What Is a Frame?
a Frame — the unit of data at Layer 2 — a packet wrapped with MAC addresses and an error check so it can cross one physical network hop.
How it works
A frame's header holds destination and source MAC addresses (plus VLAN tags on trunks); its trailer carries a checksum (FCS) so corrupted frames get dropped. Frames are rebuilt at every hop: the packet inside stays the same, but each router strips the old frame and writes a new one for the next link.
Why it matters
This re-framing per hop — constant IPs, changing MACs — is one of the most-tested concepts in CCNA and interviews. Switches live entirely in the frame world, forwarding on destination MACs.
Frequently asked questions
What addresses does a frame use?
Source and destination MAC addresses — Layer 2 hardware addresses valid only for the current network segment.
Do frames change as data crosses networks?
Yes — every router removes the incoming frame and builds a new one for the next hop; the IP packet inside is what persists.
What is the FCS in a frame?
The Frame Check Sequence — a checksum in the trailer used to detect corrupted frames, which are then discarded.
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