Reference Guide
○ 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 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 only).
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.
○ 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, flow control, IPv6
dynamic routing, 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, ingress
and egress QOS.
● VLT and VRRP interoperability
○ In a VLT domain, VRRP interoperates with virtual link trunks that carry traffic to and from access devices (refer to
Overview). The VLT peers belong to the same VRRP group and are assigned master and backup roles. Each peer actively
forwards L3 traffic, reducing the traffic flow over the VLT interconnect.
○ VRRP elects the router with the highest priority as the master in the VRRP group. To ensure VRRP operation in a VLT
domain, configure VRRP group priority on each VLT peer so that a peer is either the master or backup for all VRRP
groups configured on its interfaces. For more information, refer to Setting VRRP Group (Virtual Router) Priority.
○ To verify that a VLT peer is consistently configured for either the master or backup role in all VRRP groups, use the
show vrrp command on each peer.
○ Also configure the same L3 routing (static and dynamic) on each peer so that the L3 reachability and routing tables are
identical on both VLT peers. Both the VRRP master and backup peers must be able to locally forward L3 traffic in the
same way.
○ In a VLT domain, although both VLT peers actively participate in L3 forwarding as the VRRP master or backup router, the
show vrrp command output displays one peer as master and the other peer as backup.
● Failure scenarios
Virtual Link Trunking (VLT)
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