(DR) if they are incorrectly hashed. In addition to being first-hop or last -hop routers, the peer node can
also act as an intermediate router.
The VLT peer nodes can also act as normal PIM routers on Layer 3 ports and on VLANS that do not have
any VLT port members. In addition to being first-hop or last-hop routers, the peer node can also act as
an intermediate router.
To route traffic to and from the multicast source and receiver that are connected to VLT ports, enable
PIM-Sparse mode on the VLANs to which the VLT ports belong using the ip pim sparse-mode
command. If IGMP Snooping is configured on these VLANs, the VLTi must be configured as a static
multicast router port on both VLT peers.
To verify the PIM neighbors on the VLT VLAN and on the multicast port, use the show ip pim
neighbor, show ip igmp snooping mrouter, and show running config commands.
You can configure virtual link trunking (VLT) peer nodes as rendezvous points (RPs) in a Protocol
Independent Multicast (PIM) domain.
If the VLT node elected as the designated router fails, traffic loss occurs until another VLT node is elected
the designated router.
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.
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