Users Guide

● 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 Technologies 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 Technologies recommends not to change the PIM designated router by configuring a non-default
value using the ip pim dr-priority command.
● In large-scale multicast deployments, you might see frequent bursts of multicast control traffic. For such deployments, Dell
Technologies recommends that you increase the burst size for queue 2 on all PIM routers using control-plane policing. For
more information about how to configure a QoS policy to rate limit control-plane traffic, see Configure control-plane policing.
Example: Spanned L3 VLAN IIF
This section describes how to configure VLT multicast routing in a four-node setup—core, AG1, AG2, and ToR—using the
topology shown in the following figure:
● Core, AG1, and AG2 are multicast routers in a VLT domain.
● ToR is an IGMP-enabled L2 switch.
● OSPF is the unicast routing protocol.
Multicast
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