Cisco CCNA Exam Tutorial And Case Study VLANs and IP Connectivity
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Cisco CCNA Exam Guide: Understanding VLANs and IP Connectivity
Summary:
Enhance your grasp of VLAN and switching theory through this practical case study by Chris Bryant, CCIE
12933.
---
In this CCNA case study, we'll apply basic switching and trunking concepts using two routers (R2 and R3) alongside two switches (SW1 and SW2). R2 connects to SW1 on interface Fast 0/2, while R3 connects to SW2 on interface Fast 0/3. Both routers are part of the 172.12.23.0/24 network.
For successful communication, the switches need to connect properly. We have two Cisco 2950 switches linked by crossover cables. Let's first ensure the trunk link between the switches is active using the `show interface trunk` command.
Checking Trunk Status
```plaintext
SW2
show interface trunk
Port Mode Encapsulation Status Native VLAN
Fa0/11 desirable 802.1q trunking 1
Fa0/12 desirable 802.1q trunking 1
```
These switches utilize dynamic desirable trunking mode by default, so no additional configuration was needed for the trunk setup.
VLAN Configuration Overview
The `show vlan brief` command reveals that, by default, all switch ports are in VLAN 1 (except trunk ports).
R2 and R3 already have their Ethernet addresses configured, and with the operational trunk line, both ports reside in VLAN 1. We'll now test IP connectivity by pinging R2’s Ethernet interface from R3 and vice versa.
Conducting Ping Tests
From R2 to R3:
```plaintext
R2
ping 172.23.23.3
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms
```
From R3 to R2:
```plaintext
R3
ping 172.23.23.2
Sending 5, 100-byte ICMP Echos to 172.23.23.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms
```
Pings work correctly, indicating successful IP connectivity.
Testing Connectivity Across Different VLANs
Cisco CCNA theory teaches that devices in separate VLANs cannot communicate without a Layer 3 device. Let’s test this by switching R2 to VLAN 23. (VTP is already active on these switches.)
Configuring R2 on VLAN 23:
```plaintext
SW1
conf t
SW1(config)int fast 0/2
SW1(config-if)switchport mode access
SW1(config-if)switchport access vlan 23
SW1(config-if)^Z
```Now, test the communication between R2 and R3:
Ping Tests After VLAN Change:
- R2 to R3:
```plaintext
R2
ping 172.23.23.3
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
```
- R3 to R2:
```plaintext
R3
ping 172.23.23.2
Sending 5, 100-byte ICMP Echos to 172.23.23.2, timeout is 2 seconds:
.....
```
As expected, they cannot communicate because they are in different VLANs.
Restoring Connectivity
To regain communication, place R3’s port into VLAN 23.
Configuring R3 on VLAN 23:
```plaintext
SW2
conf t
SW2(config)interface fast0/3
SW2(config-if)switchport mode access
SW2(config-if)switchport access vlan 23
```Retesting Connectivity:
- R3 to R2:
```plaintext
R3
ping 172.23.23.2
!!!!!```
- R2 to R3:
```plaintext
R2
ping 172.23.23.3
!!!!!```
With both R2 and R3 in the same VLAN, connectivity is restored. This exercise confirms the need for a Layer 3 device for inter-VLAN communication. Stay tuned to learn about configuring router-on-a-stick in our next tutorial!
You can find the original non-AI version of this article here: Cisco CCNA Exam Tutorial And Case Study VLANs and IP Connectivity.
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