EtherChannel

Definition

EtherChannel is a networking technology that combines multiple physical Ethernet links into a single logical link called a Port Channel.

It provides increased bandwidth, fault tolerance, and load sharing by grouping multiple links between network devices.

Key Points

• EtherChannel combines multiple physical links into one logical connection.

• It is also known as:

  • Port Channel
  • Channeling
  • Link Aggregation (LAG)
  • NIC Teaming

• EtherChannel can be used between:

  • Switch to Switch
  • Server to Switch
  • Switch to Router
  • Router to Router (Layer 3 EtherChannel)

• EtherChannel hides individual physical links from upper-layer protocols.

• STP views an EtherChannel as a single logical link.

EtherChannel Objectives

Increased Bandwidth

• Multiple links are combined to increase available bandwidth.

Example:

2 × 1Gbps links = 1 Logical EtherChannel

Increased Availability

• If one physical link fails, the remaining links continue working.
• The EtherChannel remains operational as long as one member link is active.

Load Sharing

• Traffic can be distributed across multiple physical links.
• Different traffic flows may use different member links.

Auto Configuration

• Configuration applied to the EtherChannel interface is automatically applied to member interfaces.

Rapid Convergence

• EtherChannel provides faster recovery when links fail.

Cost Efficiency

• Multiple lower-speed links can replace an expensive high-speed link.

Example:

4 × 10Gbps links may cost less than 1 × 40Gbps link

Flexible Operation

EtherChannel can operate as:

• Access channel
• Trunk channel
• Layer 3 routed channel

EtherChannel Protocols

EtherChannel can be formed using dynamic negotiation protocols:

1. PAgP
2. LACP

PAgP (Port Aggregation Protocol)

Definition

PAgP is a Cisco proprietary protocol used to automatically create EtherChannels.

Characteristics

• Cisco proprietary.
• Supports half-duplex and full-duplex devices.
• Supports maximum of 8 ports per EtherChannel.
• Provides misconfiguration detection.
• Modes:
  • Auto
  • Desirable

LACP (Link Aggregation Control Protocol)

Definition

LACP is an IEEE standard protocol used to create EtherChannels.

Standard:

IEEE 802.3ad

Characteristics

• Open standard.

• Supports only full-duplex devices.

• Supports up to 16 ports:

  • 8 active ports
  • 8 standby ports

• Provides misconfiguration protection.

• Supports standby links.

• Modes:

  • Active
  • Passive

EtherChannel Protocol Modes

PAgP Modes

LACP Modes

EtherChannel Formation Rules

Important Note

PAgP, LACP, and ON modes are not compatible with each other.

Examples:

PAgP + LACP = No EtherChannel

PAgP + ON = No EtherChannel

LACP + ON = No EtherChannel

EtherChannel Components

EtherChannel contains two main parts:

STP and EtherChannel

Without EtherChannel:

Switch
 |
 |---- Link 1
 |
 |---- Link 2

STP may block one link to prevent loops.

With EtherChannel:

Switch
 |
 |==== EtherChannel ====
 |

STP sees it as one logical link.

Advantages:

• All member links can forward traffic.
• STP does not block individual links.
• Provides redundancy and bandwidth improvement.

Advantages and Disadvantages

Advantages

• Low-cost bandwidth upgrade.
• Provides Layer 2 redundancy.
• Uses existing network infrastructure.
• Improves link availability.
• Allows multiple links to operate together.

Disadvantages

Individual Link Limitation

EtherChannel does not create a single high-speed pipe.

Example:

2 × 1Gbps EtherChannel

Does not mean:

One 2Gbps connection

It means:

Two separate 1Gbps links sharing traffic

Traffic Polarization

• A single traffic flow may use only one member link.
• EtherChannel provides load distribution, not true load balancing.

EtherChannel Port Selection

When more than 8 ports are configured:

• The switch with the lowest system ID becomes the decision maker.
• System ID consists of:

System Priority + Switch MAC Address

Example:

SW1:
32768 + MAC Address

SW2:
100 + MAC Address

SW2 becomes the decision maker because it has the lower ID.

The selected switch chooses active ports based on:

1. Lowest port priority.
2. Lowest port number.

Remaining ports become standby ports.

Layer 2 EtherChannel

Definition

Layer 2 EtherChannel operates at the Data Link Layer.

Features:

• Uses switch ports.
• Can operate as access or trunk.
• Provides redundancy between switches.

Limitation:

• Flooded traffic may reach connected devices and increase CPU usage.

Layer 3 EtherChannel

Definition

Layer 3 EtherChannel is a routed EtherChannel where the logical channel operates as a Layer 3 interface.

Used for:

• Router-to-router connections.
• Layer 3 switch routing links.

Advantages:

• Supports routing protocols.
• Provides redundancy between routers.

EtherChannel Load Distribution

EtherChannel uses hashing algorithms to decide which physical link carries traffic.

Traffic can be distributed based on:

• Source MAC address
• Destination MAC address
• Source and destination MAC addresses
• Source IP address
• Destination IP address
• Source and destination IP addresses

Important:

• EtherChannel distributes traffic per flow.
• A single flow normally stays on one physical link.

Common Mistakes

• Mixing PAgP and LACP protocols.
• Configuring different EtherChannel modes on both sides.
• Assuming EtherChannel creates one high-speed link.
• Forgetting that member interfaces must have matching configurations.
• Using different trunk/access settings on member ports.
• Expecting perfect traffic distribution between all links.
• Ignoring the maximum active port limit.

Short Exam Notes

• EtherChannel: Combines multiple physical links into one logical link.

• Also called:

  • Port Channel
  • LAG
  • Link Aggregation

• Protocols:

  • PAgP: Cisco proprietary.
  • LACP: IEEE 802.3ad standard.

• PAgP modes:

  • Auto
  • Desirable

• LACP modes:

  • Active
  • Passive

• Maximum ports:

  • PAgP: 8 ports.
  • LACP: 16 ports (8 active + 8 standby).

• STP views EtherChannel as one logical link.

• EtherChannel provides:

  • Increased bandwidth
  • Redundancy
  • Load sharing

• It provides load distribution, not true load balancing.