Network Implementation · Topic 5 of 8
Switch Port Features
Configure switch port features including speed and duplex, flow control, jumbo frames, port mirroring, PoE, MDI-X, the MAC address table and ARP table.
- 5 min
- Full duplex sends and receives at once. A duplex mismatch causes errors and poor speed.
- Auto negotiation sets speed and duplex. Match both ends if you set them manually.
- Jumbo frames use an MTU of about 9000 bytes and must be supported end to end.
What it is
Beyond virtual LANs (VLANs) and spanning tree, the exam expects you to know the settings and features of an individual switch port. These control how fast the port runs, how it handles congestion and large frames, how it powers devices and how traffic is copied for analysis.
You also need to understand the two tables that explain local delivery: the Media Access Control (MAC) address table on the switch and the Address Resolution Protocol (ARP) table on a host or router.
How it works
Speed and duplex
Speed is the data rate of the port, such as 100 Mbps or 1 Gbps. Duplex describes whether the link can send and receive at the same time. Full duplex does both at once, while half duplex takes turns and can have collisions.
Auto negotiation lets the two ends agree on the best common speed and duplex. A duplex mismatch, where one side is full duplex and the other is half duplex, causes slow performance and many errors, so settings should match on both ends.
Auto MDI-X
Medium dependent interface crossover (MDI-X) lets a switch port detect the cable pair arrangement and swap its transmit and receive pairs automatically. This means a straight through or crossover cable works either way on most modern ports.
Flow control
Flow control lets a receiver ask a sender to pause when its buffers are full. On Ethernet this uses pause frames defined in Institute of Electrical and Electronics Engineers (IEEE) standard 802.3x. It prevents frame loss during short bursts but can slow other traffic that shares the link.
Jumbo frames
A standard Ethernet frame carries up to 1500 bytes of payload, which is the default maximum transmission unit (MTU). Jumbo frames carry larger payloads, commonly around 9000 bytes, which lowers overhead for large transfers such as storage traffic.
Every device on the path must support the same larger MTU. If one device cannot, large frames are dropped or fragmented, causing performance problems.
Port mirroring
Port mirroring copies traffic from one or more source ports to a destination port. A protocol analyzer or an intrusion detection system connected to the destination port can then inspect the traffic without being in the path.
Mirroring is also called SPAN (Switched Port Analyzer) on some vendors' switches. The mirror port normally does not carry regular user traffic.
Power over Ethernet
Power over Ethernet (PoE) sends electrical power with data over the same twisted pair cable. The switch is the power sourcing equipment (PSE), and the phone, access point or camera is the powered device (PD).
| Standard | Name | Approximate power at the port |
|---|---|---|
| IEEE 802.3af | PoE | About 15 watts |
| IEEE 802.3at | PoE+ | About 30 watts |
| IEEE 802.3bt | PoE++ | About 60 watts (Type 3) or 100 watts (Type 4), for devices such as large access points |
The switch has a total power budget. If the budget is used up, extra devices will not power on, even if ports are free.
MAC address table
A switch learns the source MAC address of each frame and records the port it came from. When a frame arrives for a known MAC address, the switch sends it out of that single port. If the address is unknown or is a broadcast, the switch floods the frame to all other ports in the VLAN.
Entries age out after a period of inactivity. This table is often called the CAM (content addressable memory) table.
ARP table
ARP maps an Internet Protocol version 4 (IPv4) address to a MAC address on the local network. A host sends an ARP request as a broadcast, and the owner answers with its MAC address. The result is stored in the ARP table.
| Table | Maps | Found on |
|---|---|---|
| MAC address table | MAC address to switch port | Switch |
| ARP table | Internet Protocol (IP) address to MAC address | Hosts and routers |
What to remember for the exam
- Full duplex sends and receives at once. A duplex mismatch causes errors and poor speed.
- Auto negotiation sets speed and duplex. Match both ends if you set them manually.
- Jumbo frames use an MTU of about 9000 bytes and must be supported end to end.
- Port mirroring copies traffic to an analyzer port. It is called SPAN on some switches.
- PoE: 802.3af is about 15 watts, 802.3at (PoE+) is about 30 watts, and 802.3bt (PoE++) goes up to about 60 or 100 watts.
- A switch has a power budget, and the PSE is the switch while the PD is the device.
- The MAC address table maps MAC addresses to ports. The ARP table maps IP addresses to MAC addresses.
- Unknown unicast and broadcast frames are flooded within the VLAN.
- Flow control uses pause frames (802.3x) to slow a sender.
A switch that connects an access point but does not power it often has an exhausted PoE budget or a mismatch between the PoE standard and the device. Check both before replacing the cable.
Common mistakes
Setting one end manually and leaving the other on auto. This often causes a duplex mismatch. Set both ends the same way.
Enabling jumbo frames on one device only. The whole path needs the same larger MTU.
Mixing up the MAC table and the ARP table. One maps to ports, and the other maps IP addresses to MAC addresses.
Putting the analyzer on a normal port. Traffic is visible to it only if it is on a mirror destination port or on a tap.
Assuming PoE works on every cable. PoE needs twisted pair cable, and the switch must have power budget remaining.
Do not leave port mirroring enabled when it is not needed. It copies sensitive traffic, and a spare mirrored port can expose it.
Check yourself
Test your recall with these three.
What problem does a duplex mismatch cause?
Poor performance and many errors, because one side sends at the same time the other is receiving.
What does port mirroring do?
It copies traffic from source ports to a destination port so an analyzer can inspect it.
What does a switch do with a frame for an unknown destination MAC address?
It floods the frame out of all other ports in the same VLAN.
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