The device PC is sending a known unicast packet to a server. The device formats the packet from transmission with the following Ethernet header, illustrated in the following figure:
RBridge A receives the packet and does a look up in its FDB table. The FDB table entry would have an FDB Server MAC address, PC VLAN, and RBridge D nickname table entry and the FDB entry would point to the next hop RBridge B‘s MAC address. The RBridge outer MAC DA is set to the functional All-RBridges-Multicast MAC address. Since RBridge A is an edge boundary RBridge, the received packet from the PC is encapsulated with a TRILL header as follows. The ingress RBridge nickname is set to RBridge A‘s nickname and the egress RBridge nickname is set to RBridge D‘s nickname.
RBridge B receives the TRILL formatted data packet. Because the packet‘s DA is RBridge B‘s MAC Address and has a TRILL Ethertype, RBridge B looks in the TRILL header to determine if the egress RBridge Nickname in the TRILL header matches its local RBridge nickname. Since it does not match, it merely does an RBridge nickname lookup for RBridge D and finds the next hop RBridge MAC address (which happens to be RBridge D‘s MAC address). RBridge B changes the RBridge SA to its MAC address and sets the RBridge DA to that of RBridge D. It also decrements the hop count in the TRILL header.
RBridge D receives the TRILL formatted packet and determines that the egress RBridge for the packet is itself. Since the RBridge is located at the TRILL egress boundary, RBridge D must decapsulate the PC data packet by removing the TRILL header and performs an L2 lookup of the Server DA located in the encapsulated Ethernet packet header. The Server DA lookup returns the egress port for the Server and the Ethernet packet is sent.
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