Multicast switching and "ip multicast source-timeout"

[michiel]

Hi,

Anyone know exactly what the significance of this settings is? And why it exists/what it aims to achieve?

We've discovered that a device that has previously send packets to a multicast address will have it's unicast source IP listed under show ip multicast source. And if stops sending to that multicast address for 30 seconds (default) it will be removed from that list. Same for show ip multicast forward.

Being removed from those lists however means that the next packet that the device sends towards that multicast address will be dropped! Even if that device is listed as a receiver (granted, not a sender) under show ip multicast group. The second packet is allowed through however.
This messes up some devices where they first send one packet and wait for an acknowledging packet before sending more packets.
Even though you can increase this timeout with ip multicast source-timeout I find this weird.

So my questions:
What is the use of this filtering?
Why drop the first packet if you are going to allow subsequent packets anyway? Why not just forward the first packet as well?
Is this standard multicast switching behaviour or Alcatel specific? (I'm having a hard time finding this feature on other brands?)
If standard across brands: is this described in any RFC?


From the manual (this is the same and works the same for any Omniswitch, even 6350 which does not have any multicast routing capability, so this is definitely a multicast SWITCHING feature):

Modifying the Source Timeout
The default source timeout (the expiry time of IP multicast sources) is 30 seconds. The following subsections describe how to configure a user-specified source timeout value and restore it by using the
ip multicast router-timeout command.
Configuring the Source Timeout
You can modify the source timeout from 1 to 65535 seconds by entering ip multicast source-timeout
followed by the new value. For example, to set the source timeout to 360 seconds on the system if no
VLAN is specified, enter:
-> ip multicast source-timeout 360
You can also modify the source timeout on the specified VLAN by entering:
-> ip multicast vlan 2 source-timeout 360