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– 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 be
synced. Dell Networking recommends retaining the default MTU allocation (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.
– In order that the chassis backup link does not share the same physical path as the interconnect
trunk, Dell Networking recommends using the management ports on the chassis and traverse an
out-of-band management network. The backup link can use user ports, but not the same ports
the interconnect trunk uses.
– The chassis backup link does not carry control plane information or data traffic. Its use is restricted
to health checks only.
• Virtual link trunks (VLTs) between access devices and VLT peer switches
– To connect servers and access switches with VLT peer switches, you use a VLT port channel, as
shown in Overview. Up to 48 port-channels are supported; up to 16 member links are supported
in each port channel between the VLT domain and an access device.
– The discovery protocol running between VLT peers automatically generates the ID number of the
port channel that connects an access device and a VLT switch. The discovery protocol uses LACP
properties to identify connectivity to a common client device and automatically generates a VLT
number for port channels on VLT peers that connects to the device. The discovery protocol
requires that an attached device always runs LACP over the port-channel interface.
– VLT provides a loop-free topology for port channels with endpoints on different chassis in the VLT
domain.
– VLT uses shortest path routing so that traffic destined to hosts via directly attached links on a
chassis does not traverse the chassis-interconnect link.
Virtual Link Trunking (VLT)
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