Trunking vs Etherchannel

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Noticed several colleagues of mine are still confused with "trunking" and "etherchannel". These two -- some would argue grammatically similar -- have very different functions and implications.

Trunking refers to (multiple) VLANs that are going through an interface. That is, for example, if you say that "a Cat6500 with GigabitEthernet1/1 trunks VLANs 1, 50, and 100", it means that these VLANs (and only these VLANs) are permitted to go through Gi1/1. The implication is that this forms a basic security feature, by permitting only specific VLANs; other traffic belonging to other VLANs will be permitted on this trunk interface.

Etherchannel -- aka channel-group, port-channel, or port group -- on the otherhand, refers to multiple (physical) interfaces bundled together into a single (logical) interface. That is, for example, if you say that "Cat6500 Port-channel100 includes GigabitEthernet2/1-4", these means that Gi2/1 to Gi2/4 are configured together as a single logical interface. The implication is that the bandwidth of these interfaces are combined and shared -- hence, in this example, Port-channel has a total bandwidth of 4Gbps (1Gbps per interface x 4 interfaces ).

So to state the basic difference between the two:

Trunking "bundles" VLANs together that are permitted on any single interface.
Etherchannel "bundles" (physical) interfaces together into a single logical interface.


Question: Can you combine an interface to have both trunking and etherchannel?

Answer: This is question is a little vague. In a way, yes. Here's a typical scenario actually being used in many companies:

Topology:
DeviceA. GigabitEthernet1/1 <=====> DeviceB.GigabitEthernet1/1
DeviceA. GigabitEthernet1/2 <=====> DeviceB.GigabitEthernet1/2

Requirement:
Permit only VLANs 100 and 200 through the link between DeviceA and DeviceB. Also, increase the link bandwidth between these two devices to 2Gbps. To summarize:

DeviceA.GigabitEthernet1/1 - trunking for vlans 100 and 200, share bandwidth with Gi1/2;
DeviceA.GigabitEthernet1/2 - trunking for vlans 100 and 200, share bandwidth with Gi1/1;

DeviceB.GigabitEthernet1/1 - trunking for vlans 100 and 200, share bandwidth with Gi1/2; DeviceB.GigabitEthernet1/2 - trunking for vlans 100 and 200, share bandwidth with Gi1/1;

To achieve this, we arbitrarily assign a channel group number IDs. For this example, we select 300 for DeviceA, so we have Port-Channel300 which will group Gi1/1 and Gi1/2. The basic configuration for DeviceA, then, would look like the following:

interface GigabitEthernet1/1
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
channel-group 300 mode desirable
!
interface GigabitEthernet1/2
switchport switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
channel-group 300 mode desirable
!
interface Port-Channel300
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
!

The device on the remote end, DeviceB, should be configured similarly as well.

Similarly, we select Port-Channel500 for DeviceB.

interface GigabitEthernet1/1
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
channel-group 500 mode desirable
!
interface GigabitEthernet1/2
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
channel-group 500 mode desirable
!
interface Port-Channel500
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 100,200
!

With that, you're done!

Here are some points you should take note of:

Important Note #1: The channel-group ID only has local significance. In this example, DeviceA assigns Port-channel300 while DeviceB uses Port-channel500.

Important Note #2: The VLANs trunked on the port-channel interface and the member interfaces should match. If there is a mismatch, the status of the interfaces will be down.

Important Note #3: The actual syntax would vary, depending on the IOS image currently being used. Here's a general hint: IOS 12.0 uses the "port group" interface command, while IOS 12.1 onwards uses the "channel-group" interface command.