Packet Processing with Static Routes
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.
The following describes the packet forwarding process with static routes, as demonstrated in the animation:
- The packet arrives on the GigabitEthernet 0/0/0 interface of R1.
- R1 does not have a specific route to the destination network, 192.168.2.0/24. Therefore, R1 uses the default static route.
- 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.
- The frame is forwarded out of the Serial 0/1/0 interface. The packet arrives on the Serial 0/1/0 interface on R2.
- 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.
- 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.
- The frame is forwarded out of the Serial 0/1/1 interface. The packet arrives on the Serial 0/1/1 interface on R3.
- 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.
- 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.
- 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.
- 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
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?
Refer to the exhibit. True or False? R2 will forward frames to R3 with an all 1s Layer 2 address.
Refer to the exhibit. True or False? R1 must encapsulate received packets into new frames before forwarding them to R2.
Glossary: If you have doubts about any special term, you can consult this computer network dictionary.
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