Configure and verify EtherChannels

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

Under the exam number 300-115 switch there is a topic that is "how to configure and verify ether channels". We will in this chapter explain in details about this topic so that the readers can get a better idea of the topic. The topic is a huge one but we have tried to discuss those points that are important from the CCNP exam point of view.

The Ether Channels have one main purpose and that is to provide with switches, routers and severs that are high on speed as well as on tolerating faults. It can be used anywhere in the network where you feel that bottle necks can occur. If a link in a network fails the Ether Channels will automatically redirect the traffic.

The Ether Channels contains a number of individual Gigabit Ethernet and fast Ethernet that are made into a bundle to create a single logical link. Each of the Ether Channels has the capacity to contain up to eight Ethernet interfaces that have been properly configured. All the Ether Channels will generally have the same speed. You must also keep in mind that the Ether Channels must be configured either in layer 3 or layer 2 interfaces.

LACP, PAgP, manual

We will now discuss a little about the packets that you will come across as you deal with the Ether Channels. The PAgP(physical learners and aggregate port learners) packets are allowed to be interchange packets only with interfaces that are configured with auto and desirable modes. There are some switch interfaces that will exchange LACP (link aggression protocol) packets but this is only with the partner interface that is either configured into an active or a passive mode.

If the interface is in an "on" mode then it will not exchange the packets PAgP and LACP packets. If a device is not capable of PAgP interface then a silent mode is used. The Ether Channels cannot be configured in both the LACP and the PAgP modes. You must know here that both the passive and active LACP modes will allow an interface to negotiate with a partner interface to see if they can form an Ether Channel base.

You must know one thing here and that is an interface that is in a passive mode cannot form Ether Channel with an interface that too is in a passive mode. An Ether Channels cannot be configured on both LACP and PAgP modes at the same time.

Layer 2, Layer 3

Both layer 2 and three need logical interface. The layer 3 interface will create a logical interface using the interface port-channel configuration command. When the layer 2 interface is created logical interface is also dynamically created. Using the channel group interface configuration command we can manually assign the interface to the EtherChannel.

This command will bind the logical and the physical ports together. If a port joins the EtherChannel the physical interface of the port will automatically shut down. If the port leaves the port channel then the physical interface is brought up. The interface will have the same configuration that it had before it joined the EtherChannel. If a configuration was changed in the logical interface of the EtherChannel it does not mean that the change will apply to all the member ports of the channel.

Some points that you must keep in mind as you configure layer 2 Ether Channels are as follows:

  1. The entire interface in the Ether Channels must be assigned to the same VLAN. You can also configure them as trunks. If the interface have different native then VLANs cannot form the Ether Channels.
  2. When the Ether Channels are configured from a trunk interface you must verify the trunking mode. If the trunk modes are not same then the results may not be what you are expecting.
  3. The Ether Channels will support the same allowed range of VLANs on all the interfaces that are in the trunking layer 2 Ether Channels. If the allowed range of VLANs is not the same the Ether Channels will not form the Ether Channels even if the PAgP is setup.

If you want to configure layer 3 Ether Channels then you must configure the Ether interface with the channel group interface configuration command. This will create the port channel logical interface. Keep this point in mind that you cannot create layer 2 interfaces into a port channel interface that was created manually.

Load balancing

To configure the load balancing Ether Channels you will first have to enter the global configuration mode. Using the command port - channel - load -balance (dst-mac |sec- mac) command you can configure the Ether Channels. The src-mac is a default setting. When this is enabled then the load distribution that is based on the destination and source IP address is also enabled.

In step three using the end command you can return to the privileged EXEC mode. As the step four you can verify the entries made using the command "show Ether Channel load- balance&rdquo. The last step that we are about to discuss is an optional one. In this you can save the entries that were made by you in a configuration file.

EtherChannel misconfiguration guard

It is a clear logic that the frame that is received by a link in an Ether Channel bundle must not be sent back through the links that make up the bundle. Well the logic does not work all the time in this manner as a result you may come across errors in the network.

The EtherChannel misconfiguration guard if used will tell you where exactly the error is happening so that you can take the necessary action to correct the error. It can also point out errors of cables.

We hope that you found this chapter on "how to configure and verify ether channels" interesting and informative. Do keep the preparation for the CCNP exam on these lines and you will surely be able to get a good score. All the best for the exams!

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