Concept Guide

– If the primary chassis fails, the secondary chassis takes on the operational role of the primary.
• The SNMP MIB reports VLT statistics.
Primary and Secondary VLT Peers
Primary and secondary VLT peers are supported to prevent issues when connectivity between peers is lost on the switch.
You can elect or congure the Primary Peer. By default, the peer with the lowest MAC address is selected as the Primary Peer. You can
congure another peer as the Primary Peer using the VLT primary-priority command.
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 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 link to determine the cause of the
failure. If the failed peer can still transmit heartbeat messages, the Secondary Peer disables all VLT member ports and any Layer 3
interfaces attached to the VLAN associated with the VLT domain. If heartbeat messages are not received, the Secondary Peer forwards
trac assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer reassigned as the Primary Peer retains
this role and the other peer must be reassigned as a Secondary Peer. Peer role changes are reported as SNMP traps.
RSTP and VLT
VLT 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 topology changes
are distributed to the entire layer 2 network, which can cause a network-wide ush 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 or
incorrect congurations. To minimize possible topology changes after link or node failure, RSTP is useful for potential loop detection.
Congure RSTP using the following specications.
The following recommendations help you avoid these issues and the associated trac loss caused by using RSTP when you enable VLT on
both VLT peers:
• Congure any ports at the edge of the spanning tree’s operating domain as edge ports, which are directly connected to end stations or
server racks. Disable RSTP on ports connected directly to Layer 3-only routers not running STP or congure them as edge ports.
• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has the second-best bridge ID in the network. If
the primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems with spanning tree port
state changes that occur when a VLT node fails or recovers.
• Even with this conguration, if the node has non-VLT ports using RSTP that you did not congure as edge ports and are connected to
other Layer 2 switches, spanning tree topology changes are still detected after VLT node recovery. To avoid this scenario, ensure that
you congure any non-VLT ports as edge ports or disable RSTP.
VLT Bandwidth Monitoring
When bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (shown in the following message) 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 )
When the bandwidth usage drops below the 80% threshold, the system generates another syslog message (shown in the following
message) 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 status
Virtual Link Trunking (VLT)
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