Intelligent Multicasting
Problem: Intelligent Multicasting can block protocols to non-multicast routers. If you have enabled Intelligent Multicasting and configured a VLAN attached to one or more non-multicast routers or multicast-capable endstations, Intelligent Multicasting will configure router ports where multicast-enabled routers reside. These multicast router ports are necessary to allow all multicast packets to the adjacent multicast routers. Non- multicast enabled routers will not be considered router ports, and will not receive multicast traffic for which an Intelligent Multicast session was created. The problem can arise when multiple IP multicast addresses map to the same multicast MAC address, resulting in protocol packets not being sent to the adjacent non-multicast enabled routers.
EXAMPLE:
The unicast routing protocol in use on all connected routers is OSPF, and all ports are on the same VLAN. An endstation joins the IP multicast group 226.128.0.5 on port 1. The MAC address for the group is 01:00:5E:00:00:05. IGMP snooping creates a session for this MAC address, with port 1 as the client port. There is a non-multicast OSPF router attached to port 2. OSPF uses the IP multicast link-scoped group 224.0.0.5, which also maps to a MAC address of 01:00:5E:00:00:05. Because port 2 is not a router port, and it is not part of the 01:00:5E:00:00:05 session, the switch will only pass OSPF messages out port 1.
Other protocols, such as the Service Location Protocol (RFC 2608), use 224.0.1.22 and 224.0.1.35, which can be blocked by endstations joining sessions that map to the same MAC address.
Workaround: Make certain that all ports connected to a router are configured as router ports to ensure that all router-to-router messages will not be blocked. If other non- router protocols such as the Server Location service are in use, create static sessions as needed. Also, do not create static sessions that will conflict with the protocols in use on your network. For a complete list of internet multicast addresses recognized by the IANA, go to http://www.isi.edu/in-notes/iana/assignments/multicast-addresses.
NOTE: By default, the default “Rate Limiting” state for 10/100 Megabit ports is Enabled, and multicast traffic is rate-limited (to 20%) on 10/100 Megabit ports. When transmitted from these ports, multicast traffic is rate-limited unless Intelligent Multicasting is enabled. If Intelligent Multicasting is enabled, the multicast traffic for which the Intelligent Multicast session was created will not be subject to rate limiting unless the rate limiting state is set to Enabled (all multicast included). If you do not want to enable rate limiting of multicast traffic on a port, either 1)Enable Intelligent Multicasting, or 2) Disable Rate-limiting on the port.
