Bridge port extenders (BPE) are initially discovered using a type-length-value (TLV) extension to the Link Layer Discovery Protocol (LLDP) along with certain LLDP-MED TLVs that specify serial number, model name, hardware version, and firmware version. After initial discovery, the controlling bridge (CB) uses the Port Extender Control and Status Protocol (PE-CSP) to control and manage the BPE. PE-CSP is a simple command/response protocol (see Port Extension Control and Status Protocol) that runs on top of the Edge Control Protocol (ECP), which provides reliable delivery (see Edge Control Protocol).
The BPE receives Ethernet packets on its extended ports, inserts an E-TAG with an E-CID identifying the source port, and forwards it on its upstream ports towards the CB. When the CB receives a packet on a cascaded port, it uses the E-CID within the E-TAG to identify the source virtual port and performs the L2/L3/policy forwarding decisions for the packet. Based on these forwarding decisions, the CB inserts an E-TAG to identify the destination and forwards the packet out the corresponding cascaded port. If the forwarding decision is to forward to another extended port, the CB inserts an E-Tag. The BPE receives the packet on its upstream port, processes the associated E-TAG, and forwards the packet out the corresponding egress extended port(s).
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