Users Guide

• Software features supported on VLT port-channels
• In a VLT domain, the following software features are supported on VLT port-channels: 802.1p, ingress and egress ACLs, BGP, DHCP
relay, IS-IS, OSPF, active-active PIM-SM, PIM-SSM, VRRP, Layer 3 VLANs, LLDP, ow 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 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, see the following VLT and VRRP interoperability section.
• For information about conguring IGMP Snooping in a VLT domain, see 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, IPv6 dynamic routing, ow
control, port monitoring, and jumbo frames.
• 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.
• Software features not supported with VLT
• In a VLT domain, the following software features are not supported on non-VLT ports: 802.1x, DHCP snooping, and FRRP.
• VLT and VRRP interoperability
• In a VLT domain, VRRP interoperates with virtual link trunks that carry trac to and from access devices (see 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, see 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.
• In a VRRP group, packets may be carried to the secondary VLT peer due to the LACP hash algorithm regardless of CAM table
settings. Some packets may be routed through the VLTi trunk if one of the VLT LAG ports or an uplink link fails.
• 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.
Virtual Link Trunking (VLT)
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