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● If you enable static ARP on only one VLT peer, entries may be overwritten during bulk sync.
● For multiple VLT LAGs configured on the same VLAN, if a host is learned on one VLT LAG and there is a station move
between LAGs, the link local address redirects to the VLTi link on one of the peers. If this occurs, clear the link local address
that is redirecting to the VLTi link.
● VLT Heartbeat is supported only on default VRFs.
● In a scenario where one hundred hosts are connected to a Peer1 on a non-VLT domain and traffic flows through Peer1 to
Peer2; when you move these hosts from a non-VLT domain to a VLT domain and send ARP requests to Peer1, only half
of these ARP requests reach Peer1, while the remaining half reach Peer2 (because of LAG hashing). The reason for this
behavior is that Peer1 ignores the ARP requests that it receives on VLTi (ICL) and updates only the ARP requests that it
receives on the local VLT. As a result, the remaining ARP requests still points to the Non-VLT links and traffic does not
reach half of the hosts. To mitigate this issue, ensure that you configure the following settings on both the Peers (Peer1 and
Peer2): arp learn-enable and mac-address-table station-move refresh-arp.
● 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 device in the VLT domain is assigned a primary role; the other device 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 manually.
○ In a VLT domain, the peer switches must run the same Dell EMC 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 can consist of 10G or 40G ports. A maximum of eight ports are supported. A combination of 10G
and 40G 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 EMC Networking strongly recommends configuring a static LAG for VLTi.
○ The VLT interconnect synchronizes L2 and L3 control-plane information across the two chassis.
Virtual Link Trunking (VLT)
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