/8s and filtering

bdragon at gweep.net bdragon at gweep.net
Tue Dec 10 18:26:52 UTC 2002

> Here is one reason for some to care :
> If you want to do interdomain multicast, and your
> address space is not announced globally (say because you have a /24 that 
> is
> not from the swamp), you are likely to be black-holed
> due to RPF failures (as your unicast and multicast routing is likely to 
> be different).
> This has caused problems from time to time...
> Regards
> Marshall Eubanks

Until such time as vendor C determines that RPF lookups should
use the MRIB first, then the URIB, rather than the final RIB,
the following will help:

router bgp
 distance mbgp 19 19 19
ip mroute bgp Null0 20

or if using 12.1:
router bgp
 address-family ipv4 multicast
 distance bgp 19 19 19
ip mroute bgp Null0 20

not having looked at vendor J's multicast, I can't speak to it.

This does, however, make absolutely certain that only MBGP routes
determine RPF choices. If you expect or desire unicast routes
to drive multicast routing decisions, the above is not for you.

I had to do this to get around a bunch of RPF hedge cases.

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