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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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VLT Multicast Routing

VLT multicast routing is a type of VLT peer routing that provides resiliency to multicast routed traffic during the multicast routing protocol convergence period after a VLT link or VLT peer fails using the least intrusive method (PIM) and does not alter current protocol behavior.

Unlike VLT unicast routing, a normal multicast routing protocol does not exchange multicast routes between VLT peers. When you enable VLT multicast routing, the multicast routing table is synced between the VLT peers. Only multicast routes configured with a Spanned VLAN IP as their IIF are synced between VLT peers. For multicast routes with a Spanned VLAN IIF, only OIFs configured with a Spanned VLAN IP interface are synced between VLT peers.

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 multicast until 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 on VLT ports.
  • Optimal VLTi forwarding — Only one copy of the incoming multicast traffic is sent on the VLTi for routing or forwarding to any orphan ports, 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 all ports must be open.
  • ECMP is not compatible on VLT nodes using VLT multicast. You must use a single VLAN.

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