Multicast Properties

Multicast forwarding enables one-to-many information dissemination. Multicast applications are useful for dissemination of information to multiple clients, where clients do not require reception of the entire content. A typical application is a cable-TV-like service, where clients can join a channel in the middle of a transmission, and leave before it ends.

The data is sent only to relevant ports. Forwarding the data only to the relevant ports conserves bandwidth and host resources on links. By default, all Multicast frames are flooded to all ports of the VLAN. It is possible to selectively forward only to relevant ports and filter (drop) the Multicast on the rest of the ports by enabling the Bridge Multicast filtering status in this section.

Multicast addresses have the following properties:

  • Each IPv4 Multicast address is in the address range 224.0.0.0 to 239.255.255.255.

  • The IPv6 Multicast address is FF00:/8.

  • To map an IP Multicast group address to an Layer 2 Multicast address:

For IPv4, this is mapped by taking the 23 low-order bits from the IPv4 address, and adding them to the 01:00:5e prefix. By standard, the upper nine bits of the IP address are ignored, and any IP addresses that only differ in the value of these upper bits are mapped to the same Layer 2 address, since the lower 23 bits that are used are identical. For example, 234.129.2.3 is mapped to a MAC Multicast group address 01:00:5e:01:02:03. Up to 32 IP Multicast group addresses can be mapped to the same Layer 2 address.

For IPv6, this is mapped by taking the 32 low-order bits of the Multicast address, and adding the prefix of 33:33. For example, the IPv6 Multicast address FF00::1122:3344 is mapped to Layer 2 Multicast 33:33:11:22:33:44.

To enable Multicast filtering, and select the forwarding method, follow these steps:

Procedure

Step 1

Click Multicast > Properties.

Step 2

Enter the parameters.

Bridge Multicast Filtering Status

Select Enable to enable filtering.

VLAN ID

Select the VLAN ID to set its forwarding method.

Forwarding Method for IPv6

Set one of the following forwarding methods for IPv6 addresses:

  • MAC Group Address—Forward packets according to the MAC Multicast group address

  • IP Group Address—Forward packets according to the IPv6 Multicast group address

  • Source-Specific IP Group Address—Forward packets according to the source IPv6 and IPv6 Multicast group address. If an IPv6 address is configured on the VLAN, the operational forwarding methods for IPv6 Multicast are IP Group Address.

    Note

    For IPv6 IP Group and Source-Specific IP Group Address modes, the device checks a match only for 4 bytes of the destination Multicast and source address. For the destination Multicast address, the last 4 bytes of group ID are matched. For the source address, the last 3 bytes + the 5th from the last byte are matched.

Forwarding Method for IPv4

Set one of the following forwarding methods for IPv4 addresses:

  • MAC Group Address—Forward packets according to the MAC Multicast group address

  • IP Group Address—Forward packets according to the IPv4 Multicast group address

  • Source-Specific IP Group Address—Forward packets according to the source IPv4 and IPv4 Multicast group address. If an IPv4 address is configured on the VLAN, the operational forwarding method for IPv4 Multicast are IP Group Address.

Step 3

Click Apply. The Running Configuration file is updated.