168.24.0/30 area 0
  network 192.168.24.4/30 area 0
  network 192.168.24.48/28 area 0
!
line vty
!

Finally, stop and restart the R5 container to start the ospfd daemon.

Traffic generation and results analysis

In PC1 terminal execute the following commands:
```
ping 192.168.24.82 -c 2
```
and verify that answers are received from PC2.
```
ping 192.168.24.98 -c 2
```
and verify that answers are received from PC3.
```
ping 192.168.24.114 -c 2
```
and verify that answers are received from PC4.
```
ping 192.168.24.50 -c 2
```
and verify that answers are received from PC5.

PC1-pings

In PC1 terminal execute the following commands:
```
trace 192.168.24.82 -P 1
```
and verify that packets to PC2 are routed through R0 and R1 (shortest path).
```
trace 192.168.24.98 -P 1
```
and verify that packets to PC3 are routed through R0, R4, R5 and R2 (shortest path).
```
trace 192.168.24.114 -P 1
```
and verify that packets to PC4 are routed through R0, R4, R5 and R3 (shortest path).
```
trace 192.168.24.50 -P 1
```
and verify that packets to PC5 are routed through R0, R4 and R6 (shortest path).

PC1-traces

Right-click on the link connecting R0 and R1 and suspend the link.
Wait about 30 seconds until OSPF converges to an alternative route.
In PC1 terminal execute the following command:
```
trace 192.168.24.82 -P 1
```
and verify that packets are now routed through R0, R4 and R1.

PC1-traces-alt2

Right-click on the link connecting R4 and R5 and suspend the link.
Wait about 30 seconds until OSPF converges to an alternative route.
In PC1 terminal execute the following command:
```
trace 192.168.24.98 -P 1
```
and verify that packets are now routed through R0, R4, R6, R5 and R2.

PC1-traces-alt3

Right-click on the link connecting R3 and R5 and suspend the link.
Wait about 30 seconds until OSPF converges to an alternative route.
In PC1 terminal execute the following command:
```
trace 192.168.24.114 -P 1
```
and verify that packets are now routed through R0, R4, R6, R5, R2 and R3.

PC1-traces-alt4

Right-click on the suspended links and resume them.
Wait about 30 seconds until OSPF converges again.

In R0 auxiliary console execute the following commands:

vtysh
R0# show ip route

The output should be as follows.

C>* 1.1.1.1/32 is directly connected, lo, 01:51:11
O>* 192.168.24.0/30 [110/20000] via 192.168.24.17, eth2, weight 1, 00:56:56
O>* 192.168.24.4/30 [110/30000] via 192.168.24.17, eth2, weight 1, 00:42:02
O>* 192.168.24.8/30 [110/20000] via 192.168.24.17, eth2, weight 1, 00:03:25
O   192.168.24.16/30 [110/10000] is directly connected, eth2, weight 1, 01:51:11
C>* 192.168.24.16/30 is directly connected, eth2, 01:51:12
O>* 192.168.24.20/30 [110/20000] via 192.168.24.17, eth2, weight 1, 00:03:30
  *                              via 192.168.24.26, eth1, weight 1, 00:03:30
O   192.168.24.24/30 [110/10000] is directly connected, eth1, weight 1, 00:03:30
C>* 192.168.24.24/30 is directly connected, eth1, 01:51:12
O>* 192.168.24.32/30 [110/30000] via 192.168.24.17, eth2, weight 1, 00:03:32
O>* 192.168.24.36/30 [110/30000] via 192.168.24.17, eth2, weight 1, 00:03:32
O>* 192.168.24.40/30 [110/40000] via 192.168.24.17, eth2, weight 1, 00:03:32
O>* 192.168.24.48/30 [110/30000] via 192.168.24.17, eth2, weight 1, 00:34:17
O   192.168.24.64/28 [110/10000] is directly connected, eth0, weight 1, 01:51:11
C>* 192.168.24.64/28 is directly connected, eth0, 01:51:12
O>* 192.168.24.80/28 [110/20000] via 192.168.24.26, eth1, weight 1, 00:03:30
O>* 192.168.24.96/28 [110/40000] via 192.168.24.17, eth2, weight 1, 00:03:32
O>* 192.168.24.112/28 [110/40000] via 192.168.24.17, eth2, weight 1, 00:03:18

In R0's routing table there is an entry for each of the 14 subnets.
Three subnets are directly connected to R0 interfaces.
All the others can be reached either via R1 (192.168.24.26) or R4 (192.168.24.17).
In particular, subnet 192.168.24.20/30 can be reached with the same cost via either one of R1 or R4.

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Copyright (c) 2024 - Roberto Canonico

Last updated: October 3, 2024 by Roberto Canonico