Core Layer & Routing Configuration

The Core Layer is the high-speed backbone of the campus network. In this topology, the core routers are responsible for three major tasks: Inter-VLAN routing, assigning dynamic IP addresses via DHCP, and routing traffic between the Main Campus and the Branch Campus.

Without these configurations, the campus devices would be completely isolated and unable to communicate.

1. Core Router Interface Design Strategy

When configuring IP addresses on the Core Routers, we must treat connections differently based on what the router is connecting to. In our Smart Campus design, the router handles two distinct types of links:

• WAN Links (Router-to-Router): The red serial cables connect the Main Campus router to the Branch Campus router and the Cloud. Because this is a direct point-to-point connection between exactly two devices, we configure a single IP address directly onto the physical interface using a /30 subnet mask (which provides exactly 2 usable IP addresses, preventing waste).

• LAN Links (Router-to-Distribution Switch): The Gigabit cable connects the router down to the Layer 3 Distribution Switch. The switch sends traffic from 8 different departments (VLANs) across this single trunk link. If we assigned a single IP address directly to the physical interface (e.g., gig0/0), only one department would be able to access the router. Therefore, we leave the main physical interface blank and divide it into multiple Virtual Sub-Interfaces.

2. Configuring Serial WAN Links (Connecting Campuses)

Based on our strategy, we assign the /30 IP addresses directly to the physical serial interfaces. One side of a serial cable acts as the DCE (Data Communications Equipment) and must provide a clocking signal to set the data transmission speed.

The Configuration:

enable

configure terminal

interface serial 0/1/0

ip address 10.10.10.1 255.255.255.252

clock rate 64000

no shutdown

exit

do wr

3. Inter-VLAN Routing (Router-on-a-Stick)

The Problem: By default, VLANs are designed to isolate network traffic. A PC in the Admin VLAN (VLAN 10) cannot talk to a PC in the HR VLAN (VLAN 20), even if they are on the same switch.

The Solution: To allow different departments to communicate, the traffic must go up to a Router. Following our LAN link strategy, we chop that single physical interface (gig0/0) into multiple virtual sub-interfaces (e.g., gig0/0.10). Each sub-interface acts as the Default Gateway for its specific VLAN.

The Configuration:

(Note: Always turn on the main physical interface first before creating sub-interfaces).

interface gig0/0

no shutdown

exit

interface gig0/0.10

encapsulation dot1Q 10

ip address 192.168.1.1 255.255.255.0

exit

do wr

interface gig0/0.20

encapsulation dot1Q 20

ip address 192.168.2.1 255.255.255.0

exit

do wr

4. DHCP Server (Dynamic Host Configuration Protocol)

The Problem: In a Smart Campus, there are thousands of devices: student laptops, administrative PCs, smart cameras, and IoT sensors. Manually typing an IP address into every single device is impossible.

The Solution: The Core Router is configured to act as a DHCP Server. When a device connects to the network, the router automatically leases out an IP address from a configured DHCP Pool, along with the correct Default Gateway and DNS server.

The Configuration:

service dhcp

ip dhcp pool admin_pool

network 192.168.1.0 255.255.255.0

default-router 192.168.1.1

dns-server 192.168.1.1

exit

do wr

5. Dynamic Routing using RIPv2

The Problem: Inter-VLAN routing solves communication inside the Main Campus. However, the Main Campus Router has no idea how to reach the Branch Campus Router, which is 20 miles away.

The Solution: Routers need a map of the network. RIPv2 (Routing Information Protocol version 2) is a dynamic protocol that allows routers to "talk" to each other and advertise all the networks they know about. Version 2 is used because it supports Subnetting (VLSM).

The Configuration:

config terminal

router rip

version 2

network 10.0.0.0

network 192.168.1.0

network 192.168.2.0

exit

do wr

6. Static Routing for External Servers (Cloud)

An external Email/Web server is hosted on the cloud. While RIPv2 handles the internal campus network, a Default Static Route is required to send any unknown traffic out to the internet/cloud.

The Configuration:

ip route 0.0.0.0 0.0.0.0 10.10.10.6

(Note: 0.0.0.0 0.0.0.0 creates a default route, meaning "if a destination IP is not in my routing table, send it to 10.10.10.6".)

7. Verifying Core Configurations

After applying routing and DHCP configurations, use these commands to ensure everything is working correctly before saving.

show ip route

show ip dhcp binding

copy running-config startup-config

• show ip route: Displays the routing table. You should successfully see C (Connected), R (RIP), and S* (Static Default) routes.

• show ip dhcp binding: Shows the list of IP addresses that the DHCP server has successfully assigned to campus devices and IoT sensors.