Administrator Guide

– Unknown, multicast, and broadcast trac can be ooded across the VLT interconnect.
– MAC addresses for VLANs congured across VLT peer chassis are synchronized over the VLT interconnect on an egress port such
as a VLT LAG. MAC addresses are the same on both VLT peer nodes.
– ARP entries congured across the VLTi are the same on both VLT peer nodes.
– If you shut down the port channel used in the VLT interconnect on a peer switch in a VLT domain in which you did not congure a
backup link, the switch’s role displays in the show vlt brief command output as Primary instead of Standalone.
– When you change the default VLAN ID on a VLT peer switch, the VLT interconnect may ap.
– In a VLT domain, the following software features are supported on VLTi: link layer discovery protocol (LLDP), ow control, port
monitoring, jumbo frames, and data center bridging (DCB).
– When you enable the VLTi link, the link between the VLT peer switches is established if the following congured information is true
on both peer switches:
◦ the VLT-system MAC address (if congured) matches.
◦ the VLT unit-id (if congured) is not identical.
NOTE: If the VLT-system MAC address or VLT unit-id is not congured on both VLT peer switches, VLT
automatically sets the default VLT-system MAC address and unit-id on each peer.
– 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 congure 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, pre-congure the switch with the VLT-system MAC address, unit-id, and other VLT parameters (if
applicable) 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 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 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.
– In case of dual RPM, congure the virtual IP address as backup link. This is needed so that the backup link wont ap duirng RPM
failover scenarios. See Conguring a Virtual IP Address.
• 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 eight member links are supported in each port channel between the VLT domain and an access
device.
– 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 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 (refer to
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.
Virtual Link Trunking (VLT)
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