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● If you reboot both VLT peers in BMP mode and the VLT LAGs are static, the DHCP server reply to the DHCP discover offer
may not be forwarded by the ToR to the correct node. To avoid this scenario, configure the VLT LAGs to the ToR and the
ToR port channel to the VLT peers with LACP. If supported by the ToR, enable the lacp-ungroup feature on the ToR
using the lacp ungroup member-independent port-channel command.
● If the lacp-ungroup feature is not supported on the ToR, reboot the VLT peers one at a time. After rebooting, verify that
VLTi (ICL) is active before attempting DHCP connectivity.
● When you enable IGMP snooping on the VLT peers, ensure the value of the delay-restore command is not less than the
query interval.
● When you enable Layer 3 routing protocols on VLT peers, make sure the delay-restore timer is set to a value that allows
sufficient time for all routes to establish adjacency and exchange all the L3 routes between the VLT peers before you enable
the VLT ports.
●
Only use the lacp ungroup member-independent command if the system connects to nodes using bare metal
provisioning (BMP) to upgrade or boot from the network.
● Ensure that you configure all port channels where LACP ungroup is applicable as hybrid ports and as untagged members of a
VLAN. BMP uses untagged dynamic host configuration protocol (DHCP) packets to communicate with the DHCP server.
● If the DHCP server is located on the ToR and the VLTi (ICL) is down due to a failed link when a VLT node is rebooted in
BMP mode, it is not able to reach the DHCP server, resulting in BMP failure.
● If the source is connected to an orphan (non-spanned, non-VLT) port in a VLT peer, the receiver is connected to a
VLT (spanned) port-channel, and the VLT port-channel link between the VLT peer connected to the source and TOR is
down, traffic is duplicated due to route inconsistency between peers. To avoid this scenario, Dell Networking recommends
configuring both the source and the receiver on a spanned VLT VLAN.
● In a topology in which two VLT peer nodes that are connected by a VLTi link and are connected to a ToR switch using a
VLT LAG interface, if you configure an egress IP ACL and apply it on the VLT LAG of both peers using the deny ip any
any command, the traffic is permitted on the VLT LAG instead of being denied. The correct behavior of dropping the traffic
on the VLT LAG occurs when VLT is up on both the peer nodes. However, if VLT goes down on one of the peers, traffic
traverses through VLTi and the other peer switches it to the VLT LAG. Although egress ACL is applied on the VLT nodes to
deny all traffic, this egress ACL does not deny the traffic (switching traffic is not denied owing to the egress IP ACL). You
cannot use egress ACLs to deny traffic properly in such a VLT scenario.
● To support Q-in-Q over VLT, ICL is implicitly made as vlan-stack trunk port and the TPID of the ICL is set as 8100.
● Layer 2 Protocol Tunneling is not supported in VLT.
Configuration Notes
When you configure VLT, the following conditions apply.
● 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 operating system (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 must consist of either 10G or 40G ports. A maximum of sixteen 10G or 40G ports are supported. A
combination of 10G and 40G ports are not supported.
○ A VLT interconnect over 1G ports is not supported.
848
Virtual Link Trunking (VLT)