Network switches are used to make the network bigger and provide better connectivity throughout the company. When the business is developing there should be involved mission-critical applications and devices to the daily operation. Hence, it needs to have high availability and reliability. In this article, we are going to talk about, how to provide high bandwidth and reliability between switches.
Switch port Aggregation with EtherChannel
The switches can use different types of network cables like Ethernet, Fast Ethernet, and Gigabit. It can provide link speed. Cisco facilitates a technology called EtherChannel. It can enhance the bandwidth of the link by aggregating or bundling parallel links.
How the link aggregation increases the bandwidth between switches?
Two to eight links can be bundled in the case of Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet as 10-Gigabit EtherChannel respectively. For an instance, Fast Ethernet can provide a full duplex BW of up to 1600 Mbps (1 Fast Ethernet 100 Mbps, 8 bundles,s and then 16 full-duplex equal to 1600).
In another word, this is a link to BW expansion as increases the capacity between switches without purchasing additional hardware. Using the same cable, let’s say Fast Ethernet (200 Mbps throughput) can be incrementally expanded up to eight, 1600 Mbps as Fast EtherChannel. If you need more BW, you can use Gigabit Ethernet (2 Gbps through).
Is there a possibility to occur bridging loops and undesirable conditions when bundling?
EtherChannel avoids those conditions by bundling the parallel links to a single, logical link which can act as either an access or trunk link. Here, the end device presented at the end of the link should support EtherChannel for proper operation.
If you have a 2 Fast Ethernet bundle, you will have 400 Mbps throughput. However, a frame that goes through the Fast EtherChannel doesn’t divide into similar parts and send the frame to another switch or end device. In other words, the load isn’t always distributed equal amount of data through each individual link. This load balancing with discussed later.
What about the redundancy?
If one link fails, then the traffic is automatically moved to an adjacent link. Failover occurs in less than milliseconds and it is transparent to the end user. If the link is restored, then the traffic will automatically share with active links.
The initial requirement to bundle the link
EtherChannel supports up to eight links. The same Ethernet media type should use. Some configuration restrictions exist. Hence make sure to use similar configuration links to be bundled. All ports must belong to the same VLAN. If used as a trunk, bundle ports must be in trucking mode.
Each port has the same speed and duplex setting before being bundled. Bundled ports also must be configured with spanning tree settings.
Distributing traffic in EtherChannel
Traffic will distribute through all the links, but the traffic is not equally balancing through each individual link. Traffic is forwarded through links as per the hashing algorithm. The algorithm can be source/destination IP, or a combination of them, source/destination MAC or TCP/UDP port numbers. The hash algorithm computes and selects a link number to carry each frame. (Many details in the PDF)
EtherChannel Negotiation Protocols
Two protocols are used. Port Aggregation Protocol (PAgP Cisco proprietary) and Link Aggregation Control Protocol (LACP).
|
Negotiation |
Mode |
Negotiation Packets Sent? |
Characteristics |
|
PAgP |
LACP |
|
|
|
On |
On |
No |
All ports
channeling |
|
Auto |
Passive |
Yes |
Waits to
channel until asked |
|
Desirable |
Active |
Yes |
Actively asks
to form a channel |
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