Administrator Guide

• For detailed information about how to use VRRP in a VLT domain, see the following VLT and VRRP interoperability section.
• For information about configuring IGMP Snooping in a VLT domain, see VLT and IGMP Snooping.
• All system management protocols are supported on VLT ports, including SNMP, RMON, AAA, ACL, DNS, FTP, SSH, Syslog, NTP,
RADIUS, SCP, TACACS+, Telnet, and LLDP.
• Enable Layer 3 VLAN connectivity VLT peers by configuring a VLAN network interface for the same VLAN on both switches.
• Dell EMC Networking does not recommend enabling peer-routing if the CAM is full. To enable peer-routing, a minimum of two local
DA spaces for wild-card functionality are required.
• RSPAN and ERSPAN are supported on VLT.
• FRRP is supported only on the VLTi. This feature enables configuration of an FRRP ring through VLTi. However, FRRP is not
supported on any other VLT port-channel except for VLTi.
• Software features supported on VLT physical ports
• In a VLT domain, the following software features are supported on VLT physical ports: 802.1p, LLDP, flow control, IPv6 dynamic
routing, port monitoring, DHCP snooping, and jumbo frames.
• Software features not supported with VLT
• In a VLT domain, the following software features are not supported on VLT ports: 802.1x, GVRP, and BFD.
• VLT and VRRP interoperability
• In a VLT domain, VRRP interoperates with virtual link trunks that carry traffic to and from access devices (see Overview). The VLT
peers belong to the same VRRP group and are assigned master and backup roles. Each peer actively forwards L3 traffic, reducing
the traffic flow over the VLT interconnect.
• VRRP elects the router with the highest priority as the master in the VRRP group. To ensure VRRP operation in a VLT domain,
configure VRRP group priority on each VLT peer so that a peer is either the master or backup for all VRRP groups configured on
its interfaces. For more information, see Setting VRRP Group (Virtual Router) Priority.
• To verify that a VLT peer is consistently configured for either the master or backup role in all VRRP groups, use the show vrrp
command on each peer.
• Configure the same L3 routing (static and dynamic) on each peer so that the L3 reachability and routing tables are identical on
both VLT peers. Both the VRRP master and backup peers must be able to locally forward L3 traffic in the same way.
• In a VLT domain, although both VLT peers actively participate in L3 forwarding as the VRRP master or backup router, the show
vrrp command output displays one peer as master and the other peer as backup.
• Failure scenarios
• On a link failover, when a VLT port channel fails, the traffic destined for that VLT port channel is redirected to the VLTi to avoid
flooding.
• 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
Primary and secondary VLT peers are supported to prevent issues when connectivity between peers is lost on the switch.
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 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
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Virtual Link Trunking (VLT)