Administrator Guide

VLT domain, congure the port interfaces on each VLT peer as hybrid ports before adding them to the port channel (refer to
Connecting a VLT Domain to an Attached Access Device (Switch or Server)). To congure a port in Hybrid mode so that it can
carry untagged, single-tagged, and double-tagged trac, use the portmode hybrid command in Interface Conguration
mode as described in Conguring Native VLANs.
• For example, if the DHCP server is on the ToR and VLTi (ICL) is down (due to either an unavailable peer or a link failure),
whether you congured the VLT LAG as static or LACP, when a single VLT peer is rebooted in BMP mode, it cannot 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 ports: 802.1X with multi-supplicant dynamic VLAN
assignments and MAC Authentication Bypass, VRRP, Layer 3 VLANs, IGMP Snooping, FRRP, DHCP Snooping, DHCP relay, sFlow,
ingress and egress QoS, ingress and egress ACLs, DCB and Layer 2 control protocols such as RSTP (see Conguring Rapid
Spanning Tree).
NOTE: PVST+ passthrough is supported in a VLT domain. PVST+ BPDUs does not result in an interface shutdown.
PVST+ BPDUs for a nondefault VLAN is ooded out as any other L2 multicast packet. On a default VLAN, RTSP is
part of the PVST+ topology in that specic VLAN (default VLAN).
• For detailed information about how to use VRRP in a VLT domain, refer to the following VLT and VRRP interoperability section.
• For information about conguring IGMP Snooping in a VLT domain, refer to VLT and IGMP Snooping.
• All system management protocols are supported on VLT ports, including SNMP, RMON, AAA, ACL, DNS, FTP, SSH, Syslog, NTP,
RADIUS, SCP, TACACS+, Telnet, and LLDP.
• Enable Layer 3 VLAN connectivity VLT peers by conguring a VLAN network interface for the same VLAN on both switches.
• Software features supported on VLT port-channels:
• In a VLT domain, the following software features are supported on VLT physical ports: 802.1p, LLDP, ow control, port monitoring,
and jumbo frames.
• Software features supported on non-VLT ports:
• In a VLT domain, the following software features are supported on non-VLT ports: OSPF, BGP, IS-IS, DHCP relay, sFlow, ingress and
egress QOS, ingress and egress ACLs, 802.1x, and all protocols currently supported in Dell networking OS.
• VLT and VRRP interoperability
• In a VLT domain, VRRP interoperates with virtual link trunks that carry trac to and from access devices (refer to Overview). The
VLT peers belong to the same VRRP group and are assigned master and backup roles. Each peer actively forwards L3 trac,
reducing the trac ow over the VLT interconnect.
• VRRP elects the router with the highest priority as the master in the VRRP group. To ensure VRRP operation in a VLT domain,
congure VRRP group priority on each VLT peer so that a peer is either the master or backup for all VRRP groups congured on its
interfaces. For more information, refer to Setting VRRP Group (Virtual Router) Priority.
• To verify that a VLT peer is consistently congured for either the master or backup role in all VRRP groups, use the show vrrp
command on each peer.
• Also congure the same L3 routing (static and dynamic) on each peer so that the L3 reachability and routing tables are identical on
both VLT peers. Both the VRRP master and backup peers must be able to locally forward L3 trac in the same way.
• In a VLT domain, although both VLT peers actively participate in L3 forwarding as the VRRP master or backup router, the show
vrrp
command output displays one peer as master and the other peer as backup.
• Failure scenarios
• On a link failover, when a VLT port channel fails, the trac destined for that VLT port channel is redirected to the VLTi to avoid
ooding.
• When a VLT switch determines that a VLT port channel has failed (and that no other local port channels are available), the peer
with the failed port channel noties the remote peer that it no longer has an active port channel for a link. The remote peer then
enables data forwarding across the interconnect trunk for packets that would otherwise have been forwarded over the failed port
channel. This mechanism ensures reachability and provides loop management. If the VLT interconnect fails, the VLT software on the
primary switch checks the status of the remote peer using the backup link. If the remote peer is up, the secondary switch disables
all VLT ports on its device to prevent loops.
• If all ports in the VLT interconnect fail, or if the messaging infrastructure fails to communicate across the interconnect trunk, the
VLT management system uses the backup link interface to determine whether the failure is a link-level failure or whether the remote
peer has failed entirely. If the remote peer is still alive (heartbeat messages are still being received), the VLT secondary switch
disables its VLT port channels. If keepalive messages from the peer are not being received, the peer continues to forward trac,
assuming that it is the last device available in the network. In either case, after recovery of the peer link or reestablishment of
message forwarding across the interconnect trunk, the two VLT peers resynchronize any MAC addresses learned while
communication was interrupted and the VLT system continues normal data forwarding.
986
Virtual Link Trunking (VLT)