Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

• VLT domain
  • A VLT domain supports two chassis members, which appear as a single logical device to network access devices connected to VLT ports through a port channel.
  • A VLT domain consists of the two core chassis, the interconnect trunk, backup link, and the LAG members connected to attached devices.
  • Each VLT domain has a unique MAC address that you create or VLT creates automatically.
  • ARP tables are synchronized between the VLT peer nodes.
  • VLT peer switches operate as separate chassis with independent control and data planes for devices attached on non-VLT ports.
  • One chassis in the VLT domain is assigned a primary role; the other chassis takes the secondary role. The primary and secondary roles are required for scenarios when connectivity between the chassis is lost. VLT assigns the primary chassis role according to the lowest MAC address. You can configure the primary role.
  • In a VLT domain, the peer switches must run the same Dell Networking OS software version.
  • Separately configure each VLT peer switch with the same VLT domain ID and the VLT version. If the system detects mismatches between VLT peer switches in the VLT domain ID or VLT version, the VLT Interconnect (VLTi) does not activate. To find the reason for the VLTi being down, use the show vlt statistics command to verify that there are mismatch errors, then use the show vlt brief command on each VLT peer to view the VLT version on the peer switch. If the VLT version is more than one release different from the current version in use, the VLTi does not activate.
  • The chassis members in a VLT domain support connection to orphan hosts and switches that are not connected to both switches in the VLT core.
• VLT interconnect (VLTi)
  • The VLT interconnect supports a maximum of two 10G ports.
  • In VLT mode, port 33 and 37 are dedicated to VLT interconnect ports.
  • A VLT interconnect over 1G ports is not supported.
  • The port channel must be in Default mode (not Switchport mode) to have VLTi recognize it.
  • The system automatically includes the required VLANs in VLTi. You do not need to manually select VLANs.
  • VLT peer switches operate as separate chassis with independent control and data planes for devices attached to non-VLT ports.
  • Port-channel link aggregation (LAG) across the ports in the VLT interconnect is required; individual ports are not supported. Dell Networking strongly recommends configuring a static LAG for VLTi.
  • The VLT interconnect synchronizes L2 and L3 control-plane information across the two chassis.
  • The VLT interconnect is used for data traffic only when there is a link failure that requires using VLTi in order for data packets to reach their final destination.
  • Unknown, multicast, and broadcast traffic can be flooded across the VLT interconnect.
  • MAC addresses for VLANs configured 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 configured 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 configure 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 flap.
  • In a VLT domain, the following software features are supported on VLTi: link layer discovery protocol (LLDP), flow 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 configured information is true on both peer switches:
    • the VLT system MAC address matches.
    • the VLT unit-id is not identical.
    • NOTE: If you configure the VLT system MAC address or VLT unit-id on only one of the VLT peer switches, the link between the VLT peer switches is not established. Each VLT peer switch must be correctly configured to establish the link between the peers.
  • 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 configure 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, preconfigure the switch with the VLT system MAC address, unit-id, and other VLT parameters 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 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, ingress and egress QOS.
• 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.