EP

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.