918 | Virtual Link Trunking (VLT)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m VLT domain - This domain includes both VLT peer devices, the VLT interconnect, and all of the portchannels in the VLT connected to the attached devices. It is also associated to the configuration mode thatmust be used to assign VLT global parameters.VLT peer device - One of a pair of devices that are connected with the special port channel known as theVLT interconnect (VLTi).Configuring Virtual Link TrunkingVLT requires that you enable the feature and then configure the VLT domain, backup link, and VLTinterconnect on both peer switches.Important Points to Remember• VLT port channel interfaces must be switch ports.• If RSTP is included on the system, it must be configured before VLT. See RSTP Configuration.• Dell Force10 strongly recommends that the VLTi (VLT interconnect) must be a static LAG and thatLACP should be disabled on the VLTi.• The spanning tree root bridge should be at the Aggregation layer. If RSTP is enabled on the VLTdevice, refer to RSTP and VLT for guidelines to avoid traffic loss.• If both VLT peers are rebooted in BMP mode and VLT LAGs are static, the DHCP server reply to theDHCP 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. Ifsupported by the ToR, enable the lacp-ungroup feature on the ToR using the command lacp ungroupmember-independent port-channel.• If the lacp-ungroup feature is not supported on the ToR, VLT peers should be rebooted one at a time.After rebooting, verify that VLTi (ICL) is active before attempting DHCP connectivity.• When IGMP snooping is enabled on the VLT peers, ensure the value of the delay-restore command isnot less than the query interval.• When Layer 3 routing protocols are enabled on VLT peers, make sure the delay-restore timer is set to avalue that allows sufficient time for all routes to establish adjacency and exchange all the L3 routesbetween the VLT peers before the VLT ports are enabled.• The lacp ungroup member-independent command should only be used if the system connects to nodesusing BMP to upgrade or boot from the network.• Ensure all port channels where LACP ungroup is applicable are configured as hybrid ports and asuntagged members of a VLAN. BMP uses untagged DHCP packets to communicate with the DHCPserver.• If the DHCP server is located on the ToR and the VLTi (ICL) is down due to a failed link when a VLTnode is rebooted in BMP mode, it will not be able to reach the DHCP server, resulting in BMP failure.
