Sometimes it can be beneficial to aggregate links between layer 2 or layer 3 switches for the purpose of creating more bandwidth, and at the same time even to provide resilience by having multiple physical links.
Link Aggregation is a method of combining multiple physical network links into a single logical link, thereby increasing the capacity and availability of a single physical and logical communications channel between devices, either switches or end stations such as servers.
Link Aggregation takes advantage of existing Fast Ethernet and Gigabit Ethernet technology and STP (Spanning Tree Protocol) to manage loops. Two or more Fast Ethernet or Gigabit Ethernet connections are combined in order to increase the bandwidth capability and to create resilience and redundant links. Sets of multiple parallel physical links between two devices are grouped together to form a single logical trunk link.
Most link aggregation protocols also provide for load balancing, where the processing and communications activity is distributed across several links in a trunk, so that no single physical link is overloaded.
Some of the benefits of these aggregation protocols and technologies include:
High Link Availability
By aggregating a number of physical links into a single logical link, we prevent the failure of any single component link from disrupting communications between any communicating end devices in the network. Instead of the failure of a link causing a loss of communications, the aggregated link merely suffers a loss of available capacity without actually interrupting the data flow.
Increased Link Capacity
Performance can be significantly improved due to the increased capacity afforded by the use of multiple physical Ethernet links in the trunk channel. A single Ethernet links will normally be rated at 10Mbps, 100Mbps or 1000Mbps, but with aggregation we can increase that capacity up to 8 times. The tradeoff is that we need up to 8 available ports on each switch at the end of the trunk channel. As previously stated, as well as increasing capacity, we also gain from the fact that no single physical link will cause a failure of the trunk channel.
Aggregating can sometimes be implemented rather than a full equipment upgrade
If the link capacity is to be increased, there are usually two options, either upgrade the link capacity by employing devices with faster ports (Fast Ethernet to Gigabit Ethernet, for example), or use an aggregate multiple links (4 x Gigabit Ethernet links can form a 4 Gigabit aggregated trunk link).
There are a number of situations where Link Aggregation is commonly employed:
- Switch to Switch Connections
- Switch to Station (Server or Router) Connections
- Station to Station Connections
Switch to Switch Connections
This is the most common use for link aggregation, where complete workgroups are connected together by an aggregated trunk link.
This type of configuration will require the dedicated use of multiple ports on switches at each end of the trunk, which will reduce the number of ports available for connection to external devices.
Switch to Server or Switch to Router Connections
Busy Servers can easily saturate a single 100Mbps or even 1000Mbps link, therefore making the link capacity the limiting factor when it comes to server or application performance.
From the perspective of a switch, a router is seen simply as an end station, much in the same way as a server or workstation is, so we can aggregate links between a switch and a router or server to improve both capacity and, with the right configuration, performance.
Server to Server or Router to Router Connections
The Servers or Routers can have multiple Network Interfaces, much in the same way that an Ethernet Switch does. We can therefore aggregate connection using these Network Interfaces. The improved high-capacity, logical trunk links provide the servers or routers with the bandwidth needed to ensure high performance, with the added bonus of redundancy.
Look out for a follow-up article on LACP (Link Aggregation Control Protocol), for a description of one method of link aggregation.