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Dell EMC Configuration Guide for the S4048T–ON System 9.14.2.4

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Important Points to Remember

  • You cannot enable stacking simultaneously with VLT. If you enable both at the same time, unexpected behavior can occur.
  • VLT port channel interfaces must be switch ports.
  • If you include RSTP on the system, configure it before VLT. Refer to Configure Rapid Spanning Tree.
  • If you include PVST on the system, configure it before VLT. Refer to PVST Configuration.
  • Dell EMC Networking strongly recommends that the VLTi (VLT interconnect) be a static LAG and that you disable LACP on the VLTi.
  • 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 with static VLT LAGs, the DHCP server reply to the DHCP discover offer may not be forwarded by the ToR to the correct node. To avoid this issue, 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 a VLAN is created on VLTi peers, the VLTi port-channel is added automatically to the VLAN, whether the vlan have members or not. A VLAN creation or deletion message from the VLT peer is the trigger for adding or removing VLTi port-channels to or from a VLAN. You can manually add or remove a VLTi port-channel to a VLAN. In case a VLTi port-channel is manually removed from a VLAN, it is added back to the VLAN after reload of the VLTi peers.
  • Use the lacp ungroup member-independent command only if the system connects to nodes using bare metal provisioning (BMP) to upgrade or boot from the network. Otherwise, when the member links flap, the links become independent switch ports and forward the traffic even before LACP is formed on that port.
  • Ensure that you configure all port channels as hybrid ports and as untagged members of a VLAN where the LACP ungroup option is applicable.
  • BMP uses untagged dynamic host configuration protocol (DHCP) packets to communicate with the DHCP server.
  • o disable this feature on VLT and port channels, use no lacp ungroup member-independent {vlt | port-channel} command under the configuration mode.
  • 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 EMC 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.

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