Packet Processing with Static Routes
Packet Processing with Static Routes

Packet Processing with Static Routes

Packet Processing with Static Routes
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Summary

This topic explain how a router processes packets when a static route is configured. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Module 16 of the Cisco CCNA 2 course, for a better follow up of the course you can go to the CCNA 2 section to guide you through an order.

Static Routes and Packet Forwarding

Before diving into the troubleshooting portion of this module, this topic provides a brief review of how packets are forwarded in static routes. In the figure, click Play to see the animation where PC1 is sending a packet to PC3.

Demo Static Routes and Packet Forwarding
Demo Static Routes and Packet Forwarding

The following describes the packet forwarding process with static routes, as demonstrated in the animation:

  1. The packet arrives on the GigabitEthernet 0/0/0 interface of R1.
  2. R1 does not have a specific route to the destination network, 192.168.2.0/24. Therefore, R1 uses the default static route.
  3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an “all 1s” address for the Layer 2 destination address.
  4. The frame is forwarded out of the Serial 0/1/0 interface. The packet arrives on the Serial 0/1/0 interface on R2.
  5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to 192.168.2.0/24 out of the Serial 0/1/1 interface.
  6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an “all 1s” address for the Layer 2 destination address.
  7. The frame is forwarded out of the Serial 0/1/1 interface. The packet arrives on the Serial 0/1/1 interface on R3.
  8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to 192.168.2.0/24 out of the GigabitEthernet 0/0/0 interface.
  9. R3 looks up the ARP table entry for 192.168.2.10 to find the Layer 2 Media Access Control (MAC) address for PC3. If no entry exists, R3 sends an Address Resolution Protocol (ARP) request out of the GigabitEthernet 0/0/0 interface, and PC3 responds with an ARP reply, which includes the PC3 MAC address.
  10. R3 encapsulates the packet in a new frame with the MAC address of the GigabitEthernet 0/0/0 interface as the source Layer 2 address, and the MAC address of PC3 as the destination MAC address.
  11. The frame is forwarded out of GigabitEthernet 0/0/0 interface. The packet arrives on the network interface card (NIC) interface of PC3.

Check Your Understanding – Packet Processing with Static Routes

Packet Processing with Static Routes
Packet Processing with Static Routes

Refer to the exhibit. What action will R3 take to forward a frame if it does not have an entry in the ARP table to resolve a destination MAC address?

Correct! Wrong!

Refer to the exhibit. True or False? R1 must encapsulate received packets into new frames before forwarding them to R2.

Correct! Wrong!

Refer to the exhibit. True or False? R2 will forward frames to R3 with an all 1s Layer 2 address.

Correct! Wrong!

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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