Link aggregation is a technique for inverse multiplexing two or more Ethernet circuits between two devices. Link aggregation was standardized in IEEE 802.3ad, which has since been incorporated into the main IEEE 802.3 standard.
The Need for IEEE 802.3ad Link Aggregation
Since it's inception, Ethernet speeds have increased by orders of magnitude, from 10 Mbps to 100 Mbps to 1 Gbps to 10 Gbps. There was even an older 1 Mbps standard (now obsolete). For many application environments, leaping by a factor of ten is inconsequential. After all, if the port costs only a few dollars, the fact that someone might be wasting a significant percentage of the bandwidth becomes irrelevant. However, is something drives up the price of a circuit, or if speeds are needed above the highest available rate, suddenly inverse multiplexing several circuits into one communications channel can look attractive. The former can happen for new transmission rates (before costs drop due to economies of scale), or when the circuit is across a MAN or WAN environment. The latter can happen in application environments that have high throughput demands, like storage area networking and imaging or video applications.
It should be noted that it is possible for two (or more) Ethernet circuits to be deployed between two pieces of equipment without aggregating them into a single logical communications channel. However, there are consequences. If the links are placed between two Ethernet switches, for example, it will create the potential for broadcast storms between the switches. To resolve this, the switches implement the Spanning Tree Protocol (STP). This eliminates the broadcast storms by eliminating the redundancy (i.e., blocking all but one of the redundant paths). The extra Ethernet circuits are only activated (but never more than one) if the active circuit fails. If the multiple links are placed between routers, then their use is under the control of the routing protocol, putting more burden and complexity into the routing algorithms. Aggregating the links avoids all of these problems.
Features of IEEE 802.3ad Link Aggregation
IEEE 802.3ad provides for the following features:
- Load sharing: All of the links aggregated into a group are used to transmit frames. However, the technology does not provide for proper load balancing (i.e., does not ensure that all circuits in the group are used equally).
- Automatic configuration: Devices can be configured to automatically detect multiple links and automatically aggregate them into a common group. Link Access Groups (LAG) will typically resolve in less than one second.
- Deterministic: The technique is predictable and controllable.
- Frame order: Link aggregation is designed to ensure that frame order is preserved. This is achieved by sending all of the frames between two specific MAC addresses across the same circuit within the group.
Benefits of IEEE 802.3ad Link Aggregation
Some of the benefits of IEEE 802.3 Link Aggregation include:
- Higher throughput: Obviously, if you combine multiple circuits into one inverse multiplexed group, the group has greater throughput than the individual circuits could. Link aggregation also avoids the problem of some of the circuits being blocked by STP.
- Increased availability: Members of a group can be removed from the group rapidly if they fail. If the circuits are diversely routed, this can enhance the survivability of an inter-device link.
- Reduced cost: This is not always a given, but it can certainly be true. For example, when a new Ethernet transmission rate is introduced, interface cards can easily be $1,000 or more. It takes time for them to drop. Meanwhile, the previous speed is usually already down in the low hundreds, if not tens, of dollars. It may be less expensive to aggregate a few of these interfaces than to jump to the new speed. It may also be true for MAN and WAN connections, where the carrier charges by the bandwidth every month. This is becoming less true, however, as carriers offer rate-limited Ethernet services.
Limitations of IEEE 802.3ad Link Aggregation
Some of the limitations of the technology include:
- Same two devices: Multipoint aggregations are not supported, so link aggregation only works between two devices.
- Same speeds on links: Although it is possible, according to the standard, for aggregated links to operate at difference speeds, this practice is not typically recommended and most products don't support it.
- Same duplex mode: The standard requires that all of the members of a group are either half duplex or full duplex. Most products require all links to be full duplex.
- No load balancing: This is a consequence of requiring all frames between the same two MAC addresses to cross the same link withing the LAG. With this requirement in place, there is no way to ensure that traffic is being balanced across all members of the LAG. A bandwidth intensive session can heavily burden one link while the other links are carrying lighter sessions. To further complicate things, the load sharing (distribution) algorithm is not defined in the standard and is left to the vendors’ discretion. The application of an algorithm could be scenario-dependent. Some possibilities include distribution based on source MAC address, destination MAC address, or perhaps some higher layer information such as Transport Layer socket numbers.
Applications for IEEE 802.3ad Link Aggregation
Although link aggregation can be implemented between any two devices, the most common environments where it is found include:
- Switch-to-Switch: to enhance throughput and/or survivability between Ethernet switches
- Switch-to-Router: to enhance throughput and/or survivability between an Ethernet switch and a local router
- Switch-to-Server: to provide additional capacity or survivability between a network and the attached servers
- Server-to-server: to provide an out-of-band path by which servers can mirror or backup information
- Metro Ethernet: to provide a resilient connection between the subscriber and the service
Competitors to IEEE 802.3ad Link Aggregation
The IEEE 802.3ad Link Aggregation standard is not the only game in town. Other options include:
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