1. A method for selecting between Layer 2 adapters, the method comprising: caching a received Layer 3 packet; reading the Layer 3 packet; extracting the type and parameter value of a requirements from the Layer 3 packet; transmitting the obtained results; acquiring instant metric parameter values of each of the Layer 2 adapters; transmitting obtained results; calculating an adapter identifier and a Layer 3 packet metric according to the extracted type and parameter value of the requirements and each calculated adapter metric value, transmitting the resultant adapter identifier and the Layer 3 packet metric, determining whether a link corresponding to an external destination Layer 2 adapter is a unidirectional link or a bidirectional link according to the obtained adapter identifier, and if the link is a unidirectional link, performing parameter negotiation through a bidirectional link to obtain a unidirectional link destination address mapping parameter, and transmitting the mapping parameter to an external destination Layer 2 adapter through the bidirectional link; acquiring a Layer 2 address of each adapter associated with a destination Layer 3 address; calling an external Layer 2 adapter driver according to the adapter identifier and Layer 3 packet metric, wherein the Layer 2 adapter driver performs packaging and sending of the packet.