Link Aggregation

Bandwidth bottlenecks and redundancy

In a corporate network, many user PCs can connect to an access switch that uses one uplink cable to a distribution switch. That link carries the traffic of all connected users and creates two problems:

• Bandwidth limitation: With 24 users on 1 Gbps connections and one 1 Gbps uplink to the core, the uplink limits the traffic reaching the rest of the network. The traffic cannot exceed the capacity of that one link. Replacing it with a 10 Gbps connection can require new ports, modules, and cabling.
• Single point of failure: A failed uplink cable or port disconnects the access switch and its users from the rest of the network. The lack of redundancy removes a spare path for that traffic.

Adding a second parallel physical link between the switches increases the available paths. With parallel links that are not aggregated, Spanning Tree Protocol (STP) keeps the topology loop-free. Parallel links can then be treated as redundant and some remain blocked. This provides a spare path, while both links do not carry current traffic at the same time. A LAG presents several connections as one logical STP port.

Link aggregation and the LAG group

Link aggregation combines multiple physical network connections into one logical interface. The group can provide higher aggregate throughput and tolerate the failure of one member between two devices, typically switches, servers, or routers.

Two 1 Gbps links can operate as members of one group with a total theoretical capacity of 2 Gbps. The switch presents the group as one logical port, so STP can treat it as one path. A single flow can still use one member, depending on the distribution algorithm. Documentation uses several names:

• EtherChannel: Cisco's name for a link aggregation solution.
• Port trunking or port teaming: Terms used by vendor documentation. Port trunking can also mean a VLAN trunk, so its meaning depends on context.
• Link Aggregation Group (LAG): The group name used in the IEEE standard.
• IEEE 802.3ad and 802.1AX: 802.3ad was the earlier link aggregation standard that defined LACP, while 802.1AX defines the current standard.

Effects of link aggregation

Link aggregation affects the group's throughput and connection availability:

1. Higher aggregate throughput:

   LAG members contribute their capacities to the group. Two 1 Gbps links provide 2 Gbps of total theoretical capacity, and four 10 Gbps links provide 40 Gbps. These values describe the sum available to multiple flows. A single session can be assigned to one port by the load-balancing algorithm. Actual throughput depends on frame overhead, device load, and the member-selection method. If a device has spare ports, this increase can avoid immediately replacing one port with a faster standard.

2. Group availability and redundancy:

   The group can remain usable after one member fails if the platform permits operation with the remaining ports. After detecting the failure, the device removes the port from distribution and sends new frames through the remaining members. Reaction time depends on the physical layer, LACP, and the configured minimum member count. STP treats an active group as one logical port, so the failure does not require a separate path election for each cable. The service can still lose part of its available capacity.

Traffic distribution in a LAG group

A LAG usually does not assign one large transfer to all members at once. For example, a 2x1 Gbps LAG can provide 2 Gbps of aggregate capacity, while one file flow often remains on one link.

A single session or flow between two devices is usually sent through one LAG member. The switch must choose a port deterministically so that frames from one flow retain their order. It uses a load-balancing algorithm for that choice.

The algorithm reads selected frame-header fields and passes them to a hashing function. The result selects a physical link from the LAG. Available fields depend on the platform and balancing mode:

• Source MAC address: With source-only selection, traffic from one computer maps to the same member.
• Destination MAC address: With destination-only selection, traffic to one computer maps to the same member.
• Source and destination MAC addresses: A pair of addresses can deterministically select one member for traffic in a given direction.
• Source and destination IP addresses: A Layer 3 criterion separates flows between different address pairs.
• Source and destination TCP/UDP port numbers: A Layer 4 criterion can let different applications between the same hosts select different members, depending on the implementation.

Implementation requirements and protocols

IEEE 802.1AX describes aggregation of parallel, full-duplex point-to-point links operating at the same data rate. Device configuration must also keep member parameters compatible:

• Same speed: Ports in a LAG must operate at the same rate. A 100 Mbps port cannot be mixed with a 1 Gbps port in one group.
• Same duplex mode: Members use the same mode, normally full-duplex. LACP does not form a group from half-duplex ports.
• Point-to-point: Members connect the same two systems and create parallel paths between them.
• Consistent configuration: Both ends must use matching Layer 2 parameters, such as VLAN membership or the same allowed VLAN range on a trunk.

To form a LAG, both ends must be configured. The logical interface takes over functions such as Layer 3 addressing or VLAN parameters, depending on the device mode. Group state should be checked for each member, the partner, and the logical interface separately. There are two basic approaches:

1. Static configuration (on): The administrator assigns ports to an EtherChannel on both devices. The mode exchanges no partner state and does not identify a cabling error or configuration mismatch.
2. Dynamic negotiation protocols: A negotiation protocol lets switches exchange information, check compatibility, and add or remove ports after a state change. Cisco equipment and standards-based implementations use:
   • PAgP (Port Aggregation Protocol): A protocol developed by Cisco.
   • LACP (Link Aggregation Control Protocol): An IEEE standard first described in 802.3ad and now in 802.1AX, intended to interoperate across vendors.