Concept Guide

– If the size of the MTU for VLTi members is less than 1496 bytes, MAC addresses may not synchronize between VLT peers. Dell
EMC Networking does not recommend using an MTU size lower than the default of 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 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 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 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 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 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 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, conguration les, 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, congure 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 congure a port in Hybrid mode so that it can
carry untagged, single-tagged, and double-tagged trac, use the portmode hybrid command in Interface Conguration
mode as described in
Conguring 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 congured 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, ow 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 ooded out as any other L2 multicast
packet. On a default VLAN, RTSP is part of the PVST+ topology in that specic 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.
– For detailed information about how to use VRRP in a VLT domain, see the following VLT and VRRP interoperability section.
– For information about conguring IGMP Snooping in a VLT domain, see 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 EMC 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.
– RSPAN and ERSPAN are supported on VLT.
– FRRP is supported only on the VLTi. This feature enables conguration of an FRRP ring through VLTi. However, FRRP is not
supported on any other VLT port-channel except for VLTi.
Virtual Link Trunking (VLT)
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