About this interactive
The lesson's routing process, which matches RFC 1812's forwarding order:
1. The frame arrives; the router checks it is addressed to its own MAC (and its frame check sequence).
2. It strips the Layer 2 header to reach the packet.
3. It looks up the destination IP address in its routing table for the next hop.
4. It lowers the TTL by one; at zero the packet is dropped, which stops routing loops.
5. The header changed, so it recomputes the header checksum.
6. It builds a new Layer 2 header for the next hop.
7. Layer 1 puts the new frame on the wire as signals.
What you'll learn
- Order the steps a router takes to forward one packet: accept the frame, strip Layer 2, look up the destination, lower the TTL, recompute the checksum, build a new frame and send it
- Explain why each step must come where it does: the checksum after the TTL changes, the new frame after the next hop is known
Aligned to
Cisco CCNA
3.2
Determine how a router makes a forwarding decision by default
CompTIA Network+
2.1
Explain characteristics of routing technologies.
Cisco CCST Networking
4.4
Explain basic routing concepts
Key terms
- Router
- A network device that forwards data packets between networks based on IP addresses.
- Routing Table
- A database stored in a router that lists known network destinations, the metric (cost) to reach them, and the next-hop address or outgoing interface to forward packets toward those destinations. Routers consult this table for every packet they forward.
- Time to Live
TTL
- A value in a packet that limits its lifespan on a network, preventing it from circulating indefinitely.
- Checksum
- A value calculated from packet header data and used to detect errors; recalculated by a router after modifying the IP header fields such as TTL.
- Frame Check Sequence
FCS
- A cyclic redundancy check value appended to the end of a data link frame that allows the receiver to detect transmission errors. The sending device calculates the FCS from the frame contents, and the receiving device recalculates it; a mismatch means the frame was corrupted and should be discarded.
Topics
Router
Routing Process
Ttl
Checksum
Layer 3
Interactive
Ordering
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