CCNA Exploration: LAN Switching
Case Study: Inter-VLAN Routing
Objective:
Consolidate the VLAN and Router-on-a-Stick concept and configurations
Intro:
ABC Enterprise, a small three departmental Asset Management firm in San Francisco wants to
optimize their network and asked you to lead the project.
The Scenario:
ABC increased the number of computers on their network and because of that, they have decided to
ensure their network will support it with no impact on the performance. ABC also has no plans to
buy new equipment now; therefore they want to use the gear they already have: one Cisco 2811
router and one Cisco 2960 switch. ABC desires 100% data communication between employees in
the three departments (Management, Marketing, and Finance) when necessary.
Your job is to design and implement the best converged network topology solution for ABC’s
network needs and requirements. After a study on ABC network devices, you decide to implement
the topology described below.
Topology:
198.168.10.0 /24
198.168.30.0 /24
198.168.20.0 /24
CCNA Exploration: LAN Switching
Inter-VLAN Routing Case Study
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The topology will use 3 different VLANs in the switch to separate traffic:
VLAN 10: Management
VLAN 20: Marketing
VLAN 30: Finance
R1: Cisco 2811 Router to route between VLANs.
SW1: Cisco 2960 Switch to Configure VLANs
Step 1 – Creating a Solution
The Solution: Router-on-a-Stick
The Cisco 2960 switch ABC already has is a layer 2 switch. Since VLANs are in different IP
subnets, this switch will not be able to route layer 3 packets between them. From the Inter-VLAN
lesson description, in order to route layer 3 packets, a layer 3 network device must be used. The
Cisco 2811 router will be used to accomplish this and which also supports the 802.1Q trunk
protocol.
The idea is to configure R1’s fastEthernet0/0 interface to speak 802.1Q trunk protocol. This will
create an 802.1Q trunk link between the switch (SW1) and the router (R1). From this link, all traffic
from all VLANs will flow. In order to separate VLAN traffic into R1, sub-interfaces must be
created in R1. Once each VLAN has its own sub-interface, R1 will see each VLAN as a regular
interface, place its network into its routing table as a direct connected route and will be able to route
between them as usual.
When an ABC employee computer needs to communicate to other user device within the same
VLAN (department), the switch will forward the data with no R1’s help. When devices under
different VLANs must communicate (VLAN 10 sending packets to VLAN 30, for example) the
switch will use the trunk link to send the frame to R1. R1 will receive the packets via its subinterface fastEthernet0/0.10 (sub-interface which represents VLAN 10) and, after check its routing
table, will realize that to reach the destination address, it must forward the packet via sub-interface
fastEthernet0/0.30. Even though fastEthernet0/0.10 and fastEthernet0/0.30 are part of the same
physical interface (fastEthernet0/0), from R1’s routing stand point, fa0/0.10 and fa0/0.30 are regular
interfaces.
Note: Router-on-a-Stick is only possible if the router supports 802.1Q trunk protocol. Always check
with Cisco’s IOS documentation when working with routers other than Cisco 2811 routers.
Step 2 – Configuring the Switch
You decide to begin the configuration by SW1. You connect the console cable to SW1 console port
and create all 3 VLANs: VLAN10, VLAN20 and VLAN30 and name them respectively. Once the
VLANs are created, you assign the switch ports to the correct VLAN. Since port 24 will be the port
CCNA Exploration: LAN Switching
Inter-VLAN Routing Case Study
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connected to R1, it must be configured as an 802.1Q link. For Cisco 2960 switches, you accomplish
this by trunking the specified port. The VLAN mapping to be used in SW1 is shown below:
VLAN ID
10
20
30
Trunk Link
Port
1, 2, 3, 4, 5, 6, 7, 8
9, 10, 11, 12, 13, 14
15, 16, 17, 18, 19, 20
24
Network ID (Subnet)
198.168.10.0/24
198.168.20.0/24
198.168.30.0/24
Step 3 – Testing the Network
Once SW1 and R1 are configured to perform router-on-a-stick, you check the user PCs and devices
to ensure they all have proper IP configuration (IP address, default gateway, subnet mask, etc) of
the VLAN it belongs. All user devices must use R1’s sub-interface representing its VLAN as
default gateway.
After everything is set, you issue a few pings within the same VLAN and between different VLANs
and all of them should flow successfully.
Helpful Resources
CONFIGURATION OF VLANS:
MY GOOGLE SITE: http://sites.google.com/site/arguetacourses/home
MY BLOG: http://www.efazio.com