Westermo RedFox Series - Manual (Page 649)

Westermo OS Management Guide
Version 4.17.0-0
29.1.1 Overview of IP multicast
Multicast is an efficient data distribution mechanism for purposes of reaching
more than one receiver. IP multicast applications, such as a camera, need only
send one packet to reach a group of receivers.
The network infrastructure,
switches and routers, send a copy of the packet to each subscriber of the group.
A multicast group is an IP address. In IPv4 the entire 224.0.0.0/4 block is reserved,
i.e., 224.0.0.0 – 239.255.255.255. However, not all address are available to the
end-user and some use-cases may not provide the most optimal distribution in
switched (layer-2) networks.
The 224.0.0.0/24 subnet (224.0.0.*) is reserved for control protocols, e.g., IGMP,
RIPv2 and OSPF.
Like regular IP addresses IP multicast groups must be translated to Ethernet (LAN)
MAC addresses. However, the range of reserved MAC multicast addresses is too
small, see RFC1112[
6 ] for details.
The lack of reserved multicast MAC addresses may be a problem in switched
networks where the switch fabric often only supports IGMP Snooping (
Sec. 18.1 ),
i.e., filtering, per MAC address. E.g., subscribers of group 224.1.2.3 will also
receive all traffic sent to group 225.1.2.3.
This is due to the mapping to MAC addresses, in our case
❼ 224.1.2.3 maps to 01:00:5e:01:02:03
❼ 225.1.2.3 maps to 01:00:5e:01:02:03
❼ etc.
On a per LAN basis (layer-2) IP multicast is managed by IGMP (routers) and IGMP
Snooping (switches). Managing multicast on this level is important due to its
inherent broadcast nature. Knowledge of this can be very important when de-
bugging multicast (re)distribution and routing.
Routing of IP multicast can be done either dynamically (e.g., DVMRP, PIM) or
statically. WeOS currently only supports the latter.
29.1.2 Static multicast routing
Contrary to static unicast, multicast has a separate routing table and is handled
a little bit differently. To be able to route multicast you need the following:
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