Learning objectives
- Explain Ethernet frame structure.
- Describe dynamic MAC learning and forwarding.
- Distinguish collision and broadcast domains.
Core concepts
Ethernet uses frames with destination and source MAC addresses, a Type/Length field, payload, and Frame Check Sequence. A VLAN tag may be inserted by 802.1Q on trunks. The switch learns from source MAC addresses and looks up destination MAC addresses to decide where frames go.
How it works and worked example
Known unicast traffic is forwarded only toward the learned port. Unknown unicast and broadcast traffic are flooded within the VLAN, excluding the ingress port. Each switch port is its own collision domain; VLANs define broadcast domains. Full duplex permits simultaneous transmission in both directions.
A frame from AA-AA arriving on Gi1/0/1 teaches the switch AA-AA → Gi1/0/1. If BB-BB is known on Gi1/0/8, forward only there; if unknown, flood within the VLAN.
IOS XE verification
show mac address-table dynamic show interfaces status show interfaces counters errors
Knowledge check
Choose one answer. Submit for per-option explanations.