How to Configure EtherChannel Easily? - A Demonstration

Ethernet is a LAN technology based on the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard. Ethernet is a shared medium between the networking devices and provides the specific bandwidth to the end users. Ethernet also evolves as the users and their need of accessing the resources increases.  

The evolution helps end-users’ effective access to resources, both on- and off- the campus network. However, today's mission-critical applications and services demand networks that provide high availability and reliability. This article will focus on the technology that you can use in your network to provide higher bandwidth and reliability between switches.

Switch Port Aggregation with EtherChannel

We need to expand our network, to accommodate the increasing number of end-users. And the selection of the switch port speed also depends on the factors of, how many users, applications type, etc.

In the above graphic (Figure-1), 4-PCs, PC1 to PC4, want to access the File Server (File_Server-FS1). There can be more end devices too. But, there is only one link between the switches SW1 and SW2. If that link goes down, the communication between the devices will be no more. For the high availability and reliability, we need more than one link between the devices. Right?

We added three more links between SW1 and SW2. If any link will go down, another will forward the traffic. It might seem logical to simply add more links between two switches to scale the bandwidth incrementally.

What do you think, now the available bandwidth will be four time?
No, because each link acts independently.

In case of redundant links between the switches, Spanning Tree Protocol is responsible for providing the layer 2, loop-free network. For this, STP will place only one link in the forwarding state and rest of the links will be in the blocking state.

In this case, only one link is forwarding the traffic out of three links. It means that our resources are underutilized.

How can we use all these four links, without occurring the bridging loop?

Cisco offers a technology of "EtherChannel", which provides the method of logically aggregating or bundling the parallel links. Two to eight links of either Fast Ethernet (FE), Gigabit Ethernet (GE), or 10-Gigabit Ethernet (IOGE) can be bundled as one logical link of Fast EtherChannel (FEC), Gigabit EtherChannel (GEC), or 10-Gigabit Etherchannel (IOGEC), respectively.

As the above graphic presents, the four 1Gbps Physical links, bundled into a single 4Gbps EtherChannel Logical link. Now for STP only one link exists and it will be in forwarding state, i.e. all the links in this bundle will be in forwarding state.

Distribution of the traffic across the links, is dependent upon the what Load-balancing hash algorithm will be in use.

EtherChannel also known as:

• Port Channel
• Channeling
• Link Aggregation (LAG)
• NIC Teaming

Working of EtherChannel or Port Channel

EtherChannel can be configured statically or dynamically. Statically configured EtherChannel will not exchange any negotiation information of the Port Channel. And for dynamically configuration, there are two types of EtherChannel negotiation protocols:

1. Port Aggregation Protocol (PAgP) is a Cisco proprietary solution
2. Link Aggregation Control Protocol (LACP) is IEEE 802.3 AD a standards based

PAgP Modes

PAgP negotiates EtherChannel formation and maintenance:

• On: channel member without negotiation (no protocol)
• Desirable: actively ask if the other side can/will
• Auto: passively wait for other side to ask
• Off: EtherChannel not configured on interface

LACP Modes

• On: channel member without negotiation (no protocol)
• Active: actively ask if the other side can/will
• Passive: passively wait for other side to ask
• Off: EtherChannel not configured on interface

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7-Steps for EtherChannel or Port Channel Configuration 

Before starting the configuration of EtherChannel, we should consider the following guidelines:

1. Port-channel interface configuration changes affect the EtherChannel.
2. The physical interface configuration changes affect the interface only.
3. EtherChannel cannot be used if SPAN is a destination port.
4. All interfaces within an EtherChannel must have same configuration:

• Same speed and duplex.
• Same mode (access or trunk).
• Same native and allowed VLANs on trunk ports.
• Same access VLAN on access ports.
• Configure these parameters on the port-channel interface.

Let us use the same topology for the configuration of EtherChannel.

Step-1 : Enable RSTP

Decide which interfaces do you want to aggregate in the Port-Channel, in our example we are going to use E3/0 - E3/3. In the below figure you can see STP is blocking three ports on SW2, only port E3/0 is in the forwarding state.

However, this step is optional, we should look to configure the RSTP for faster convergence as compare to STP.

Run these commands on both switches:

Step-2 : Create VLAN

We will create the VLAN 100 on both switches and assign the ports to that VLAN as per the topology.

These ports will be trunk ports.

Step-3 : Configure Port Channel

We can see, SW1 and SW2 successfully created a Port-Channel Po1

Step-4 : Observation of Traffic Distribution

You can change the selection of the Load-balancing method as per the switch model and requirement of the traffic flow in the network. In this step we will observe, the traffic distribution behavior in the Po1.

PC-1 is sending the ICMP packets to File_Server-FS1

I have used the 'clear counters' command, for clearing the counter. Then I run the ping command on the PC-1 (source) to File_Server-FS1 (destination).

Now we will check, which interface will transfer the traffic between the PC-2 (source) to File_Server-FS1 (destination).

PC-2 is sending the ICMP packets to File_Server-FS1.

PC-3 is sending the ICMP packets to File_Server-FS1

In step 4, we can observe that current method of load-balancing is not distributing the traffic among the links. So, we need to select the another method of load-balancing.

Step-5 : Etherchannel Load Balancing

We need to implement the proper method of Load-Balancing which is suitable for our network and can transfer the traffic between the links. Here is the list of possible methods of EtherChannel Load-Balancing.

It is locally significant and outbound, i.e. on the SW1, we can implement the different load-balancing method as compare to the SW2.

In our topology, when the traffic is going from the PCs to File_Server-FS1, we have many sources and one destination.

In this situation, we can pick one of the load-balancing method given below:

• src-mac
• src-ip
• Src-dst-mac
• Src-dst-ip

On the other hand, when traffic is going from the File_Server-FS1 to PCs, we have one source and many destinations.

In this situation, we can select one of the load-balancing method given below:

• dst-mac
• dst-ip
• Src-dst-mac
• Src-dst-ip

We are going to use 'src-ip' EtherChannel Load-Balancing method on SW1 and 'dst-ip' on SW2.

Step-6 : Verification of Load Balancing from PCs to Server

PC-1 is sending the ICMP packets to File_Server-FS1

PC-2 is sending the ICMP packets to File_Server-FS1

PC-3 is sending the ICMP packets to File_Server-FS1

PC-4 is sending the ICMP packets to File_Server-FS1

 In the step 6, we can see this load-balancing method is distributing the traffic between the links. If we will use more PCs or end devices, the traffic can be distributed among all the links.

Step-7 : Verification of Load Balancing from Server to PCs

File_Server-FS1 is sending the ICMP packets to PC-1

File_Server-FS1 is sending the ICMP packets to PC-2

File_Server-FS1 is sending the ICMP packets to PC-3

File_Server-FS1 is sending the ICMP packets to PC-4

Finally, we can say that properly check the flow of the traffic among the links. If the existing load-balancing method is not working, then use the another.

What do you think about this demonstration of EtherChannel configuration?

Kindly let us know your opinion.

Thanks.