Connectivity Guide

Table Of Contents
Virtual Link Trunking
Virtual Link Trunking (VLT) is a Layer 2 (L2) aggregate protocol between end devices such as servers connected to different
network devices. VLT reduces the role of Spanning Tree Protocols (STPs) by allowing link aggregation group (LAG) terminations
on two separate distributions or core switches.
VLT:
Allows a single device to use a LAG across two upstream devices
Provides a loop-free topology
Eliminates STP-blocked ports
Optimizes using all available uplink bandwidth
Guarantees fast convergence if either a link or device fails
Enhances optimized forwarding with Virtual Router Redundancy Protocol (VRRP)
Provides link-level resiliency
Assures high availability
VLT provides L2 multipathing, creating redundancy through increased bandwidth, enabling multiple parallel paths between nodes
and load-balancing traffic where alternative paths exist.
VLT presents a single logical L2 domain from the perspective of attached devices that have a virtual link trunk terminating on
separate nodes in the VLT domain. The two VLT nodes are independent Layer2/ Layer3 (L2/L3) switches for devices in the
upstream network. L2/L3 control plane protocols and system management features function normally in VLT mode.
VLT configurations must be identical on both sides of a trunk. External switches or servers with LACP see the VLT switches as
a single virtual switch.
VLT physical
ports
802.1p, 802.1q, LLDP, flow control, port monitoring, and jumbo frames are supported on VLT physical
ports.
System
management
protocols
All system management protocols are supported on VLT ports SNMP, RMON, AAA, ACL, DNS, FTP,
SSH, syslog, NTP, RADIUS, SCP, and LLDP.
L3 VLAN
connectivity
Enable L3 VLAN connectivity, VLANs assigned with an IP address, on VLT peers by configuring a VLAN
interface for the same VLAN on both devices.
Optimized
forwarding with
VRRP
To ensure the same behavior on both sides of the VLT nodes, VRRP requires state information
coordination. VRRP Active-Active mode optimizes L3 forwarding over VLT. By default, VRRP Active-
Active mode is enabled on all the VLAN interfaces. VRRP Active-Active mode enables each peer to locally
forward L3 packets, resulting in reduced traffic flow between peers over the VLTi link.
Spanning-Tree
Protocol
VLT ports support RSTP, RPVST+, and MSTP.
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