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○ 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 .
○ 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.
● Software features supported on VLT port-channels
○ 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, 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, IPv6
dynamic routing.
● Failure scenarios
○ On a link failover, when a VLT port channel fails, the traffic destined for that VLT port channel is redirected to the VLTi
to avoid flooding.
○ When a VLT switch determines that a VLT port channel has failed (and that no other local port channels are available),
the peer with the failed port channel notifies the remote peer that it no longer has an active port channel for a link. The
remote peer then enables data forwarding across the interconnect trunk for packets that would otherwise have been
forwarded over the failed port channel. This mechanism ensures reachability and provides loop management. If the VLT
interconnect fails, the VLT software on the primary switch checks the status of the remote peer using the backup link. If
the remote peer is up, the secondary switch disables all VLT ports on its device to prevent loops.
○ If all ports in the VLT interconnect fail, or if the messaging infrastructure fails to communicate across the interconnect
trunk, the VLT management system uses the backup link interface to determine whether the failure is a link-level failure
or whether the remote peer has failed entirely. If the remote peer is still alive (heartbeat messages are still being
received), the VLT secondary switch disables its VLT port channels. If keepalive messages from the peer are not being
received, the peer continues to forward traffic, assuming that it is the last device available in the network. In either case,
after recovery of the peer link or reestablishment of message forwarding across the interconnect trunk, the two VLT
peers resynchronize any MAC addresses learned while communication was interrupted and the VLT system continues
normal data forwarding.
○ If the primary chassis fails, the secondary chassis takes on the operational role of the primary.
● The SNMP MIB reports VLT statistics.
PMUX Mode of the IO Aggregator
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