Users Guide

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).
– In a VLT domain, ingress and egress QoS policies are supported on physical VLT ports, which can be members of VLT port
channels in the domain.
* Ingress and egress QoS policies applied on VLT ports must be the same on both VLT peers.
* You should apply the same ingress and egress QoS policies on VLTi (ICL) member ports to handle failed links.
– 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.
– Dell Networking does not recommend enabling peer-routing if the CAM is full. To enable peer-routing, a minimum of two local
DA spaces for wild card functionality are required.
• Software features supported on VLT physical ports
– In a VLT domain, the following software features are supported on VLT physical ports: 802.1p, LLDP, ow control, IPv6
dynamic routing, port monitoring, and jumbo frames.
• Software features not supported with VLT
– In a VLT domain, the following software features are not supported on VLT ports: 802.1x, DHCP snooping, FRRP, GVRP,
ERSPAN, RSPAN, VXLAN, ingress and egress QOS.
• 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.
– 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.
– If the primary chassis fails, the secondary chassis takes on the operational role of the primary.
• The SNMP MIB reports VLT statistics.
Virtual Link Trunking (VLT)
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