# Day 11: Routing Fundamentals # Routing Fundamentals & Static Routing ## Overview What is Routing? Routing is the process that routers use to determine the path that IP packets should take over a network to reach their destination. Core Concept: - Routers forward packets between different networks (across network boundaries) - Switches forward frames within the same LAN (within a network) - A router reads the destination IP address and determines where to send the packet next Key Difference from Switching: Switches (Layer 2): \- Examine destination MAC address \- Forward within the same LAN \- Fast forwarding to connected devices # Routers (Layer 3): \- Examine destination IP address \- Forward between different networks \- Use routing tables to determine best path # --- ## Routing Tables ### What is a Routing Table? A routing table is a database of routes that tells the router where to send packets destined for specific networks or IP addresses. Stored on: Every router in the network Content: Network destinations and how to reach them ### Route Components Each route entry contains: Destination Network → Next-Hop Router (or directly connected) # Example: Destination: 192.168.2.0/24 → Send to next-hop router: 10.0.0.2 Destination: 10.1.1.0/24 → Send directly out interface G0/1 Destination: 172.16.0.0/16 → Send to next-hop router: 10.0.0.3 # ### Route Types When a router receives a packet, it can take one of three actions: 1\. Forward to Next-Hop Router Route: "To reach 192.168.1.0/24, send to router 10.0.0.2" # Router receives packet destined for 192.168.1.50 ↓ Looks in routing table: "I have a route to 192.168.1.0/24" ↓ Action: Forward packet to 10.0.0.2 (next-hop IP) ↓ Router 10.0.0.2 receives and continues forwarding # 2\. Forward Directly to Destination (Directly Connected) Route: "192.168.1.0/24 is directly connected to my G0/1 interface" # Router receives packet destined for 192.168.1.100 ↓ Looks in routing table: "192.168.1.0/24 is on my G0/1 interface" ↓ Action: Send packet directly out G0/1 to destination # 3\. Receive for Itself Route: "10.0.0.0/24 is my own interface" # Router receives packet destined for 10.0.0.1 (its own IP) ↓ Looks in routing table: "This is MY IP address" ↓ Action: Don't forward, receive packet and process it # ### Viewing the Routing Table Router# show ip route # This displays all routes the router knows about. Output includes: Gateway of last resort is not set # 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks C 10.0.0.0/24 is directly connected, GigabitEthernet0/0 L 10.0.0.1/32 is directly connected, GigabitEthernet0/0 192.168.0.0/16 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, GigabitEthernet0/1 L 192.168.1.254/32 is directly connected, GigabitEthernet0/1 S 192.168.2.0/24 \[1/0\] via 10.0.0.2 # Legend: C = Connected (directly connected) L = Local (router's own IP) S = Static (manually configured) D = Dynamic (learned from routing protocol) R = RIP, O = OSPF, B = BGP (dynamic protocols) # --- ## Routing Methods There are two main ways routers learn routes and populate their routing tables: ### 1. Static Routing What it is: - Network engineer/administrator manually configures routes - Routes do NOT change automatically - Engineer must manually update routes if network changes Advantages: - Simple to understand and implement - No overhead (no routing protocol traffic) - Secure (no routing information shared automatically) - Good for small networks Disadvantages: - Must manually configure every route - Doesn't adapt if network fails or changes - Not scalable for large networks - Time-consuming to maintain When to use: - Small networks (< 10 routers) - Point-to-point links (few destinations) - Labs and testing - When security requires manual control ### 2. Dynamic Routing What it is: - Routers automatically share routing information with each other - Routers learn and update routes automatically - Routes adapt when network conditions change Advantages: - Automatic route discovery and updates - Adapts to network failures - Scalable for large networks - Less manual configuration - Load balancing across multiple paths Disadvantages: - More complex to configure and understand - Uses network bandwidth and CPU (routing protocol overhead) - Slower initial convergence - Security concerns (routing information shared) Common Dynamic Routing Protocols:
Protocol Type Purpose
RIP (Routing Information Protocol) Distance Vector Simple, outdated
OSPF (Open Shortest Path First) Link State Industry standard, widely used
EIGRP (Enhanced Interior Gateway Routing Protocol) Hybrid Cisco proprietary, powerful
BGP (Border Gateway Protocol) Path Vector Internet routing
When to use: - Medium to large networks - Complex topologies - Multiple paths to destinations - Networks with redundancy - When automatic failover is needed --- ## WANs (Wide Area Networks) ### What is a WAN? A Wide Area Network (WAN) is a network that extends over a large geographic area — typically connecting multiple locations (cities, countries, or across the Internet). Characteristics: - Connects different LANs together - Uses routers to connect - Often uses leased lines or Internet as backbone - Lower bandwidth and higher latency than LANs - Can span great distances Example WAN Architecture: \[LAN A\] | Router A ←→ Router B ← WAN Link (Internet or Leased Line) | | \[LAN B\] \[LAN C\] # Routers A and B are connected via WAN Each router serves its local LAN Users in LAN A can reach LAN B/C through routers # ### LAN vs WAN
Feature LAN WAN
Size Single building/floor Multiple locations
Speed Fast (100 Mbps - 10 Gbps) Slower (1-100 Mbps)
Devices Switches, local routers Routers, modems, firewalls
Latency Low (< 1ms) Higher (10-100+ ms)
Ownership Single organization Often shared/rented
Cost Lower per Mbps Higher per Mbps
--- ## Static Routing Configuration ### Basic Static Route Syntax Router(config)# ip route <destination-network> <subnet-mask> <next-hop-ip> # Components: - destination-network — The network you want to reach (e.g., 192.168.2.0) - subnet-mask — Netmask of that network (e.g., 255.255.255.0) - next-hop-ip — IP address of the next router (e.g., 10.0.0.2) ### Example 1: Basic Static Route Scenario: - Router A at 10.0.0.1 needs to reach network 192.168.2.0/24 - Router B (next-hop) is at 10.0.0.2 on a shared link with Router A Configuration: RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2 # Translation: "To reach 192.168.2.0/24, send packets to my neighbor at 10.0.0.2" Result in routing table: S 192.168.2.0/24 \[1/0\] via 10.0.0.2 # ### Example 2: Multiple Static Routes Scenario: Router A needs to reach two different networks: RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2 RouterA(config)# ip route 172.16.0.0 255.255.0.0 10.0.0.3 # Result: - Packets destined for 192.168.2.x go to 10.0.0.2 - Packets destined for 172.16.x.x go to 10.0.0.3 ### Static Route Configuration with Exit Interface Alternative Syntax (for point-to-point links): Router(config)# ip route <destination-network> <subnet-mask> <exit-interface> # Example: RouterA(config)# ip route 192.168.2.0 255.255.255.0 Serial0/0/0 # Translation: "To reach 192.168.2.0/24, send out my Serial0/0/0 interface" When to use: - Point-to-point WAN links (Serial, PPP) - When only one neighbor on the link - More efficient on point-to-point connections Difference:
Method Use Case
Next-Hop IP Multi-access networks (Ethernet, Frame Relay with many neighbors)
Exit Interface Point-to-point links (Serial, PPP)
### Example 3: Next-Hop vs Exit Interface Next-Hop (for Ethernet): RouterA(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2 ↑ specific neighbor IP # Exit Interface (for Serial): RouterA(config)# ip route 192.168.2.0 255.255.255.0 Serial0/0/0 ↑ output interface # --- ## Default Route ### What is a Default Route? A default route is a "catch-all" route that handles packets destined for networks the router doesn't have specific routes for. Use: Send unknown destinations to a central router or ISP gateway ### Default Route Configuration Syntax: Router(config)# ip route 0.0.0.0 0.0.0.0 <next-hop-ip> # Or in CIDR notation: Router(config)# ip route 0.0.0.0/0 <next-hop-ip> # Translation: "For any destination I don't have a more specific route for, send to this router" Common notation: The default route is often written as 0.0.0.0/0 (the "default route") ### Example: Setting a Default Route Scenario: Small office network with single ISP connection Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1 # Translation: "For any destination not in my routing table, send to the ISP gateway at 203.0.113.1" Result: S\* 0.0.0.0/0 \[1/0\] via 203.0.113.1 \* indicates this is the default route # ### Route Precedence (Most Specific Wins) When multiple routes match a destination, the router uses the most specific route: Routing table: 0.0.0.0/0 → 203.0.113.1 (default route) 192.168.2.0/24 → 10.0.0.2 (specific route) 192.168.2.0/25 → 10.0.0.3 (more specific) # Packet destined for: # 192.168.2.10 → Uses 192.168.2.0/25 (longest prefix match — /25 most specific) 192.168.3.10 → Uses 0.0.0.0/0 (default — no other match) 172.16.0.1 → Uses 0.0.0.0/0 (default — no other match) # --- ## Static Routing Configuration Example ### Complete Scenario Network setup: \[PC A\] | \[Switch\] | Router A ────WAN Link──── Router B ────\[Switch\]──── \[PC B\] 10.0.0.1 (Serial) 10.0.0.2 192.168.1.0/24 Router A is in: 10.0.0.0/24 Router B is in: 10.0.0.0/24 and 192.168.1.0/24 # Requirements: - PC A (in 10.0.0.0 LAN) needs to reach PC B (in 192.168.1.0 LAN) - Need static routes on both routers Configuration on Router A: RouterA(config)# ip route 192.168.1.0 255.255.255.0 10.0.0.2 RouterA(config)# end # RouterA# show ip route S 192.168.1.0/24 \[1/0\] via 10.0.0.2 # Configuration on Router B: RouterB(config)# ip route 10.0.0.0 255.255.255.0 10.0.0.1 RouterB(config)# end # RouterB# show ip route S 10.0.0.0/24 \[1/0\] via 10.0.0.1 # Result: - Router A knows how to reach 192.168.1.0/24 (sends to Router B) - Router B knows how to reach 10.0.0.0/24 (sends to Router A) - PC A can now ping and communicate with PC B ### Verifying Routes Check routing table: Router# show ip route # Test connectivity: Router# ping 192.168.1.100 # Trace packet path: Router# traceroute 192.168.1.100 # --- ## Comparison: Static vs Dynamic Routing ### When to Use Static Routing ✅ Good for: - Small networks (< 10 routers) - Simple, stable topologies - Point-to-point links - High security requirements - Lab/testing environments - When you want complete control ❌ Not good for: - Large networks - Complex topologies - Frequent topology changes - Networks requiring redundancy - Automatic failover ### When to Use Dynamic Routing ✅ Good for: - Large networks (> 10 routers) - Complex topologies - Multiple paths to destinations - Automatic redundancy/failover - Networks that change frequently - Production enterprise networks ❌ Not good for: - Very small networks (overhead not worth it) - When bandwidth is extremely limited - When security isolation is critical - Simple point-to-point links --- ## Static Route Commands Reference ### Configuration
Command Purpose
ip route <dest> <mask> <next-hop> Add static route
ip route <dest> <mask> <exit-int> Add route via interface
ip route 0.0.0.0 0.0.0.0 <next-hop> Configure default route
no ip route <dest> <mask> <next-hop> Delete static route
### Verification
Command Purpose
show ip route Display entire routing table
show ip route static Show only static routes
show ip route 192.168.1.0 Show route for specific destination
ping <ip> Test connectivity
traceroute <ip> Trace path to destination
### Example Session Router# configure terminal Router(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2 Router(config)# ip route 192.168.3.0 255.255.255.0 10.0.0.2 Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1 Router(config)# exit # Router# show ip route 10.0.0.0/24 is directly connected, GigabitEthernet0/0 S 192.168.2.0/24 \[1/0\] via 10.0.0.2 S 192.168.3.0/24 \[1/0\] via 10.0.0.2 S\* 0.0.0.0/0 \[1/0\] via 203.0.113.1 # Router# ping 192.168.2.100 Type escape sequence to abort. Sending 5, 100-byte ICMP Echoes to 192.168.2.100, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/5 ms # --- ## Key Differences: Switches vs Routers in Network
Feature Switch Router
Layer Layer 2 (Data Link) Layer 3 (Network)
Forwards within/between Within LANs Between LANs/Networks
Uses MAC addresses IP addresses
Table MAC address table Routing table
Decision Destination MAC Destination IP
Broadcasts Floods to all ports Blocks/filters
Default behavior Active Interfaces down by default
--- ## Key Takeaways for CCNA ✅ Routing is how packets move between different networks using destination IP addresses ✅ Routing table stores all known routes; router uses it for forwarding decisions ✅ Static routing = manual configuration; good for small, stable networks ✅ Dynamic routing = automatic discovery; good for large, complex networks ✅ WAN connects separate LANs over large geographic distances ✅ Default route (0.0.0.0/0) catches packets with no other matching route ✅ Static route syntax: ip route <destination> <mask> <next-hop-ip> ✅ Exit interface method used for point-to-point links; next-hop method for multi-access ✅ Routes point to next-hop IP or exit interface — tells router where to send packets ✅ Most specific route wins when multiple routes match a destination (longest prefix match) ✅ Router must have routes in both directions for bidirectional communication ✅ Directly connected routes appear as 'C' in routing table; don't need to be configured ✅ Local routes (L) are the router's own IP addresses; handled automatically ✅ show ip route displays routing table; includes source and metric information ✅ Routers separate broadcast domains — broadcasts don't cross router interfaces