Configuration Notes - Dell S4820T Configuration Manual

• 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 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)
Virtual Link Trunking (VLT)
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