an Aggregated Link (also commonly known as Link Aggregation, Port Aggregation, EtherChannel [Cisco proprietary], Bonding, or NIC Teaming) refers to the practice of combining multiple physical network connections (typically Ethernet cables) into a single logical connection.

This technique is used to achieve two primary goals:

1. Increased Bandwidth (Throughput): By bundling multiple physical links, the aggregated link can provide a combined bandwidth that is the sum of the individual links. For example, aggregating two 1 Gigabit Ethernet links results in a logical link with a theoretical maximum throughput of 2 Gigabits per second.

2. Redundancy and High Availability: If one of the physical links within the aggregated group fails, traffic can automatically and transparently be redistributed across the remaining active links without any loss of connectivity. This provides fault tolerance and minimizes downtime.

How Aggregated Links Work:

While multiple physical cables are connected, the networking devices (switches, servers, routers) treat them as a single logical connection. Here’s a simplified breakdown:

1. Link Aggregation Group (LAG): The group of physical ports that are combined together is called a Link Aggregation Group (LAG) or a Port Channel.

2. Logical Interface: The switch or server creates a logical interface that represents this LAG. All configuration (IP address, VLANs, etc.) is applied to this logical interface, not the individual physical ports.

3. Load Balancing: When traffic needs to be sent across the aggregated link, a hashing algorithm is used to determine which physical link within the LAG should carry a particular frame or packet. Common hashing methods involve:

   • Source MAC address

   • Destination MAC address

   • Source and/or Destination IP address

   • Source and/or Destination TCP/UDP port number

   It’s important to understand that individual flows (e.g., a single file transfer between two specific hosts) typically use only one of the links within the bundle. This is to prevent out-of-order packet delivery, which can significantly degrade performance for protocols like TCP. Therefore, while the aggregate bandwidth of the LAG increases, the maximum bandwidth for any single flow is limited to the speed of one of the individual physical links. This means perfect load distribution is rarely achieved in real-world scenarios, especially with a small number of active flows.

4. Failure Detection and Recovery:

   • If a physical link within the LAG fails (e.g., cable cut, port failure), the Link Aggregation Control Protocol (LACP) or proprietary mechanisms quickly detect this.

   • The failed link is automatically removed from the active bundle.

   • Traffic is then redistributed across the remaining operational links, ensuring continuous connectivity without manual intervention or disruption to higher-layer protocols.

Protocols for Link Aggregation:

There are two main ways to configure aggregated links:

1. Static (Manual) Link Aggregation:

   • The administrator manually configures the ports on both ends of the link to be part of an aggregation group.

   • There’s no negotiation protocol between the devices. If a link fails, it’s removed, but there’s no dynamic communication to ensure both sides agree on the status of the links. This is simpler but less robust.

2. Dynamic Link Aggregation (using LACP or PAgP):

   • LACP (Link Aggregation Control Protocol) – IEEE 802.3ad: This is the industry-standard protocol for dynamic link aggregation. Devices exchange LACPDU (Link Aggregation Control Protocol Data Unit) messages to negotiate and manage the bundling of links. LACP automatically detects link failures and can even identify misconfigurations (e.g., one side configured for aggregation and the other not). It provides active monitoring and fault detection.

   • PAgP (Port Aggregation Protocol) – Cisco Proprietary: This is Cisco’s proprietary protocol for dynamic link aggregation, similar in function to LACP but only works between Cisco devices.

Benefits of Aggregated Links:

• Increased Bandwidth: Provides higher data throughput for high-traffic connections (e.g., between switches, or between a server and a switch).

• Redundancy and High Availability: Ensures network uptime by providing backup paths if individual links fail.

• Load Balancing: Distributes network traffic across multiple paths, preventing a single link from becoming a bottleneck (though load distribution depends on the hashing algorithm and traffic patterns).

• Cost-Effective: Often more economical than upgrading to a single, higher-speed interface (e.g., two 10GbE links can be cheaper than a single 40GbE link initially).

• Simplified Management: The bundle is managed as a single logical interface, simplifying configuration and troubleshooting compared to managing multiple independent links with Spanning Tree Protocol (STP).

• STP Compatibility: Link aggregation presents multiple physical links as one logical link to Spanning Tree Protocol, preventing STP from blocking redundant paths that are part of the bundle.

Common Use Cases:

• Switch-to-Switch Connections: Creating high-bandwidth, redundant uplinks between core, distribution, and access layer switches.

• Server Connectivity: Connecting high-traffic servers (e.g., file servers, database servers, virtualization hosts) to the network with increased throughput and resilience.

• Firewall/Router Connections: Providing redundant and higher-capacity connections to network security devices or routing platforms.

Aggregated links are a fundamental technology in modern network design, essential for building robust, high-performance, and resilient network infrastructures.