EtherChannel (or Port Channel) is a method to aggregate bandwidth or buldle ports between networking devices to provide or increase bandwidth capacity.
The actual number of links which can be aggregated depends on the hardware and software capabilities. Typically up to 8 links can be aggregated and it is recommended that an even number of links are configured in the Ether Channel.
When links are aggregated the traffic is distributed across the links of the Ether Channel according to an Ether Channel Load balancing algorithm. The default is typically source-and-destination MAC address forwarding.
Link Aggregation Protocol (LACP) is defined in IEE 802.3ad and is the standards based solution. It is recommended for cross-stack Ether Channel. LACP can be used with the following platforms and versions:
•Catalyst 6500/6000 series with Cisco IOS Software Release 12.1(11b)EX and later
•Catalyst 4500/4000 Series with Cisco IOS Software Release 12.1(13)EW and later
•Catalyst 2960/ 3750 Series with Cisco IOS Software Release 12.2(25)SEC and later
Port Aggregation Protocol (PAgP) is the Cisco implementation, which pre-dates LACP. This may be required, for example when connecting a new switch to an existing switch with an older IOS.
Cross-Stack Ether Channel
Cross-Stack Ether Channel is supported on 3750 and 2960-S stacks. It allows the creation of an Ether Channel with member ports across different stack members.
Currently Each EtherChannel can consist of up to eight compatibly configured Ethernet ports. All ports in each Ether Channel must be configured as either Layer 2 or Layer 3 ports. The number of EtherChannels on a Catalyst 3750 is limited to 48.