Concept Guide

NOTE: If you congure 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 congured 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 congure 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, precongure 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 congure 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 trac. 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 .
– 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 dierent chassis in the VLT domain.
– VLT uses shortest path routing so that trac 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.
• Software features supported on VLT port-channels
– For information about conguring IGMP Snooping in a VLT domain, refer to VLT and IGMP Snooping.
– All system management protocols are supported on VLT ports, including SNMP, RMON, AAA, ACL, DNS, FTP, SSH, Syslog, NTP,
RADIUS, SCP, TACACS+, Telnet, and LLDP.
– Enable Layer 3 VLAN connectivity VLT peers by conguring a VLAN network interface for the same VLAN on both switches.
– Dell Networking does not recommend enabling peer-routing if the CAM is full. To enable peer-routing, a minimum of two local DA
spaces for wild card functionality are required.
• Software features supported on VLT physical ports
– In a VLT domain, the following software features are supported on VLT physical ports: 802.1p, LLDP, ow control, port monitoring,
and jumbo frames.
• Software features not supported with VLT
– In a VLT domain, the following software features are supported on non-VLT ports: 802.1x, , DHCP snooping, FRRP, IPv6 dynamic
routing.
• Failure scenarios
– On a link failover, when a VLT port channel fails, the trac destined for that VLT port channel is redirected to the VLTi to avoid
ooding.
– When a VLT switch determines that a VLT port channel has failed (and that no other local port channels are available), the peer
with the failed port channel noties the remote peer that it no longer has an active port channel for a link. The remote peer then
enables data forwarding across the interconnect trunk for packets that would otherwise have been forwarded over the failed port
channel. This mechanism ensures reachability and provides loop management. If the VLT interconnect fails, the VLT software on the
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PMUX Mode of the IO Aggregator