CCNA | Static Routing | 11

Overview of Routers and Traffic Forwarding

• Routers: Devices that forward traffic between different networks (LANs).

• Switches: Devices that forward traffic within a LAN.

IP Routing Process

• Initial Steps:

  • An end host wants to send a packet to another end host in a different network.

  • Concept of a default gateway for routing traffic.

• Routing Table:

  • Each router maintains a routing table listing known destinations and methods to reach them.

  • Vital for network engineers to understand; uses the command show IP route frequently.

Network Topology for Demonstration

• LANs & IP Addresses:

  • LAN 1 (connected to Router R1): 192.168.1.0/24

    • PC1: 192.168.1.1

    • Router R1: 192.168.1.254 (g0/2 interface)

  • LAN 2 (connected to Router R4): 192.168.4.0/24

    • PC4: 192.168.4.1

    • Router R4: 192.168.4.254 (g0/2 interface)

• WAN Connections: Connects the routers across large distances using fiber optics.

  • Routers Interconnecting:

    • R1 to R2: 192.168.12.0/24

    • R2 to R4: 192.168.24.0/24

    • R1 to R3: 192.168.13.0/24

    • R3 to R4: 192.168.34.0/24

Packet Sending Process from PC1 to PC4

1. Destination Check:

   • PC1 checks if PC4 is in the same network.

   • 192.168.1.0 (PC1) vs 192.168.4.0 (PC4) => Different networks.

2. Routing Logic Activation:

   • PC1 must send the packet to its default gateway (Router R1).

   • Default gateway: Device to forward data to another network (R1 in this case).

Router R1 Processing

• Forwarding Responsibilities:

  • Upon receiving the packet, R1 compares the destination IP with its routing table.

  • Example entry for R1:

    • Destination: 192.168.4.0/24

    • Next hop: 192.168.12.2 (R2)

    • Exit interface: g0/1.

• Packet Forwarding:

  • R1 forwards the packet to R2 based on routing table entry.

Continuing Through Router R2 to R4

• Router R2 Actions:

  • Similar process; it checks its routing table for the destination IP.

  • Example entry might indicate forwarding to R4 via:

    • Destination: 192.168.4.0

    • Next hop: 192.168.24.4 (R4 IP)

    • Exit interface: g0/1.

• Final Router R4 Processing:

  • R4 checks its own routing table.

  • Directly connected to the destination: 192.168.4.0/24 on g0/2.

  • Forwards packet to Switch 4.

Understanding Layer 3 vs Layer 2

• Layer 3 Overview: Packet's source and destination IP addresses remain unchanged during the journey.

• Layer 2 Complexity: Involves MAC addresses, which change based on local network transit points.

Demonstration on Cisco Router CLI

• Routing Table Examination:

  • Issue the command show IP route in privileged exec mode to view the routing table.

  • Different codes indicate the type of routes:

    • C: Connected route.

    • L: Local route.

• Local vs Connected Routes:

  • Connected Route:

    • e.g., 192.168.1.0/24 indicates the interface network.

  • Local Route:

    • e.g., 192.168.1.1/32 indicates the specific address on the interface.

Configuring Default Gateway

• Default Route Concept:

  • Matches all possible destinations if no more specific route is available.

  • Identified by IP address of 0.0.0.0 and subnet mask 0.0.0.0.

• This setup allows the router (acting as PC in the simulation) to send traffic to all external networks as needed.