Rules, rules, rules, everything in the networking world has rules! EtherChannel is no different; it has a set of restrictions that dictate what you can and cannot do. Before running off to implement EtherChannel, know what the restrictions are so you do not get halfway through and find out it will not work the way you planned. Here are some basic guidelines on setting up EtherChannel:
You can assign up to eight ports to a channel group. Using Link Aggregation Control Protocol (LACP), you can configure 16 ports in the port group, but only eight ports can be active; the other ports are in Standby mode. This is useful when you lose links in the active group, as the standby links will activate immediately.
If you happen to configure your ports this way, then you have a very fast, very reliable inter-switch link, with a very high port cost.
You need to configure both switches for the same connection mode: Link Aggregation Control Protocol (LACP), Port Aggregation Protocol (PAgP), or EtherChannel.
PAgP is a Cisco propriety protocol, whereas LACP is an open standard. If you are creating EtherChannel with switches from other vendors, you need to use LACP.
PAgP has two configuration modes, Auto and Desirable. Auto waits for the other host to start the session, and Desirable attempts to start the session. The Auto setting will minimize the number of PAgP packets sent on the connection. An Auto-configured link can form a session with a Desirable-configured link; a Desirable can form a session with another Desirable-configured link or an Auto-configured link.
However, both ends of an inter-switch link cannot be set to Auto, or you would have two hosts on Auto looking rather dumb waiting for each other to start the session.
LACP also has two configuration modes, Active and Passive. Similar to PAgP’s Auto and Desirable, an Active link attempts to start a LACP session by sending out negotiation packets; a Passive link will respond to packets that it receives. As with Auto links in PAgP, the LACP Passive links minimizes the number of LACP packets sent on the connection.
Both ends of the link cannot be set to Passive mode, so two Active links can form a session, as can an Active link and a Passive link, but two Passive links will never send out negotiation packets and again will just be standing around looking rather dumb.
All ports in a channel group must have the same configuration for Speed and Duplex settings; otherwise, you have anarchy on your hands. Ports that are 100 Mbps Half Duplex trying to send data when the port they are paired with is a 1 Gbps Full Duplex port is like giving a German person coming to America a translator who only speaks Dutch and Russian.
All ports must be assigned to the same VLAN and have matching switchport modes.
When STP needs to send data out, it uses only one of the configured ports, instead of sending the data over all the ports.
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Source:http://www.dummies.com/how-to/content/etherchannel-basic-restrictions.html
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