A frame trace forces you to separate end-host knowledge from switch behavior. The sender creates the frame using a local destination MAC. Each switch makes a forwarding decision from its MAC table, while the frame's payload remains an IP packet. When the frame crosses a router, the Layer 2 envelope is recreated for the next link.
Remember the shape, not the sentence
“Imagine two reception desks connected by a hallway. Each receptionist only needs a directory for the rooms on their side of the hallway. The contents of the parcel do not tell the receptionist which room to use; the envelope address does.”
Same-LAN path
If two hosts are in the same VLAN, the source can ARP for the destination and send the frame through one or more switches. The destination MAC remains the same across the switched path.
Routed path
When the destination is remote, the sender uses the gateway MAC. The router removes the inbound frame, examines the IP packet, chooses a route, and builds a fresh outbound frame on the next interface.
Why this matters
Packets and frames have different scopes. A packet may cross multiple networks; a frame normally exists only for one link or LAN segment at a time.
See the mechanism before you memorize it
One diagram, one causal story, one reconstruction task.
Trace a frame across 2 switches (worksheet) — causal map
Host A
Build frame: src MAC A, dst MAC B or gateway MAC.
Switch 1
Look up destination MAC and choose output port.
Switch 2
Repeat the same Layer 2 lookup on its own table.
Router, if needed
Terminate the frame, route the IP packet, then create a new frame.
Draw the idea yourself
Rebuild the core flow on a real canvas before moving to the worked walkthrough.
Walk the system
Host A
Build frame: src MAC A, dst MAC B or gateway MAC.
Switch 1
Look up destination MAC and choose output port.
Switch 2
Repeat the same Layer 2 lookup on its own table.
Router, if needed
Terminate the frame, route the IP packet, then create a new frame.
Host B
Receive the frame, validate it, pass the payload up the stack.
Hop-by-hop vs end-to-end
Use this artifact whenever an interview question tries to trick you into saying MAC and IP addresses behave the same way.
End-to-end: IP src = A → IP dst = B
Hop 1: MAC A → MAC Gateway
Hop 2: MAC Router2 → MAC Router3
Hop final: MAC Gateway → MAC BWhat usually goes wrong
Close the tab. Rebuild it from memory.
Read the question, answer aloud, then reveal.
Does a switch change the IP packet?Reveal answer
Ordinary Layer 2 switching forwards the Ethernet frame without changing the IP payload.
What changes at a router?Reveal answer
The incoming frame is removed and a new outgoing frame is created for the next link. The IP packet is forwarded, subject to routing and fields that may be updated by the router.
Why does this simplify local networking?Reveal answer
Each hop only needs local link-layer addressing while the IP layer provides a logical identity that can span multiple networks.
Make this idea travel.
Connect it next to: ipv4-cidr • routing • tcp-reliability.