Users Guide

• Ensure that the spanning tree root bridge is at the Aggregation layer. Refer to RSTP and VLT for guidelines to avoid traffic loss, if you
enable RSTP on the VLT device.
• 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.
• Bulk Sync happens only for Global IPv6 Neighbors; Link-local neighbor entries are not synced.
• If all of the following conditions are true, MAC addresses may not be synced correctly:
○ VLT peers use VLT interconnect (VLTi)
○ Sticky MAC is enabled on an orphan port in the primary or secondary peer
○ MACs are currently inactive
If this scenario occurs, use the clear mac-address-table sticky all command on the primary or secondary peer to
correctly sync the MAC addresses.
• 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.
Virtual Link Trunking (VLT)
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