Siemens RUGGEDCOM ROX II User Manual page 460

Chapter 5
Setup and Configuration
• 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.
Figure 6: Example – Combined Router and Switch IGMP In Operation
1. Producer 2. Multicast Router 1
422
P1
1
P2
3. Multicast Router 2
2
C1 C2
5
4. Switch 5. Host
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4
C3
IGMP