Ethernet Technology and MAC Addressing

Ethernet Technology and MAC Addressing

Quick Answer: Ethernet is the dominant Layer 2 technology in Huawei campus networks. You must know frame fields, how MAC unicast/multicast/broadcast work, why CSMA/CD mattered historically, and how switches and routers split collision and broadcast domains.

Ethernet (IEEE 802.3 family) carries almost all wired campus traffic you will configure on Huawei S series switches. Wireless LAN uses a related 802.11 MAC, but wired access, aggregation uplinks, and data-center-style campus cores still speak Ethernet frames. HCIA-Datacom tests both the structure of those frames and the behavior of devices that forward them.

Ethernet Frame Structure (Exam View)

A common Ethernet II frame includes:

Minimum and maximum sizes: Without VLAN tag, the minimum Ethernet frame is often cited as 64 bytes (including header and FCS) and the standard maximum payload is 1500 bytes (MTU). Jumbo frames exist in some campus cores but are not free-for-all—both ends and the path must agree.

Why the Type field matters

The EtherType tells the receiver which Layer 3 protocol is inside. IPv4 is 0x0800, ARP is 0x0806, IPv6 is 0x86DD. On a Huawei switch, you do not configure EtherType for ordinary IP traffic, but understanding it explains why ARP and IP look different to analyzers and why Layer 2 ACLs can match ethertype values on advanced platforms.

802.1Q tagging in campus trunks

Between access and aggregation switches, trunks carry multiple VLANs by inserting the 4-byte 802.1Q tag. Access ports usually send untagged frames for a single PVID/VLAN. Huawei also supports Hybrid ports that can untag some VLANs and tag others—critical for multi-service access (data + voice + AP management). Frame format knowledge makes Hybrid behavior less mysterious: the tag is either present or stripped on egress according to port configuration.

MAC Addresses: Format and Types

A MAC address is 48 bits (6 bytes), usually written as 00:1A:2B:3C:4D:5E or 001a-2b3c-4d5e (Huawei VRP style often uses hyphenated hexadecimal).

Unicast, multicast, and broadcast

Unicast MAC identifies a single interface. A switch that knows the destination MAC on a port forwards only out that port (within the VLAN).

Broadcast MAC is FF:FF:FF:FF:FF:FF. Every station in the broadcast domain (same VLAN / same Layer 2 segment) receives the frame. ARP requests for unknown hosts use broadcast. Excessive broadcasts (storms, loops without STP) saturate access links—why Huawei enables storm control and loop detection features on campus ports.

Multicast MAC has the I/G bit set and targets a group. Common examples include protocols that map IP multicast to MAC multicast (for IPv4, the mapping uses 01-00-5E plus 23 bits of the IP group address). You will see multicast more heavily in later video/IPTV campus scenarios; for HCIA fundamentals, know that switches may flood unknown multicast unless IGMP snooping constrains it.

Special addresses worth memorizing

Huawei VLANIF gateways and VRRP groups may present virtual MACs so hosts keep the same gateway MAC when the active router fails—hosts care about MAC stability for ARP entries.

How Switches Use MAC Addresses

A Layer 2 switch maintains a MAC address table (CAM table) per VLAN:

1. Learn: When a frame arrives, record source MAC → ingress port (and VLAN).
2. Forward: Look up destination MAC.
   • Known unicast: forward only to the learned port.
   • Unknown unicast: flood within the VLAN (except ingress port).
   • Broadcast / certain multicast: flood within the VLAN (unless constrained).
3. Age: Entries expire if not refreshed (default aging often hundreds of seconds; exact default can vary by platform—know the concept, not only one number).

On VRP, operators inspect tables with commands such as display mac-address. Duplicate MACs, flapping ports, or MAC moved between ports indicate loops, mis-cabling, or VM mobility issues.

CSMA/CD: Historical Context That Still Appears on Exams

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) was required when multiple stations shared the same electrical collision domain—classic coaxial Ethernet or hub-based half-duplex networks.

How it worked conceptually:

1. Carrier sense: Listen before transmit.
2. Multiple access: Shared medium; anyone may try to send.
3. Collision detection: If two stations transmit together, detect the collision, jam, back off randomly, retry.

Why it faded operationally: Full-duplex links on switches give each host a dedicated collision domain (often “collision domain of one”). With full duplex, simultaneous send and receive are allowed and CSMA/CD is not used. Modern campus ports default to auto-negotiation for speed/duplex; a duplex mismatch (one side full, one side half) still causes late collisions and poor performance—an exam-favorite root cause.

Remember the distinction for HCIA:

• CSMA/CD → wired shared Ethernet history / half duplex.
• CSMA/CA → Wi-Fi collision avoidance (WLAN chapter later).

Collision Domains vs Broadcast Domains

These two phrases appear constantly on associate-level exams. Define them tightly:

Device comparison (high-yield table)

Example: 24 PCs on a hub → 1 collision domain, 1 broadcast domain. Same 24 PCs on a switch in one VLAN → 24 collision domains, 1 broadcast domain. Same 24 PCs in two VLANs with inter-VLAN routing on a Huawei Layer 3 switch → still many collision domains, two broadcast domains; broadcasts in VLAN 10 never hit VLAN 20 without L3 help (and L3 does not forward pure L2 broadcasts between VLANs).

Ethernet Speeds and Campus Cabling Notes

Huawei campus designs commonly use:

• 1GE / Multi-GE copper at access for PCs, phones, APs
• 10GE / 25GE / 40GE / 100GE optical uplinks at aggregation and core
• PoE / PoE+ / PoE++ on access for APs and cameras (power is Physical/device feature, traffic is still Ethernet)

Auto-negotiation settles speed and duplex. Fixed settings are used when troubleshooting or when a peer cannot negotiate. Mismatched MTUs or error-disabled ports after flapping are operational issues you will later correlate with display interface counters (CRC, giants, runts).

Relating Ethernet Behavior to Huawei Scenarios

1. User cannot reach gateway: Check access VLAN, port link type, and whether the PC MAC is learned on the correct port (display mac-address).
2. Intermittent slow file copy on one floor: Look for duplex mismatch or CRC errors on the access port—not OSPF first.
3. Broadcast storm after a loop: STP/loop detection should block a port; without it, flooded broadcasts and unknown unicasts multiply and exhaust CPU/bandwidth.
4. AP on Hybrid port: Management VLAN may be untagged while service SSIDs map to tagged VLANs—frame tagging knowledge explains the design.

Exam Pitfalls

• Believing modern switches still “run CSMA/CD on every packet” in full duplex.
• Claiming a Layer 2 switch breaks broadcast domains (it does not; VLANs create separate domains, routing connects them).
• Confusing MAC multicast with IP multicast addressing without the mapping rules.
• Forgetting that source MAC learning and destination MAC forwarding are different steps.
• Thinking routers forward Ethernet broadcasts by default between interfaces—they do not.

Master frame fields and domain definitions now; VLAN, STP, and Eth-Trunk chapters build directly on this Ethernet foundation.