Administrator Guide

● If the DHCP server is located on the ToR and the VLTi (ICL) is down due to a failed link when a VLT node is rebooted in
BMP mode, it is not able to reach the DHCP server, resulting in BMP failure.
● If the source is connected to an orphan (non-spanned, non-VLT) port in a VLT peer, the receiver is connected to a
VLT (spanned) port-channel, and the VLT port-channel link between the VLT peer connected to the source and TOR is
down, traffic is duplicated due to route inconsistency between peers. To avoid this scenario, Dell Networking recommends
configuring both the source and the receiver on a spanned VLT VLAN.
● Bulk synchronization happens only for global IPv6 Neighbors; link-local neighbor entries are not synced.
● If all of the following conditions are true, MAC addresses may not be synced correctly:
○ VLT peers use VLT interconnect (VLTi)
○ Sticky MAC is enabled on an orphan port in the primary or secondary peer
○ MACs are currently inactive
If this scenario occurs, use the clear mac-address-table sticky all command on the primary or secondary peer
to correctly sync the MAC addresses.
● If static ARP is enabled on only one VLT peer, entries may be overwritten during bulk sync.
● In VLT, non-default VRF is not supported on LM/LP physical or port-channel ports.
● Routing with non-spanned VLAN is not supported.
Configuration Notes
When you configure VLT, the following conditions apply.
● With VLT, when an L3 interface is created, the local DA of that interface is added as an L2 entry pointing to the ICL
interface on the peer chassis. This ensures that the L3 packets reaching the peer, by LAG hashing on ToR, get forwarded to
the actual chassis via ICL and then get routed. When this interface is removed, the entry pointing to ICL on the peer chassis
is deleted.
● VLT domain
○ A VLT domain supports two chassis members, which appear as a single logical device to network access devices
connected to VLT ports through a port channel.
○ A VLT domain consists of the two core chassis, the interconnect trunk, backup link, and the LAG members connected to
attached devices.
○ Each VLT domain has a unique MAC address that you create or VLT creates automatically.
○ ARP tables are synchronized between the VLT peer nodes.
○ VLT peer switches operate as separate chassis with independent control and data planes for devices attached on
non-VLT ports.
○ One device in the VLT domain is assigned a primary role; the other device takes the secondary role. The primary and
secondary roles are required for scenarios when connectivity between the chassis is lost. VLT assigns the primary chassis
role according to the lowest MAC address. You can configure the primary role manually.
○ In a VLT domain, the peer switches must run the same Dell EMC Networking OS software version.
○ Separately configure each VLT peer switch with the same VLT domain ID and the VLT version. If the system detects
mismatches between VLT peer switches in the VLT domain ID or VLT version, the VLT Interconnect (VLTi) does not
activate. To find the reason for the VLTi being down, use the show vlt statistics command to verify that there
are mismatch errors, then use the show vlt brief command on each VLT peer to view the VLT version on the peer
switch. If the VLT version is more than one release different from the current version in use, the VLTi does not activate.
○ The chassis members in a VLT domain support connection to orphan hosts and switches that are not connected to both
switches in the VLT core.
● VLT interconnect (VLTi)
○ The VLT interconnect can consist of 10G or 40G ports. A maximum of eight ports are supported. A combination of 10G
and 40G ports is not supported.
○ The port channel must be in Default mode (not Switchport mode) to have VLTi recognize it.
○ The system automatically includes the required VLANs in VLTi. You do not need to manually select VLANs.
○ VLT peer switches operate as separate chassis with independent control and data planes for devices attached to
non-VLT ports.
○ Port-channel link aggregation (LAG) across the ports in the VLT interconnect is required; individual ports are not
supported. Dell EMC Networking strongly recommends configuring a static LAG for VLTi.
○ The VLT interconnect synchronizes L2 and L3 control-plane information across the two chassis.
○ The VLT interconnect is used for data traffic only when there is a link failure that requires using VLTi in order for data
packets to reach their final destination.
○ Unknown, multicast, and broadcast traffic can be flooded across the VLT interconnect.
Virtual Link Trunking (VLT)
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