Reference Guide

Virtual Link Trunking (VLT) | 297
VLT Multicast
VLT multicast provides multiple alternate paths for resiliency against link and node failures. This feature
supports inter-server multicast communication between top-of-rack (ToR) switches using an inter-VLAN
Layer 3 routing protocol (for example, PIM, IS-IS, or OSPF). It also provides traffic resiliency during
multicast routing convergence after failure without disrupting or altering multicast routing behavior.
Layer 2 protocols from the ToR to the server are intra-rack and inter-rack. No spanning tree is required, but
interoperability with spanning trees at the aggregation layer is supported. Communication is active-active,
with no blocked links. MAC tables are synchronized between VLT nodes for bridging and IGMP snooping
can be enabled.
VLT multicast is also scalable, so additional racks can be implemented in an existing system to provide
additional resiliency and resources to accommodate an increased need or to anticipate future growth. You
can enable VLT unicast and multicast routing across multiple configurations using VLT links. Protocols
such as BGP, OSPF, and PIM are compatible with VLT multicast.
Important Points to Remember:
• A VLT node cannot be configured as a rendezvous point (RP), but any PIM-SM compatible VLT node
can serve as a designated router (DR).
• Only one spanned VLAN with PIM enabled from both PIM-enabled peer routers inside the VLT
domain to an external neighboring PIM router can be used.
• If multiple spanned VLANs are connected to a PIM neighbor, or if both spanned and non-spanned
VLANs can access the PIM neighbor, ECMP can cause the PIM protocol running on each VLT peer
node to choose a different VLAN or IP route to reach the PIM neighbor. This can result in issues with
multicast route syncing between peers.
• Both VLT peers require symmetric Layer 2 and Layer 3 configurations on both VLT peers for any
spanned VLAN.
• For optimal performance, configure the VLT VLAN routing metrics to prefer VLT VLAN interfaces
over non-VLT VLAN interfaces.
• When using factory default settings on a new switch deployed as a VLT node, packet loss may occur
due to the requirement that all ports must be open.
• ECMP can be enabled on VLT nodes using VLT unicast; however, ECMP is not compatible on VLT
nodes using VLT multicast. You must use a single VLAN.
Configuring VLT Multicast
To enable and configure VLT multicast, follow these steps.
Step Task
Command
Syntax Command Mode
1 Enter interface configuration mode.
vlt domain
domain-id
CONFIGURATION
2 Enable peer-routing.
peer-routing
VLT DOMAIN