Some LAN Switches with IGMP Snooping Enabled Stop Forwarding Multicast Packets on RRAS Startup (223136)

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Switches with IGMP snooping enabled have a feature that attempts to determine which ports connect to devices that belong to a particular multicast group. If the port does not connect to a device in the multicast group, the switch does not forward packets destined to the multicast group out that port. Some switches attempt to do this smart multicast forwarding for all multicast destinations, while others do this only for non-permanent groups (groups outside the range 224.0.0.1-224.0.0.255). Switches doing this for permanent groups, such as the all-routers group 224.0.0.2, the OSPF multicast groups 224.0.0.5 and 224.0.0.6, and the RIP 2 multicast group 224.0.0.9, could cause problems on the switched network.

This behavior occurs if the switch has Cisco routers connected to it, running Hot Standby Routing Protocol, OSPF, or RIP 2, and a Windows 2000 server is connected to the switch and initialized. Other routers may be affected as well.

Before the server is brought online, the routers are communicating through the switch using one or more of the above multicast addresses. The routers never send IGMP join packets for these groups so the switch never tries to parse which ports will receive the multicast packets. When the server with RRAS comes online, it sends an IGMP join packet for the all-routers multicast group (224.0.0.2), and for the OSPF and RIP 2 groups if the protocols are running. The switch sees the join message and sends a membership query out all its ports to determine which ports have devices that also belong to this group. The routers do not respond to membership queries for these multicast groups. The switch then stops sending packets destined to these multicast groups to the router's ports, and effectively disables the routing protocol communication between routers.

Hewlett-Packard (HP) and Nortel Networks (formerly Bay) switches operate in this manner when IGMP snooping is enabled. Both switches have an option for defining filters that enable them to always forward multicast packets to all ports for specific groups. These filters must be enabled to assure that the routers will continue functioning. Other switches always forward all multicast packets for these groups to all ports without requiring filters be enabled.

The IGMP join packets sent from the Windows 2000 server with RRAS can be observed by monitoring the data sent by the server when it first initializes. Without any RRAS configuration, the server sends the IGMP join for the all-routers group (224.0.0.2). When RRAS is started and OSPF is configured the server sends the join for the OSPF groups 224.0.0.5 and 224.0.0.6. When RIP 2 is configured, the server sends the join for the RIP 2 group 224.0.0.9.