White Papers

○ 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.
○ MAC addresses for VLANs configured across VLT peer chassis are synchronized over the VLT interconnect on an egress
port such as a VLT LAG. MAC addresses are the same on both VLT peer nodes.
○ ARP entries configured across the VLTi are the same on both VLT peer nodes.
○ If you shut down the port channel used in the VLT interconnect on a peer switch in a VLT domain in which you did
not configure a backup link, the switch’s role displays in the show vlt brief command output as Primary instead of
Standalone.
○ When you change the default VLAN ID on a VLT peer switch, the VLT interconnect may flap.
○ In a VLT domain, the following software features are supported on VLTi: link layer discovery protocol (LLDP), flow
control, port monitoring, jumbo frames, and data center bridging (DCB).
○ When you enable the VLTi link, the link between the VLT peer switches is established if the following configured
information is true on both peer switches:
￭ the VLT system MAC address matches.
￭ the VLT unit-id is not identical.
NOTE:
If you configure the VLT system MAC address or VLT unit-id on only one of the VLT peer switches, the link
between the VLT peer switches is not established. Each VLT peer switch must be correctly configured to establish
the link between the peers.
○ If the link between the VLT peer switches is established, changing the VLT system MAC address or the VLT unit-id
causes the link between the VLT peer switches to become disabled. However, removing the VLT system MAC address or
the VLT unit-id may disable the VLT ports if you happen to configure the unit ID or system MAC address on only one VLT
peer at any time.
○ If the link between VLT peer switches is established, any change to the VLT system MAC address or unit-id fails if the
changes made create a mismatch by causing the VLT unit-ID to be the same on both peers and/or the VLT system MAC
address does not match on both peers.
○ If you replace a VLT peer node, preconfigure the switch with the VLT system MAC address, unit-id, and other VLT
parameters before connecting it to the existing VLT peer switch using the VLTi connection.
○ If the size of the MTU for VLTi members is less than 1496 bytes, MAC addresses may not synchronize between VLT
peers. Dell EMC Networking does not recommend using an MTU size lower than the default of 1554 bytes for VLTi
members.
● VLT backup link
○ In the backup link between peer switches, heartbeat messages are exchanged between the two chassis for health
checks. The default time interval between heartbeat messages over the backup link is 1 second. You can configure this
interval. The range is from 1 to 5 seconds. DSCP marking on heartbeat messages is CS6.
Virtual Link Trunking (VLT)
933