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Determine how router makes a forwarding decision by default

Determine how router makes a forwarding decision by default #router #decisionmaking #route In this video, we break down how routers make forwarding decisions by default. Learn how routers use the routing table, destination IP addresses, and the longest prefix match rule to forward packets efficiently. 📌 Topics Covered: Routing Table Lookup Process Longest Prefix Match Explained Role of Administrative Distance and Metrics What Happens When No Specific Route is Found Default Routes in Action Real-World Examples Using Cisco CLI Perfect for networking beginners, CCNA students, and tech enthusiasts looking to understand core routing concepts. By default, a *router makes forwarding decisions* based on the *destination IP address* in each packet and its own **routing table**. Here's how it works, step-by-step: --- 🔁 *1. Check the Destination IP Address* When a router receives a packet, it examines the *destination IP address* in the packet header. --- 📘 *2. Look Up the Routing Table* The router searches its *routing table* to find the best route to the destination. The routing table is a list of known networks and the best next hop to reach them. --- 📏 *3. Longest Prefix Match* If there are multiple possible routes, the router uses the *longest prefix match* rule — the route with the most specific subnet mask (i.e., the most number of matching bits in the network portion) is chosen. #### Example: Destination IP: `192.168.1.10` Routing table contains: `192.168.0.0/16` via Router A `192.168.1.0/24` via Router B ✅ ← *Chosen* (longer match) --- ⚖️ *4. Choose Based on Metric and Administrative Distance (if tied)* If multiple routes have the same prefix length: *Metric* (cost) is checked — lower is better. *Administrative Distance (AD)* is used if multiple routing protocols provide the same route. Lower AD is preferred. --- 📦 *5. Forward the Packet* The router forwards the packet to the *next hop IP address* or *out the interface* that corresponds to the chosen route. --- 🛣️ *6. Use the Default Route if No Match Found* If there is *no match* in the routing table, the router checks if there is a *default route* (usually `0.0.0.0/0`). If it exists, the packet is sent that way. --- ❌ *7. Drop the Packet if No Route Exists* If there’s no match and no default route, the router **drops the packet**. --- 🔄 Summary Flow: 1. Check destination IP 2. Lookup routing table 3. Apply longest prefix match 4. Use metric & AD if needed 5. Forward to next hop/interface 6. Use default route (if necessary) 7. Drop if no route --- ✅ Don’t forget to Like, Comment, and Subscribe to Network Zeal for more deep dives into networking! #Routing #Networking #CCNA #Router #IPForwarding #NetworkZeal