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○ 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 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 (refer to 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.
● Software features supported on VLT port-channels
○ In a VLT domain, the following software features are supported on VLT ports: 802.1X with multi-supplicant dynamic
VLAN assignments and MAC Authentication Bypass, VRRP, Layer 3 VLANs, IGMP Snooping, FRRP, DHCP Snooping,
DHCP relay, sFlow, ingress and egress QoS, ingress and egress ACLs, DCB and Layer 2 control protocols such as RSTP
(see Configuring Rapid Spanning Tree).
NOTE:
PVST+ passthrough is supported in a VLT domain. PVST+ BPDUs does not result in an interface shutdown.
PVST+ BPDUs for a nondefault VLAN is flooded out as any other L2 multicast packet. On a default VLAN, RTSP is
part of the PVST+ topology in that specific VLAN (default VLAN).
○ For detailed information about how to use VRRP in a VLT domain, refer to the following VLT and VRRP interoperability
section.
○ For information about configuring 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 configuring a VLAN network interface for the same VLAN on both
switches.
● Software features supported on VLT port-channels:
○ In a VLT domain, the following software features are supported on VLT physical ports: 802.1p, LLDP, flow control, port
monitoring, and jumbo frames.
● Software features supported on non-VLT ports:
○ In a VLT domain, the following software features are supported on non-VLT ports: OSPF, BGP, IS-IS, DHCP relay, sFlow,
ingress and egress QOS, ingress and egress ACLs, 802.1x, and all protocols currently supported in Dell networking OS.
● VLT and VRRP interoperability
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Virtual Link Trunking (VLT)