Primary and Secondary VLT PeersTo prevent issues when connectivity between peers is lost, you can designate Primary and Secondaryroles for VLT peers . You can elect or configure the Primary Peer. By default, the peer with the lowestMAC address is selected as the Primary Peer. You can configure another peer as the Primary Peer usingthe VLT domain domain-id role priority priority-value command.If the VLTi link fails, the status of the remote VLT Primary Peer is checked using the backup link. If theremote VLT Primary Peer is available, the Secondary Peer disables all VLT ports to prevent loops.If all ports in the VLTi link fail or if the communication between VLTi links fails, VLT checks the backup linkto determine the cause of the failure. If the failed peer can still transmit heartbeat messages, theSecondary Peer disables all VLT member ports and any Layer 3 interfaces attached to the VLANassociated with the VLT domain. If heartbeat messages are not received, the Secondary Peer forwardstraffic assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peerreassigned as the Primary Peer retains this role and the other peer must be reassigned as a SecondaryPeer. Peer role changes are reported as SNMP traps.RSTP and VLTVLT provides loop-free redundant topologies and does not require RSTP.RSTP can cause temporary port state blocking and may cause topology changes after link or nodefailures. Spanning tree topology changes are distributed to the entire layer 2 network, which can cause anetwork-wide flush of learned MAC and ARP addresses, requiring these addresses to be re-learned.However, enabling RSTP can detect potential loops caused by non-system issues such as cabling errorsor incorrect configurations. To minimize possible topology changes after link or node failure, RSTP isuseful for potential loop detection. Configure RSTP using the following specifications.The following recommendations help you avoid these issues and the associated traffic loss caused byusing RSTP when you enable VLT on both VLT peers:• Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which aredirectly connected to end stations or server racks. Disable RSTP on ports connected directly to Layer3-only routers not running STP or configure them as edge ports.• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has thesecond-best bridge ID in the network. If the primary VLT peer node fails, the secondary VLT peernode becomes the root bridge, avoiding problems with spanning tree port state changes that occurwhen a VLT node fails or recovers.• Even with this configuration, if the node has non-VLT ports using RSTP that you did not configure asedge ports and are connected to other Layer 2 switches, spanning tree topology changes are stilldetected after VLT node recovery. To avoid this scenario, ensure that you configure any non-VLTports as edge ports or disable RSTP.VLT Bandwidth MonitoringWhen bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (shown in the followingmessage) and an SNMP trap are generated.%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-channel 25)crosses threshold. Bandwidth usage (80 )856 Virtual Link Trunking (VLT)
