Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 962

RSTP Configuration RSTP is supported in a VLT domain. Before you configure VLT on peer switches, configure RSTP in the network. RSTP is required for initial loop prevention during the VLT startup phase. You may also use RSTP for loop prevention in the network outside of the VLT port channel. For information about how to configure RSTP, Rapid Spanning Tree Protocol (RSTP). Run RSTP on both VLT peer switches. The primary VLT peer controls the RSTP states, such as forwarding and blocking, on both the primary and secondary peers. Dell Networking recommends configuring the primary VLT peer as the RSTP primary root device and configuring the secondary VLT peer as the RSTP secondary root device. BPDUs use the MAC address of the primary VLT peer as the RSTP bridge ID in the designated bridge ID field. The primary VLT peer sends these BPDUs on VLT interfaces connected to access devices. The MAC address for a VLT domain is automatically selected on the peer switches when you create the domain (refer to Enabling VLT and Creating a VLT Domain). Configure both ends of the VLT interconnect trunk with identical RSTP configurations. When you enable VLT, the show spanningtree rstp brief command output displays VLT information (refer to Verifying a VLT Configuration). 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. 962 Virtual Link Trunking (VLT)