The advantages of syncing the multicast routes between VLT peers are:• VLT resiliency — After a VLT link or peer failure, if the traffic hashes to the VLT peer, the traffic continues to be routed using multicastuntil the PIM protocol detects the failure and adjusts the multicast distribution tree.• Optimal routing — The VLT peer that receives the incoming traffic can directly route traffic to all downstream routers connected onVLT ports.• Optimal VLTi forwarding — Only one copy of the incoming multicast traffic is sent on the VLTi for routing or forwarding to any orphanports, rather than forwarding all the routed copies.Important Points to Remember• 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 allports must be open.• ECMP is not compatible on VLT nodes using VLT multicast. You must use a single VLAN.Configuring VLT MulticastTo 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 modevlt domain domain-id2 Enable peer-routing.VLT DOMAIN modepeer-routing3 Configure the multicast peer-routing timeout.VLT DOMAIN modemulticast peer-routing—timeout valuevalue: 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 DesignatedRouter.5 Configure a PIM-enabled external neighboring router as a rendezvous point (RP). For more information, refer to Configuring a StaticRendezvous Point.6 Configure the VLT VLAN routing metrics to prefer VLT VLAN interfaces over non-VLT VLAN interfaces. For more information, refer toClassify Traffic.7 Configure symmetrical Layer 2 and Layer 3 configurations on both VLT peers for any spanned VLAN.Non-VLT ARP SyncARP entries (including ND entries) learned on other ports are synced with the VLT peer to support station move scenarios.NOTE: ARP entries learned on non-VLT, non-spanned VLANs are not synced with VLT peers.994 Virtual Link Trunking (VLT)
