All things related to multicast

[Matthew Davy <>]

I'm going to jump back on my soapbox about filtering the IANA Reserved  
multicast group addresses (ie 225/8-231/8 and 234/8-238/8) at all  
inter-domain borders.

Today we had another round of MSDP instability caused by a single host  
scanning the entire 224/4 address space.   The vast majority of this  
address space is IANA Reserved address space that should not be routed  
inter-domain any more than unicast IANA Reserved addresses.  Also,  
232/8 is set aside for SSM and 239/8 is "private" address space so you  
should never see MSDP SAs for groups in these blocks either.   That  
leaves just 224/8 and 233/8 to be used for inter-domain ASM.   
Therefore these are the only address blocks that are legitimate group  
addresses for MSDP SAs.

If you are responsible for multicast on a campus, statewide or  
regional network, I encourage you to update your MSDP SA filters to  
only permit SAs with group addresses in the 224/8 and 233/8 address  
ranges.   I think this filter should also be implemented on the  
Internet2 backbone.

You may ask why we don't just go do this, if this is obviously the  
right thing to do ?   Well, I just looked and it appears there are a  
few dozen group addresses in these IANA Reserved ranges that are in  
active use.   I'm basing this on the fact that there are 2 or more  
active sources from different organizations for these groups.   As you  
might expect, the groups are typically along the lines of 226.1.2.3  
and 225.3.4.5.    So by implementing this, we will likely break some  
applications, but I think the long-term benefits to the stability of  
multicast would be significant.

- Matt

Matt Davy
Chief Network Architect, Indiana University

 / 812.855.7728