Administrator Guide

On each VLAN where the VLT peer nodes act as the first hop or last hop routers, one of the VLT peer nodes is
elected as the PIM designated router. If you configured IGMP snooping along with PIM on the VLT VLANs, you
must configure VLTi as the static multicast router port on both VLT peer switches. This ensures that for first
hop routers, the packets from the source are redirected to the designated router (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
Virtual Link Trunking (VLT) 961