If the VLTi link fails, the status of the remote VLT Primary Peer is checked using the backup link. If the remote VLT Primary Peer isavailable, 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 link to determine the cause ofthe failure. If the failed peer can still transmit heartbeat messages, the Secondary Peer disables all VLT member ports and any Layer3 interfaces attached to the VLAN associated with the VLT domain. If heartbeat messages are not received, the Secondary Peerforwards traffic assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer reassigned as the PrimaryPeer retains this role and the other peer must be reassigned as a Secondary Peer. 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 node failures. Spanning tree topologychanges are distributed to the entire layer 2 network, which can cause a network-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 ascabling errors or incorrect configurations. To minimize possible topology changes after link or node failure, RSTP is useful for potentialloop detection. Configure RSTP using the following specifications.The following recommendations help you avoid these issues and the associated traffic loss caused by using RSTP when you enableVLT on both VLT peers:• Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which are directly connected to endstations or server racks. Disable RSTP on ports connected directly to Layer 3-only routers not running STP or configure them asedge ports.• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has the second-best bridge ID in thenetwork. If the primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems withspanning tree port state changes that occur when a VLT node fails or recovers.• Even with this configuration, if the node has non-VLT ports using RSTP that you did not configure as edge ports and areconnected to other Layer 2 switches, spanning tree topology changes are still detected after VLT node recovery. To avoid thisscenario, ensure that you configure any non-VLT ports as edge ports or disable RSTP.VLT Bandwidth MonitoringWhen bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (shown in the following message) and an SNMP trapare generated.%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-channel 25)crosses threshold. Bandwidth usage (80 )When the bandwidth usage drops below the 80% threshold, the system generates another syslog message (shown in the followingmessage) and an SNMP trap.%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-channel 25)reaches below threshold. Bandwidth usage (74 )VLT show remote port channel statusVLT and IGMP SnoopingWhen configuring IGMP Snooping with VLT, ensure the configurations on both sides of the VLT trunk are identical to get the samebehavior on both sides of the trunk.When you configure IGMP snooping on a VLT node, the dynamically learned groups and multicast router ports are automaticallylearned on the VLT peer node.VLT IPv6The following features have been enhanced to support IPv6:• VLT Sync — Entries learned on the VLT interface are synced on both VLT peers.752 Virtual Link Trunking (VLT)
