Flows that require flooding are handled similarly. Distribution trees can be optimized to minimize unwanted packet forwarding. These control plane optimizations to reduce flooding are discussed in more detail in Section 4.1.15. If the PC doesn‘t know the MAC address of the Server, the PC formats a broadcast packet. In the case of IP, this would be an IP ARP request, but the type of packet is irrelevant for the purpose of this example.
RBridge A receives the packet and determines that the packet is formatted as a broadcast packet. RBridge A encapsulates the packet in a TRILL header and sets the M-bit in the TRILL header to ‘1‘ to indicate that encapsulated packet is a multicast packet. The selected egress RBridge nickname represents a distribution tree and not specific egress RBridge. This instructs transit RBridges to flood the packet along the calculated tree topology so that each egress RBridge receives one copy of the packet. The ingress RBridge nickname is set to RBridge A‘s nickname and the hop count value is initialized to the configured maximum number of RBridge hops. The RBridge outer MAC DA is set to the functional All-Bridges-Multicast MAC address.
RBridge B receives the TRILL formatted packet and assumes that the egress RBridge nickname is a distribution tree nickname and the packet must be flooded, as indicated by the ‘M‘ bit in the TRILL header. RBridge B must forward the packet to both RBridge C and RBridge D. RBridge B decrements the hop count, updates the RBridge SA, and replicates the packet sending two copies, one to RBridge C and one to RBridge D.
RBridge C receives the TRILL data packet and decapsulates the packet and forwards the broadcast packet onto its local VLAN. RBridge C also learns that the PC MAC Address is associated with the nickname of RBridge A. Since the server doesn‘t reside on the local VLAN of RBridge C, no response to the broadcast packet is received. RBridge D performs the same forwarding action as RBridge C and also learns that RBridge A (by examining the ingress nickname field in the TRILL header) is the egress RBridge to reach PC MAC Address. Since the server is located on the local VLAN connected to RBridge D, after receiving the broadcast packet, the server replies with a unicast response to the PC. RBridge D forwards the unicast response back to the PC. Since RBridge D has now learned the egress RBridge for the PC MAC address, the TRILL header ingress RBridge nickname is set to RBridge D and the egress RBridge nickname is set to RBridge A. The TRILL header M-bit is set to zero, indicating that the encapsulated packet is a Unicast Packet. The RBridge next hop lookup is executed to determine the next-hop that reaches RBridge A‘s nickname and the response is returned to the PC via RBridge B and RBridge A.
Print
this page
Email this topic
Feedback
View PDF
Download EPUB