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Frame Flooding in Multicast VLANs
Multicast traffic is flooded as follows in an environment that includes multicast VLANs:
• If the destination address is a multicast address,
and
• if the destination multicast address is in the filtering database,
and
• if the destination multicast address is a specified multicast address for a multicast VLAN, then
flood the traffic on all ports that have at least one multicast VLAN in common with the desti-
nation multicast address. This is illustrated below. If any of the conditions described above
are untrue, the traffic is flooded as it is for normal AutoTracker VLANs.
12345678
123456
Omni Switch/Router
Port 2
Client A
Client B
Client C
Client D
Server 1
Server 2
Group 3
Ports 1 – 6
Multicast VLAN 10
Ports 3 and 4
Multicast address = CNN
Port 1
Port 3
Port 4
4
Multicast VLAN 20
Ports 5 and 6
Multicast address = NBC
CNN
Port 6
Port 5rt 5
CNN
NBC
NBC
Multicast
Address CNN
Multicast
Address NBC
When the Switch receives a frame with multicast destination address
CNN from Server 1, the Switch locates the CNN multicast address in
the filtering database (or places the address into the filtering data-
base if it is not there already). The Switch then uses this address to
determine the multicast VLAN(s) – if any – out which to flood the
frame. In this example, the Switch determines that multicast VLAN 10
should receive the frame, since multicast VLAN 10 spec-
ifies multicast address CNN. The Switch
then checks through the port
records to determine if any ports
are assigned to multicast VLAN
10. It finds ports 3 and 4 are so
assigned, and floods the frame
out those ports only.
For this Example, the Filtering Database is:
MAC Address Port VLAN Membership Type
CNN n/a 10 MVLAN
NBC n/a 20 MVLAN
Server 1 1 1 BRIDGE
Server 2 2 1 BRIDGE
Client A 3 1 BRIDGE
Client B 4 1 BRIDGE
Client C 5 1 BRIDGE
Client D 6 1 BRIDGE
For this Example, the Port Records are:
Port
VLAN Membership MVLAN Membership
1 1 none
2 1 none
31 10
41 10
51 20
61 20
The filtering database is a record of source
MA
C
addresses, their ports of entry into the switch, an
d
their VLAN membership. Note that the ports of entr
y
for multicast addresses CNN and NBC are irrelevan
t
in the filtering database. This table is for informa
-
tional purposes—it is not available in the UI.
The port records show the VLAN and multicast
VLAN (MVLAN) membership of each port. This
table is for informational purposes only—it is
not available as a UI command.