Users Guide

○ 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 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:
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.
▪ You should 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, 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 not supported on VLT ports: 802.1x, DHCP snooping, FRRP, GVRP, ERSPAN,
RSPAN, VXLAN, 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.
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Virtual Link Trunking (VLT)