Dell Force10 Z9000 FTOS Configuration Guide for Z9000 System - Page 921

• The chassis backup link does not carry control plane information or data traffic. Its use is restricted to health checks only. • Virtual link trunks (VLTs) between access devices and VLT peer switches: • To connect servers and access switches with VLT peer switches, you use a VLT port channel (see Figure 48-1). Up to 48 port-channels are supported; up to 8 member links are supported in each port channel between the VLT domain and an access device. • The ID number of the port channel that connects an access device and a VLT switch is automatically generated by the discovery protocol running between VLT peers. The discovery protocol uses LACP properties to identify connectivity to a common client device and automatically generate a VLT number for port channels on VLT peers that connect to the device. The discovery protocol requires that an attached device should always run LACP over the port-channel interface. • VLT provides a loop-free topology for port channels with endpoints on different chassis in the VLT domain. • VLT uses shortest path routing so that traffic destined to hosts via directly attached links on a chassis does not traverse the chassis-interconnect link. • VLT allows multiple active parallel paths from access switches to VLT chassis. • VLT supports port-channel links with LACP between access switches and VLT peer switches. Dell Force10 recommends that you use static port channels on VLTi. • If VLTi connectivity with a peer is lost but the VLT backup connectivity indicates the peer is still alive, the VLT ports on the Secondary peer are orphaned and will be shut down. In one possible topology, a switch uses the BMP feature to receive its IP address, configuration files, and boot image from a DHCP server that connects to the switch through the VLT domain. In the port-channel used by the switch to connect to the VLT domain, you must configure the port interfaces on each VLT peer as hybrid ports before adding them to the port channel (see Connect a VLT domain to an attached access device (switch or server)). To configure a port in hybrid mode so that it can carry untagged, single-tagged, and double-tagged traffic, enter the portmode hybrid command in interface configuration mode as described in Native VLANs. • For example, if the DHCP server is located on the ToR and VLTi (ICL) is down (due to either an unavailable peer or a link failure), whether the VLT LAG is configured as static or LACP, when a single VLT peer is rebooted in BMP mode, it will not be able to reach the DHCP server, resulting in BMP failure. • Software features supported on VLT port-channels: • In a VLT domain, the following software features are supported on VLT port-channels: 802.1p, designated router functionality of PIM Sparse-Mode, VRRP, Layer 3 VLANs, LLDP, flow control, port monitoring, jumbo frames, IGMP Snooping, sFlow, ingress and egress ACLs, and Layer 2 control protocols RSTP only). Note: PVST+ passthrough is supported in a VLT domain. PVST+ BPDUs will not result in an interface shutdown. PVST+ BPDUs for a non-default VLAN will be flooded out as any other L2 multicast packet. On a default VLAN, RTSP will be part of the PVST+ topology in that specific VLAN (default VLAN). • Refer to VLT and VRRP interoperability: for detailed information on how to use VRRP in a VLT domain. • Refer to VLT and IGMP Snooping for information on configuring IGMP Snooping in a VLT domain. Virtual Link Trunking (VLT) | 921