Users Guide

VLT multicast routing
OS10 supports multicast routing in a VLT domain for IPv4 networks. This feature provides resiliency to multicast-routed traffic when a
VLT peer node or the VLTi link goes down.
Multicast routing table synchronization
Multicast routing protocols do not exchange multicast routes between peer VLT nodes. Each VLT node runs the PIM protocol
independent of the peer VLT node. Hence, the PIM states do not synchronize between the nodes. However, OS10 synchronizes the
multicast routing table with routes that the PIM learns on each of the nodes between the peer VLT nodes. Multicast routing table
synchronization:
• Avoids unoptimized forwarding over VLTi links. Table synchronization allows the incoming traffic sent to the wrong peer to be routed
locally within the device.
• Provides traffic resiliency in the event of a VLT node failure. The traffic is forwarded until the PIM protocol reconverges and builds a
new tree.
IGMP message synchronization
VLT nodes use the VLTi link to synchronize IGMP messages across their peers. Any IGMP join message that is received on one of the VLT
nodes synchronizes with the peer node. Therefore, the IGMP tables are identical in a VLT domain.
Egress mask
When multicast traffic from the source arrives at one of the VLT peer nodes, it is sent to the downstream receivers using local routing or
switching and over the VLTi link. The port block at the VLTi link of the peer node drops the multicast traffic. This port block, also known as
the egress mask, avoids duplicate traffic forwarding on the VLT port channel by both VLT nodes. However, if the receiver is connected to
the peer node, the system forwards the multicast traffic to the receiver.
Spanned VLAN
Any VLAN configured on both the VLT peer nodes is known as a spanned VLAN. The VLT interconnect (VLTi) port is automatically added
as a member of the spanned VLAN. Any adjacent router connected to at least one VLT node on a spanned VLAN subnet is directly
reachable from both the VLT peer nodes at the L3 level.
• Spanned VLAN L3 interface: If you enable PIM on each of the spanned VLAN L3 interfaces on both VLT nodes, the interface is a
spanned VLAN L3 interface.
• Spanned VLT VLAN L3 interface: Includes all spanned L3 VLANs that have at least one VLT port that is configured as a port
channel member.
• Spanned non-VLT VLAN L3 interface: Includes all spanned VLANs that do not have VLT ports configured as port channel
members.
• Nonspanned L3 interface: All point-to-point interfaces or L3 VLANs that do not have VLT ports configured as port channel members.
For more information, see Deployment considerations.
Deployment considerations
Dell EMC recommends the following:
• In a VLT-enabled PIM router, multicast routing is not supported when there are multiple PIM spanned paths to reach the source or RP.
Configure only one PIM spanned path to reach any PIM router in the aggregation or spine.
• If a source is connected to a nonspanned interface of the VLT peer nodes and the RP is reachable on a spanned interface from both
the VLT nodes, the receiver might receive duplicate traffic. To avoid duplicate traffic, configure the source to be reachable on a
spanned interface.
• For better convergence, the upstream incoming interface (IIF) and the downstream outgoing interface (OIF) must be a spanned
VLAN.
• In VLT deployments, Dell EMC recommends not to change the PIM designated router by configuring a non-default value using the ip
pim dr-priority command.
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