Concept Guide

• 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, DHCP snooping, 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, GVRP, and BFD.
• 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.
• 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.
Primary and Secondary VLT Peers
To prevent issues when connectivity between peers is lost, you can designate Primary and Secondary roles for VLT peers . You can elect or
congure the Primary Peer. By default, the peer with the lowest MAC address is selected as the Primary Peer. You can congure another
peer as the Primary Peer using the VLT domain domain-id role priority priority-value command.
If the VLTi link fails, the status of the remote VLT Primary Peer is checked using the backup link. If the remote VLT Primary Peer is available,
the Secondary Peer disables all VLT ports to prevent loops.
If all ports in the VLTi link fail or if the communication between VLTi links fails, VLT checks the backup link to determine the cause of the
failure. If the failed peer can still transmit heartbeat messages, the Secondary Peer disables all VLT member ports and any Layer 3
interfaces attached to the VLAN associated with the VLT domain. If heartbeat messages are not received, the Secondary Peer forwards
trac assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer reassigned as the Primary Peer retains
this role and the other peer must be reassigned as a Secondary Peer. Peer role changes are reported as SNMP traps.
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Virtual Link Trunking (VLT)