Routing Protocols: OSPF and BGP

What You'll Cover Today

Day 3 of CCNA Prep in 5 Days is the midpoint — and often the most rewarding day. The pieces from Day 1 and Day 2 start connecting. Most students have an 'it clicks' moment on Day 3.

DHCP relay

Understanding DHCP relay is the core goal of Day 3. The concept is straightforward once you see it in practice — most confusion comes from skipping the mental model and jumping straight to implementation. Start with the model, then write the code.

# DHCP relay — Working Example
# Study this pattern carefully before writing your own version

class DHCPrelayExample:
    """
    Demonstrates core DHCP relay concepts.
    Replace placeholder values with your real implementation.
    """
    
    def __init__(self, config: dict):
        self.config = config
        self._validate()
    
    def _validate(self):
        required = ['name', 'type']
        for field in required:
            if field not in self.config:
                raise ValueError(f"Missing required field: {field}")
    
    def process(self) -> dict:
        # Core logic goes here
        result = {
            'status': 'success',
            'topic': 'DHCP relay',
            'data': self.config
        }
        return result


# Usage
example = DHCPrelayExample({
    'name': 'my-implementation',
    'type': 'dhcp relay'
})
output = example.process()
print(output)

NAT/PAT

NAT/PAT is the practical application of DHCP relay in real projects. Once you understand the underlying model, NAT/PAT becomes the natural next step.

SNMP

SNMP rounds out today's lesson. It connects DHCP relay and NAT/PAT into a complete picture. You'll use all three concepts together in the exercise below.

Common Mistakes on Day 3

• Skipping the fundamentals — DHCP relay requires understanding the underlying model before you can apply it correctly. Read the section twice if needed.
• Ignoring error messages — error messages for NAT/PAT are usually precise. Read them carefully before searching online.
• Hard-coding values — anything that might change between environments belongs in configuration, not in your source code.
• Not testing edge cases — the happy path is not enough. What happens with empty input? With unexpected types? With network failures?