Administrator Guide

○ In a VLT domain, the following software features are supported on VLT port-channels: 802.1p, ingress and egress ACLs,
BGP, DHCP relay, IS-IS, OSPF, active-active PIM-SM, PIM-SSM, VRRP, Layer 3 VLANs, LLDP, flow control, port
monitoring, jumbo frames, IGMP snooping, sFlow, ingress and egress ACLs, and Layer 2 control protocols RSTP and
PVST only.
NOTE: Peer VLAN spanning tree plus (PVST+) passthrough is supported in a VLT domain. PVST+ BPDUs does not
result in an interface shutdown. PVST+ BPDUs for a nondefault VLAN is flooded out as any other L2 multicast
packet. On a default VLAN, RTSP is part of the PVST+ topology in that specific VLAN (default VLAN).
○ In a VLT domain, ingress and egress QoS policies are supported on physical VLT ports, which can be members of VLT
port channels in the domain.
￭ Ingress and egress QoS policies applied on VLT ports must be the same on both VLT peers.
￭ Apply the same ingress and egress QoS policies on VLTi (ICL) member ports to handle failed links.
○ 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
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Virtual Link Trunking (VLT)