Dell S4048T, Configuration Manual

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• When a VLT switch determines that a VLT port channel has failed (and that no other local port
channels are available), the peer with the failed port channel notifies the remote peer that it no
longer has an active port channel for a link. The remote peer then enables data forwarding across
the interconnect trunk for packets that would otherwise have been forwarded over the failed port
channel. This mechanism ensures reachability and provides loop management. If the VLT
interconnect fails, the VLT software on the primary switch checks the status of the remote peer
using the backup link. If the remote peer is up, the secondary switch disables all VLT ports on its
device to prevent loops.
• If all ports in the VLT interconnect fail, or if the messaging infrastructure fails to communicate
across the interconnect trunk, the VLT management system uses the backup link interface to
determine whether the failure is a link-level failure or whether the remote peer has failed entirely. If
the remote peer is still alive (heartbeat messages are still being received), the VLT secondary switch
disables its VLT port channels. If keepalive messages from the peer are not being received, the peer
continues to forward traffic, assuming that it is the last device available in the network. In either
case, after recovery of the peer link or reestablishment of message forwarding across the
interconnect trunk, the two VLT peers resynchronize any MAC addresses learned while
communication was interrupted and the VLT system continues normal data forwarding.
• 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
To prevent issues when connectivity between peers is lost, you can designate Primary and Secondary roles
for VLT peers . You can elect or configure the Primary Peer. By default, the peer with the lowest MAC address
is selected as the Primary Peer. You can configure another peer as the Primary Peer using the 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 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 traffic 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 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 or incorrect
configurations. To minimize possible topology changes after link or node failure, RSTP is useful for potential
loop 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 enable VLT on both VLT peers:
Virtual Link Trunking (VLT) 1122