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Page 8 | AlliedWare™ OS How To Note: IGMP
IGMP overview > Choosing group addresses
Avoid x.0.0.y, x.0.
1
.y, x.
1
28.0.y, and x.
1
28.
1
.y
It is particularly important to avoid using any address in the ranges x.0.0.y, x.
1
28.0.y, x.0.
1
.y,
or x.
1
28.
1
.y (where x is 224-239 and y is
1
-254).
This is because x.0.0.y and x.
1
28.0.y will map to the same multicast MAC address as 224.0.0.y.
Similarly, x.0.
1
.y and x.
1
28.
1
.y will map to the same multicast MAC address as 224.0.
1
.y. Most
addresses in the ranges 224.0.0.y and 224.0.
1
.y are reserved for contacting all routers, or for
routing protocol messages, so they are always flooded out all ports in the relevant VLAN.
Therefore, all addresses in the ranges x.0.0.y, x.
1
28.0.y, x.0.
1
.y, or x.
1
28.
1
.y are flooded out
every port in the relevant VLAN. Using these addresses can significantly increase multicast
traffic in your network.
If you are debugging a situation where it seems that certain multicast groups are forwarded
when you think they shouldn’t be, check whether the choice of group addresses has violated
any of the recommendations above.