229PBNA provider bridge network (PBN) connects a PBBN to a customer network. A customer network canconnect to a PBBN directly or through a PBN.BEBA backbone edge bridge (BEB) is an edge device in a PBBN. A BEB encapsulates frames from acustomer network by using PBB. It decapsulates PBB frames from a PBBN and forwards them to acustomer network.BCBA backbone core bridge (BCB) is a core device in a PBBN. It forwards a PBB frame according to itsB-MAC and B-VLAN. A BCB only forwards frames and learns MAC addresses within the PBBN. It doesnot learn a large number of customer MAC addresses. This reduces network deployment costs, and thePBBN is more expandable.B-MAC and B-VLANA backbone MAC address (B-MAC) is a MAC address associated with a PBB bridge. A backbone VLAN(B-VLAN) is a VLAN assigned by the service provider for transmitting customer traffic on the PBBN.For customer frames to be transmitted across a PBBN, the ingress BEB encapsulates them in MAC-in-MACformat. In the outer frame header, the source MAC address is a B-MAC of the ingress BEB, and thedestination MAC is a B-MAC of the egress BEB. All devices in the PBBN forward the PBB frames basedon the destination B-MAC and B-VLAN.Uplink port and downlink portA port that connects a BEB to a PBBN is an uplink port, and a port that connects a BEB to a customernetwork is a downlink port. After frames from the customer network are encapsulated in PBB frames, theyare forwarded out of the corresponding uplink ports on the BEB. After PBB frames from the PBBN aredecapsulated, they are forwarded out of the corresponding downlink ports on the BEB according to thecustomer MAC addresses.PBB VSI and I-SIDIn a PBBN, a PBB VSI is a virtual switch provided by the service provider, and it is uniquely identified bya backbone service instance identifier (I-SID). A VSI acts as a virtual switch with all conventional Ethernetswitch functions, including source MAC address learning, MAC address aging, and flooding.PBB frame formatFigure 65 shows the format of a PBB frame.Figure 65 PBB frame format
