Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 987

Preventing Forwarding Loops in a VLT Domain During the bootup of VLT peer switches, a forwarding loop may occur until the VLT configurations are applied on each switch and the primary/secondary roles are determined. To prevent the interfaces in the VLT interconnect trunk and RSTP-enabled VLT ports from entering a Forwarding state and creating a traffic loop in a VLT domain, take the following steps. 1 Configure RSTP in the core network and on each peer switch as described in Rapid Spanning Tree Protocol (RSTP). Disabling RSTP on one VLT peer may result in a VLT domain failure. 2 Enable RSTP on each peer switch. PROTOCOL SPANNING TREE RSTP mode no disable 3 Configure each peer switch with a unique bridge priority. PROTOCOL SPANNING TREE RSTP mode bridge-priority Sample RSTP Configuration The following is a sample of an RSTP configuration. Using the example shown in the Overview section as a sample VLT topology, the primary VLT switch sends BPDUs to an access device (switch or server) with its own RSTP bridge ID. BPDUs generated by an RSTP-enabled access device are only processed by the primary VLT switch. The secondary VLT switch tunnels the BPDUs that it receives to the primary VLT switch over the VLT interconnect. Only the primary VLT switch determines the RSTP roles and states on VLT ports and ensures that the VLT interconnect link is never blocked. In the case of a primary VLT switch failure, the secondary switch starts sending BPDUs with its own bridge ID and inherits all the port states from the last synchronization with the primary switch. An access device never detects the change in primary/ secondary roles and does not see it as a topology change. The following examples show the RSTP configuration that you must perform on each peer switch to prevent forwarding loops. Configure RSTP on VLT Peers to Prevent Forwarding Loops (VLT Peer 1) Dell_VLTpeer1(conf)#protocol spanning-tree rstp Dell_VLTpeer1(conf-rstp)#no disable Dell_VLTpeer1(conf-rstp)#bridge-priority 4096 Configure RSTP on VLT Peers to Prevent Forwarding Loops (VLT Peer 2) Dell_VLTpeer2(conf)#protocol spanning-tree rstp Dell_VLTpeer2(conf-rstp)#no disable Dell_VLTpeer2(conf-rstp)#bridge-priority 0 Virtual Link Trunking (VLT) 987