Siemens RX1500 User Manual page 597

RUGGEDCOM ROX II
User Guide
Switch IGMP Operation
The IGMP Snooping feature provides a means for switches to snoop (i.e. watch) the operation of routers,
respond with joins/leaves on the behalf of consumer ports, and prune multicast streams accordingly. There are
two modes of IGMP the switch can be configured to assume: active and passive.
• Active Mode
IGMP supports a routerless mode of operation.
When such a switch is used without a multicast router, it is able to function as if it is a multicast router sending
IGMP general queries.
• Passive Mode
When such a switch is used in a network with a multicast router, it can be configured to run Passive IGMP. This
mode prevents the switch from sending the queries that can confuse the router causing it to stop issuing IGMP
queries.
NOTE
A switch running in passive mode requires the presence of a multicast router or it will be unable to
forward multicast streams at all if no multicast routers are present.
NOTE
Without a multicast router, at least one IGMP Snooping switch must be in active mode to make IGMP
functional.
IGMP Snooping Rules
IGMP Snooping adheres to the following rules:
• When a multicast source starts multicasting, the traffic stream will be immediately blocked on segments from
which joins have not been received.
• Unless configured otherwise, the switch will forward all multicast traffic to the ports where multicast routers are
attached.
• Packets with a destination IP multicast address in the 224.0.0.X range that are not IGMP are always forwarded
to all ports. This behavior is based on the fact that many systems do not send membership reports 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,
as 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 querier is the router with the
lowest IP address. All other routers become non-queriers, participating only in forwarding multicast traffic.
Switches running in active mode participate in the querier election the same as 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 does not have an assigned IP address.
NOTE
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
IGMP
Chapter 5
Setup and Configuration
565