Two Cisco 3550 switches have two links connecting them. What is the difference between 1) making each interface a trunk and then putting both interfaces in an etherchannel  and 2) placing both links into an etherchannel and making then the etherchannel a trunk?
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Hassan BesherConnect With a Mentor Commented:
Both are the same , there are guidelines you should consider:

•Configure an EtherChannel with up to eight Ethernet interfaces of the same type.

Note Do not configure a GigaStack GBIC port as part of an EtherChannel.
•Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.

•Enable all interfaces in an EtherChannel. An interface in an EtherChannel that is disabled by using the shutdown interface configuration command is treated as a link failure, and its traffic is transferred to one of the remaining interfaces in the EtherChannel.

•When a group is first created, all ports follow the parameters set for the first port to be added to the group. If you change the configuration of one of these parameters, you must also make the changes to all ports in the group:

–Allowed-VLAN list

–Spanning-tree path cost for each VLAN

–Spanning-tree port priority for each VLAN

–Spanning-tree Port Fast setting

•An EtherChannel interface that is configured as a Switched Port Analyzer (SPAN) destination port does not join the group until it is deconfigured as a SPAN destination port.Do not configure a port that belongs to an EtherChannel port group as a secure port.

•Before enabling 802.1X on the port, you must first remove it from the EtherChannel. If you try to enable 802.1X on an EtherChannel or on an active port in an EtherChannel, an error message appears, and 802.1X is not enabled. If you enable 802.1X on a not-yet active port of an EtherChannel, the port does not join the EtherChannel.

•For Layer 2 EtherChannels:

–Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks. Interfaces with different native VLANs cannot form an EtherChannel.

–If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (ISL or 802.1Q) is the same on all the trunks. Inconsistent trunk modes on EtherChannel interfaces can have unexpected results.

–An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not form an EtherChannel even when PAgP is set to the auto or desirable mode.

–Interfaces with different spanning-tree path costs can form an EtherChannel if they are otherwise compatibly configured. Setting different spanning-tree path costs does not, by itself, make interfaces incompatible for the formation of an EtherChannel.

•For Layer 3 EtherChannels, assign the Layer 3 address to the port-channel logical interface, not to the physical interfaces in the channel.
colstConnect With a Mentor Commented:
I believe it's best to start out with no config on the interfaces, add those to the channel group and then configure the etherchannel with trunking, etc.  settings pass down to the interface, which you can see in the running config. This assures config is consistent across all interfaces, and you don't have to type it multiple times.
From what I remember that's actually best practice.
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