Tion 18.1.1, Section 18.1.1 – Westermo RedFox Series User Manual
Page 382
Westermo OS Management Guide
Version 4.17.0-0
❼ treat multicast traffic as broadcast, i.e., forward it on all ports (in the same
VLAN), or
❼ limit forwarding of multicast only to subscribers
The latter method requires switches to inspect Internet Group Management Pro-
tocol (IGMP) control messages exchanged by hosts and routers to learn which
ports lead to subscribers – this mechanism is referred to as IGMP snooping[
With IGMP Snooping enabled, WeOS switches dynamically keep track of up to
2048 multicast addresses
As part of the IGMP snooping support, WeOS also enables a switch to act as IGMP
querier – a role which is usually handled by a multicast router. Having switches
with IGMP querier capabilities enables efficient distribution of IP multicast in net-
works without multicast routers.
Warning
WeOS devices can only limit the broadcast effects of multicast on a Layer-
2 basis, it is therefore important to design IPv4 multicast networks so
that groups do not overlap. For example, 225.1.2.3 and 226.1.2.3 map
to the same multicast MAC address and will effectively be treated as the
same group. This means that both groups will be forwarded by the de-
vice and potentially overloading the intended receiver.
See RFC 1112,
for details on how IP multicast
groups map to MAC multicast addresses.
18.1.1
IGMP Snooping
The switch is capable of efficiently distributing IP(v4) multicast traffic on LAN
interfaces by means of IGMP snooping. IGMP Snooping is enabled by default per
VLAN, see
❼ With IGMP snooping enabled on a VLAN, IP multicast packets are only for-
warded to ports leading to a subscriber of that IP multicast group, and to
ports leading to an IP multicast router
❼ With IGMP snooping disabled on a VLAN, multicast traffic is forwarded on all
ports in that VLAN, i.e., like broadcast traffic
1
Special restriction for DDW-142 and DDW-142-485: On these products the MAC address
database can hold at most 1000 addresses in total (unicast and multicast MAC). Thus, the up-
per limit for multicast addresses possible to keep track of is roughly 1000.
382
➞ 2015 Westermo Teleindustri AB