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Support for Multicast

This section describes how the Contrail Controller supports broadcast and multicast.

Subnet Broadcast

Multiple subnets can be attached to a virtual network when it is spawned. Each of the subnets has one subnet broadcast route installed in the unicast routing table assigned to that virtual network. The recipient list for the subnet broadcast route includes all of the virtual machines that belong to that subnet. Packets originating from any VM in that subnet are replicated to all members of the recipient list, except the originator. Because the next hop is the list of recipients, it is called a composite next hop.

If there is no virtual machine spawned under a subnet, the subnet routing entry discards the packets received. If all of the virtual machines in a subnet are turned off, the routing entry points to discard. If the IPAM is deleted, the subnet route corresponding to that IPAM is deleted. If the virtual network is turned off, all of the subnet routes associated with the virtual network are removed.

Subnet Broadcast Example

The following configuration is made:

  1. Virtual network name – vn1

  2. Unicast routing instance – vn1.uc.inet

  3. Subnets (IPAM) allocated –;;

  4. Virtual machines spawned – vm1 (; vm2 (; vm3 (; vm4 (

The following subnet route additions are made to the routing instance vn1.uc.inet.0:

  1. -> forward to NH1 (composite next hop)

  2. -> DROP

  3. -> forward to NH2

  4. The following entries are made to the next-hop table:

  5. NH1 –;;

  6. NH2 –

If traffic originates for from vm1 (, it will be forwarded to vm2 ( and vm3 ( The originator vm1 ( will not receive the traffic even though it is listed as a recipient in the next hop.

Host Broadcast

The host broadcast route is present in the host routing instance so that the host operating system can send a subnet broadcast/all-broadcast (limited-broadcast). This type of broadcast is sent to the fabric by means of a vhost interface. Additionally, any subnet broadcast/all-broadcast received from the fabric will be handed over to the host operating system.