As a logical interface with multiple member physical interfaces transparently transmitting upper-layer data, a trunk interface must comply with the following rules:
Parameters of the member physical interfaces on both ends of a trunk link must be consistent. The parameters include:
Number of member interfaces bundled on each end
Transmission rate of member interfaces
Duplex mode of physical interfaces
Traffic-control mode of physical interfaces
Data must be transmitted in sequence.
A data flow is a set of data packets with the same source and destination MAC addresses and IP addresses. For example, the traffic over a Telnet or FTP connection between two devices is a data flow.
Before a trunk is configured, frames that belong to a data flow can reach their destination in correct order because only one physical connection exists between two devices. When the trunk interface is configured, frames are transmitted by multiple physical links. If the second frame is transmitted over a different physical link than the first frame, the second frame may reach the destination before the first.
To prevent frame mis-sequence, a datagram forwarding mechanism is used to ensure the correct order of frames belonging to the same data flow. This mechanism categorizes data flows based on MAC or IP addresses. The datagrams that belong to the same data flow are transmitted over the same physical link.
After the datagram forwarding mechanism is introduced, frames are transmitted in either of the following manners:
Frames with the same source and destination MAC addresses are transmitted over the same physical link.
Frames with the same source and destination IP addresses are transmitted over the same physical link.