#TechExchange: Increasing your bandwidth through Link Aggregation

Networks across the globe continue to improve phenomenally as new technologies of multiplying bandwidth speeds continue to flood the telecommunications industry. There used a to be a time when one would have to wait for a good three hours to download a 5Mb video but the game has since been changed to current benchmarks where one can pull down a 2Gig movie within minutes when using the likes of ZOL’s fibroniks.

By Cisco Eng. Shingie Lev Muringi 

The most popular technology used by network engineers to achieve such fierce Internet speeds is configuration Link Aggregation or Ether-Channeling. Link Aggregation is the bundling of several physical links to produce a multiplied throughput on one logical link.

It is also possible to multiply the number of physical links between the switches to increase the overall speed of switch-to-switch communication.

EtherChannel technology was originally developed by Cisco as a LAN switch-to-switch technique of grouping several Fast Ethernet or Gigabit Ethernet ports into one logical channel. When an EtherChannel is configured, the resulting virtual interface is called a port channel. The physical interfaces are bundled together into a port channel interface.


A physical topology demonstration of link aggregation.

EtherChannel technology has many advantages:

  • Most configuration tasks can be done on the EtherChannel interface instead of on each individual port, ensuring configuration consistency throughout the links.
  • EtherChannel relies on existing switch ports. There is no need to upgrade the link to a faster and more expensive connection to have more bandwidth.
  • Load balancing takes place between links that are part of the same EtherChannel. Depending on the hardware platform, one or more load-balancing methods can be implemented. These methods include source MAC to destination MAC load balancing, or source IP to destination IP load balancing, across the physical links.
  • EtherChannel creates an aggregation that is seen as one logical link. When several EtherChannel bundles exist between two switches, STP may block one of the bundles to prevent switching loops. When STP blocks one of the redundant links, it blocks the entire EtherChannel. This blocks all the ports belonging to that EtherChannel link. Where there is only one EtherChannel link, all physical links in the EtherChannel are active because STP sees only one (logical) link.
  • EtherChannel provides redundancy because the overall link is seen as one logical connection. Additionally, the loss of one physical link within the channel does not create a change in the topology; therefore a spanning tree recalculation is not required. Assuming at least one physical link is present; the EtherChannel remains functional, even if its overall throughput decreases because of a lost link within the EtherChannel.

EtherChannel can be implemented by grouping multiple physical ports into one or more logical EtherChannel links.

Link Agr

Improved bandwidth speeds and fault tolerance are the major advantages of Link Aggregation

Note: Interface types cannot be mixed; for example, Fast Ethernet and Gigabit Ethernet cannot be mixed within a single EtherChannel.

The EtherChannel provides full-duplex bandwidth up to 800 Mb/s (Fast EtherChannel) or 8 Gb/s (Gigabit EtherChannel) between one switch and another switch or host. Currently each EtherChannel can consist of up to eight compatibly-configured Ethernet ports. The Cisco IOS switch can currently support six EtherChannels. However, as new IOSs are developed and platforms change, some cards and platforms may support increased numbers of ports within an EtherChannel link, as well as support an increased number of Gigabit EtherChannels. The concept is the same no matter the speeds or number of links that are involved. When configuring EtherChannel on switches, be aware of the hardware platform boundaries and specifications.

The original purpose of EtherChannel is to increase speed capability on aggregated links between switches. However, this concept was extended as EtherChannel technology became more popular, and now many servers also support link aggregation with EtherChannel. EtherChannel creates a one-to-one relationship; that is, one EtherChannel link connects only two devices. An EtherChannel link can be created between two switches or an EtherChannel link can be created between an EtherChannel-enabled server and a switch. However, traffic cannot be sent to two different switches through the same EtherChannel link.

The individual EtherChannel group member port configuration must be consistent on both devices. If the physical ports of one side are configured as trunks, the physical ports of the other side must also be configured as trunks within the same native VLAN. Additionally, all ports in each EtherChannel link must be configured as Layer 2 ports.

Note: Layer 3 EtherChannels can be configured on Cisco Catalyst multilayer switches, such as the Catalyst 3560, but these are not explored in this course. A Layer 3 EtherChannel has a single IP address associated with the logical aggregation of switch ports in the EtherChannel.

Each EtherChannel has a logical port channel interface, illustrated in the figure. A configuration applied to the port channel interface affects all physical interfaces that are assigned to that interface.

The battle for relevance continues……..follow Shingie Lev Muringi our Technology Research Specialist and Sub Editor on Twitter @ShingieMuringi1, Email [email protected] or direct Cell: 0775 380 652 for all the latest trending technological issues in and outside Zimbabwe.


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