[Xorp-hackers] RPF check in PIM-SM

Bruce Simpson bms at incunabulum.net
Mon Jul 27 07:43:14 PDT 2009

Chintan Shah wrote:
> Hello,
> We are trying to implement a fast repair mechanism for PIM-SM  
> multicast trees.
> For that I need to know when is the RPF check performed on a received 
> packet. From whatever I could gather,
> I understand that the kernel first tries to find a matching (S,G) 
> entry in the MFC table and if  it is  found then it
> matches the incoming interface for the packet with the actual 
> interface  on which the packet was received and if both are  same then 
> forwards the packet to the outgoing interfaces from the outgoing 
> interface list. My question is whether this is the only RPF check that 
> is performed on the packet or is there any
> other check involved like looking up the unicast table and then 
> checking the reverse path to the source

RPF checks normally happen in-kernel, and not under direct XORP control. 
This is one reason why the fib2mrib process exists -- to push unicast 
routes from the RIB into the MFEA.

> Could you please elaborate on how a multicast packet forwarded in the 
> xorp router. 

XORP doesn't forward packets, the host operating system does.

Both Linux and FreeBSD MROUTING have a common ancestor, they are pretty 
much equivalent code.

Good luck with your app.


More information about the Xorp-hackers mailing list