Virtual Link Trunking (VLT) | 923the network. In either case, upon recovery of the peer link or reestablishment of messageforwarding across the interconnect trunk, the two VLT peers resynchronize any MAC addresseslearned while communication was interrupted, and the VLT system continues normal dataforwarding.• If the primary chassis is rebooted, the secondary chassis takes on the operational role of theprimary. When operation of the original, primary chassis is restored, it takes on the operationalrole of the secondary chassis.• The SNMP MIB reports VLT statistics.RSTP and VLTVLT provides loop-free redundant topologies and does not require rapid spanning tree protocol (RSTP).RSTP can cause temporary port state blocking and may cause topology changes after link or node failures.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 errors orincorrect configurations. RSTP is useful for potential loop detection but should be configured using thespecifications below to minimize possible topology changes after link or node failure.The following recommendations will help you avoid these issues and the associated traffic loss caused byusing rapid spanning trees when VLT is enabled on both VLT peers:• Any ports at the edge of the spanning tree's operating domain should be configured as edge ports,which are directly connected to end stations or server racks. Ports connected directly to layer 3-onlyrouters not running STP should have RSTP disabled or be configured as edge ports.• Ensure the primary VLT node is the root bridge and the secondary VLT peer node has the second-bestbridge ID in the network. If the primary VLT peer node fails, the secondary VLT peer node becomesthe root bridge, avoiding problems with spanning tree port state changes that occur when a VLT nodefails or recovers.• Even with this configuration, if the node has non-VLT ports using RSTP that are not configured asedge ports and are connected to other layer 2 switches, spanning tree topology changes can still bedetected after VLT node recovery. To avoid this scenario, ensure that any non-VLT ports areconfigured as edge ports or have RSTP disabled.VLT Bandwidth MonitoringWhen bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (Message 1) and anSNMP trap are generated.Message 1 VLTi Bandwidth Usage Exceeding Threshold Value Error%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-channel 25)crosses threshold. Bandwidth usage (80% )
