Reference Guide

– 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.
• 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 eight 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.
– VLT allows multiple active parallel paths from access switches to VLT chassis.
– VLT supports port-channel links with LACP between access switches and VLT peer switches. Dell
Networking recommends using static port channels on VLTi.
– If VLTi connectivity with a peer is lost but the VLT backup connectivity indicates that the peer is still
alive, the VLT ports on the Secondary peer are orphaned and are shut down.
* In one possible topology, a switch uses the BMP feature to receive its IP address, configuration
files, and boot image from a DHCP server that connects to the switch through the VLT domain.
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