25. Multicast Filtering
ROX™ v2.2 User Guide
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RuggedBackbone™ RX1500
A switch running in passive mode requires the presence of a multicast router or it will not
be able to forward multicast streams at all
If no multicast routers are present, at least one IGMP Snooping switch must be configured
for Active IGMP mode to make IGMP functional.
IGMP Snooping Rules
• When a multicast source starts multicasting, the traffic stream will be immediately blocked on
segments from which joins have not been received.
• The switch will always forward all multicast traffic to the ports where multicast routers are attached
unless configured otherwise.
• Packets with a destination IP multicast address in the 224.0.0.X range which are not IGMP are always
forwarded to all ports. This behavior is based on the fact that many systems do not send joins for IP
multicast addresses in this range while still listening to such packets.
• The switch implements “proxy-reporting”, i.e. membership reports received from downstream are
summarized and used by the switch to issue its own reports.
• The switch will only send IGMP membership reports out of those ports where multicast routers are
attached because sending membership reports to hosts could result in unintentionally preventing a
host from joining a specific group.
• Multicast routers use IGMP to elect a master router known as the querier – the one with the lowest
IP address is elected to be the querier, all other routers become non-queriers, participating only
forward multicast traffic. Switches running in Active IGMP mode participate in the querier election
like multicast routers.
• When the querier election process is complete, the switch simply relays IGMP queries received from
the querier.
• When sending IGMP packets, the switch uses its own IP address, if it has one, for the VLAN on which
packets are sent, or an address of 0.0.0.0, if it doesn’t have an assigned IP address.
IGMP Snooping switches perform multicast pruning using a multicast frames’ destination
MAC multicast address which depends on the group IP multicast address. IP address
W.X.Y.Z corresponds to MAC address 01-00-5E-XX-YY-ZZ where XX is the lower 7 bits
of X and YY and ZZ are simply Y and Z coded in hexadecimal.
One can note that IP multicast addresses such as 224.1.1.1 and 225.1.1.1 will both map
onto the same MAC address 01-00-5E-01-01-01. This is indeed a problem for which the
IETF Network Working Group currently has offered no solution. Users are advised to be
aware of and avoid this problem.
IGMP and RSTP
An RSTP change of topology can render the routes selected to carry multicast traffic as incorrect. This
results in lost multicast traffic.
If RSTP detects change in the network topology, IGMP will take some actions to avoid loss of multicast
connectivity and reduce network convergence time:
• The switch will immediately issue IGMP queries (if in IGMP Active mode) to obtain potential new
group membership information.
• The switch can be configured to flood multicast streams temporarily out of all ports that are not
configured as RSTP Edge Ports.
