Westermo RedFox Series User Manual
Page 383
Westermo OS Management Guide
Version 4.17.0-0
❼ Ports shared between multiple VLANs may have different IGMP snooping
settings on different VLANs, i.e., one VLAN may have IGMP snooping enabled
and another may have it disabled. The disabled mode takes precedence on
such ports, i.e., multicast will be flooded on ports where at least one VLAN
has IGMP Snooping disabled
As part of the IGMP snooping functionality, the switch can also act as an IGMP
Querier, and settings for querier mode, and query interval are provided.
Querier mode: By default the switch has auto mode enabled. It relies on the
standard IGMP protocol to elect a designated IGMP querier on each LAN
With auto mode unknown multicast is flooded to the elected querier, which
acts as a distribution point to the rest of the network. Keep this in mind
when designing your network, for many use-cases it is valuable information.
The forced querier mode is a non-standard setting specific to WeOS that in
some cases can be more fault tolerant. When all switches on a LAN are set
to this mode they will all discard the election mechanism of the protocol and
always send queries. This not only incurs a notable penalty on the network
due to the overhead of IGMP messages flooding the LAN, but it also causes
all unknown multicast to be flooded to all switches. This makes the network
less vulnerable to the loss of one querier and all multicast is always available
to end devices. It is however not recommended due to the broadcast like
effects it causes.
In proxy mode, the switch normally only acts as a silent forwarder of IGMP
queries (and reports) between the IGMP querier and end devices. However,
to prevent loss of multicast traffic in the case when there exist no elected
IGMP querier on a LAN, the switch will initiate queries with the source IP
address 0.0.0.0
. This feature of proxy mode can be used to optimise low-
bandwidth setups, see
for more information.
On VLANs where the network interface is not assigned an IP address, the
switch will automatically fall back to proxy mode, regardless of the querier
mode setting.
Query interval: The switch can be configured to send out queries on intervals
12, 30, 70 and 150 seconds, default 12 sec. This interval is also used when
timing out multicast to end devices that for some reason stop answering the
2
The querier with the lowest IP address on each LAN is elected. Usually the gateway or multicast
router.
3
Address 0.0.0.0 is a special case and is never part of the IGMP querier election process, as
clearly stated in the standard.
➞ 2015 Westermo Teleindustri AB
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