Etherchannel allows a method of doubling (or more) bandwidth between switches without spanning tree protocol (STP) blocking one of the links.
Multiple physical links to be grouped into a single logical link, called an Etherchannel or port channel as defined in the 802.3AD link aggregation specification.
The multiple physical links that form the logical link are called member interfaces.
Spanning tree protocol does not operate on each individual member link in an etherchannel, the single logical link between the two devices. This allows all of the member links to operate giving access to the full total bandwidth between the switches.
Etherchannel can operate in this mode as Layer2 access or trunk interfaces, or Layer 3 forwarding.
Etherchannel has an advantage if a member link changes it status: the number of Spanning Tree Protocol topology changes or route recalculations sent across the network are reduced for as long as one of the member links stays online.
Etherchannel is negotiated via a static setting or negotiation using a dynamic protocol between two devices.
Dynamic negotiation is a preferred method over static. A major disadvantage to static setting is no health integrity check on a link between two devices. If the physical media degrades the link may be kept in a online state with its static setting. If a dynamic negotiation method was set instead, the missing negotiation packets across the link would not be received if were a fault – the member link would not form.
Dynamic Link Aggregation Protocols
There are two common Link Aggregation Protocols supported across Cisco Catalyst switches, Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP).
The protocols are not cross compatible, each end of the link between two devices must use the same protocol.
Port Aggregation Protocol (PAgP)
PAgP is a Cisco proprietary protocol. It was developed before LACP.
PAgP advertises messages across the link with the multicast MAC address 01:00:0C:CC:CC:CC with a protocol code of 0x0104.
PAgP operates in two modes:
- Auto mode: The interface set to this mode does not try to initiate the EtherChannel and does not send packets out of the interface. The interface only responds with PAgP if it is recieved on the interface from the neighbouring switch. This means if both switches are set to Auto mode, no packets are ever sent so an adjacency can not form
- Desirable mode: PAgP will automatically try to establish an Etherchannel by transmitting PAgP packets out of its interface. The etherchannel will only establish if the other end of the link is set to auto or desirable.
Link Aggregation Protocol (LACP)
LACP is an open industry standard protocol and was developed after PAgP. LACP advertises messages using the MAC address 01:80:C2:00:00:02.
LACP, similar to PAgP, operates in two modes:
- Passive mode: LACP does not transmit LACP packets out of its interface to try form an adjacency. If a LACP packet is received from a remote switch, the interface will respond to this and attempt to form a LACP link. In a situation where both ends of a link are configured as passive, the LACP link will never form.
- Active mode: LACP will actively attempt to establish an Etherchannel by transmitting packets out of the local interface. If the remote end has LACP configured as active or passive an aggregated link will form.