Users Guide

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.
• 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. In the port-channel used by the switch to connect to the VLT domain,
configure the port interfaces on each VLT peer as hybrid ports before adding them to the
port channel (see
Connecting a VLT Domain to an Attached Access Device (Switch or Server)). To configure a
port in Hybrid mode so that it can carry untagged, single-tagged, and double-tagged traffic,
use the portmode hybrid command in Interface Configuration mode as described in
Configuring Native VLANs.
• For example, if the DHCP server is on the ToR and VLTi (ICL) is down (due to either an
unavailable peer or a link failure), whether you configured the VLT LAG as static or LACP,
when a single VLT peer is rebooted in BMP mode, it cannot reach the DHCP server, resulting
in BMP failure.
Virtual Link Trunking (VLT) 1033