Siemens RUGGEDCOM ROX II User Manual page 337

RUGGEDCOM ROX II
CLI User Guide
• 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 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 frame's 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.
If RSTP detects a change in the network topology, IGMP will take some actions to avoid the loss of multicast
connectivity and reduce network convergence time:
• The switch will immediately issue IGMP queries (if in IGMP Active mode) to obtain potential new group
membership information.
• The switch can be configured to flood multicast streams temporarily out of all ports that are not RSTP Edge
Ports.
Combined Router and Switch IGMP Operation
The following example illustrates the challenges faced with multiple routers, VLAN support and switching.
Producer P1 resides on VLAN 2 while P2 resides on VLAN 3. Consumer C1 resides on both VLANs whereas C2 and
C3 reside on VLANs 3 and 2, respectively. Router 2 resides on VLAN 2, presumably to forward multicast traffic to a
remote network or act as a source of multicast traffic itself.
IGMP
Chapter 8
Layer 2
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