Lesson 32: Introduction to Routing Fundamentals

Layer 3 Routers are the devices that connect separate, distinct networks (subnets) and determine the best path for packets to reach a destination.

Key Router Functions

1. Forwarding Decisions: Based on the destination IP address, the router looks up the best exit interface in its Routing Table.
2. Path Determination: Routers use algorithms (static or dynamic) to learn about remote networks.
3. Broadcast/Collision Domain Isolation: Each interface on a router represents a separate broadcast domain and collision domain.
4. Packet Lifetime Control: Routers decrement the Time To Live (TTL) field in the IP header. If TTL reaches zero, the packet is dropped, preventing endless loops.

The Routing Table

The brain of the router. It contains the following essential information:

• Destination Network Address/Prefix: The network being learned.
• Next-Hop IP Address: The IP address of the next router in the path.
• Exit Interface: The local router interface used to forward the packet.
• Metric: The "cost" associated with the route (used for choosing the best path).