Administrator Guide

• 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 EMC 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
• 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 EMC 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.
• Software features supported on VLT port-channels
• In a VLT domain, the following software features are supported on VLT port-channels: 802.1p, ingress and egress ACLs, BGP,
DHCP relay, IS-IS, OSPF, active-active PIM-SM, PIM-SSM, VRRP, Layer 3 VLANs, LLDP, flow control, port monitoring, jumbo
frames, IGMP snooping, sFlow, ingress and egress ACLs, and Layer 2 control protocols RSTP and PVST only.
NOTE:
Peer VLAN spanning tree plus (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).
• In a VLT domain, ingress and egress QoS policies are supported on physical VLT ports, which can be members of VLT port
channels in the domain.
• Ingress and egress QoS policies applied on VLT ports must be the same on both VLT peers.
• Apply the same ingress and egress QoS policies on VLTi (ICL) member ports to handle failed links.
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Virtual Link Trunking (VLT)