RUGGEDCOM ROS
User Guide
Chapter 5
Setup and Configuration
IGMP Concepts
219
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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
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 IGMPv2
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 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 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.
