Westermo U200 Operator manal - Manual (Page 28)

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U/R/T200 series
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10 IGMP snooping
10.1 IP Multicast filtering
Several applications are based on multicast communication. Data is only sent once even
though the data is meant for more than one receiver. However, the multicast packets will be
sent on every drop link in the network unless the Ethernet switches support multicast filtering.
The R/T200 series support IP multicast filtering. This means that IP multicast "join" and
"leave" requests will be trapped by the switches, and the multicast filters will be set based on
which drop links where these requests are received.
10.2 Routerless operation
IGMP "Join" and "leave" request are forwarded to one or more IGMP servers (routers)
present in the network. This is one of the main properties of IGMP. The Westermo OnTime
IGMP snooping implementation does not depend on the availability of an IGMP Querier
(IGMP server) in the network. This is important from robustness or a performance point of
view. Thus, the multicast filtering feature will work even though the network connection to a
standalone IGMP Querier is lost or not, and the drop link to such standalone IGMP Querier
will not become a bandwidth bottleneck in the network, because the switch can also act as an
IGMP Querier. The IGMP Querier operation of the switch is controlled by the “Auto mode”
and “Querier” parameters. The following combinations of these two parameters are possible:
- “Auto mode” enabled + Querier enabled: the switch is able to act as an IGMP Querier
(IGMP server) and the IGMP Querier in the network is selected automatically. The
switch (with Querier support) in the network with the lowest IP address will be chosen as
the network Querier (i.e. IGMP focal point). Only one Querier will exist in the network if
all IGMP enabled switches and routers in the network have this configuration. This is
the default IGMP settings.
- “Auto mode” enabled + Querier disabled: same operation as above, but the switch
cannot act as an IGMP Querier.
- “Auto mode” disabled + Querier enabled: the switch will always act as an IGMP Querier.
Each switch/router will act as IGMP Querier if this configuration is used on each
switch/router in the network.
A switch with “Auto mode” enabled, which is not acting as the IGMP Querier, will forward
IGMP Queries received from the IGMP Querier on all ports except the port where the IGMP
Queries are received. The port where IGMP Queries are received is referred to as the
“Router port”. This port is part of every active multicast filter. The use of “Router port” is not
relevant in case “Auto mode” is disabled since the switch in this mode always is acting as a
Querier (IGMP focal point). A switch in this mode will not forward IGMP queries received.
IGMP Measurement reports for each active multicast filter on the switch will be sent back for
each IGMP Query received. This is valid for both “Auto mode” being enabled and disabled.
The interval between two IGMP query packets can also be set in the IP configuration tool.
Four intervals are possible: [12, 30, 70, 150] seconds.
The IGMP snooping implementation will also forward IGMP information (join, leave,
measurements reports) on the switch trunk ports. A trunk ports is automatically detected in
case a network redundancy protocol such as if FRNT or STP are running, but the user may
also configure manually ports as trunk ports. Manually trunk port configuration might be
relevant in case no network redundancy protocol is running on a port connected to another
IGMP snooping enabled switch. This feature is required in case the multicast producers (i.e.
Ethernet end nodes sending IP multicast packets) make no IGMP join or IGMP