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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
You cannot configure a VLT node as a rendezvous point (RP), but any PIM-SM compatible VLT node can serve as a
designated router (DR).
You can only use one spanned VLAN from a PIM-enabled VLT node to an external neighboring PIM router.
If you connect multiple spanned VLANs 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.
You can enable ECMP 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.
1. Enable VLT on a switch, then configure a VLT domain and enter VLT-domain configuration mode.
CONFIGURATION mode
vlt domain domain-id
2. Enable peer-routing.
VLT DOMAIN mode
peer-routing
3. Configure the multicast peer-routing timeout.
VLT DOMAIN mode
multicast peer-routingtimeout value
value: Specify a value (in seconds) from 1 to 1200.
4. Configure a PIM-SM compatible VLT node as a designated router (DR). For more information, refer to Configuring a
Designated Router.
5. Configure a PIM-enabled external neighboring router as a rendezvous point (RP). For more information, refer to Configuring
a Static Rendezvous Point.
6. Configure the VLT VLAN routing metrics to prefer VLT VLAN interfaces over non-VLT VLAN interfaces. For more
information, refer to Classify Traffic.
7. Configure symmetrical Layer 2 and Layer 3 configurations on both VLT peers for any spanned VLAN.
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Virtual Link Trunking (VLT)