[Xorp-users] Problems with multicast routing on Linux

[Pavlin Radoslavov]

Jon-Olov Vatn <jon-olov at ontimenet.com> wrote:

> Pavlin Radoslavov skrev:
> > If you want multicast forwarding between two segments, you MUST use
> > a multicast routing protocol like PIM-SM.
> > The purpose of IGMP is to inform the multicast routing protocol
> > about directly connected receivers. Then it is up to the multicast
> > routing protocol to modify the multicast forwarding entries in the
> > kernel.
> >   
> Is it really necessary to use a multicast routing protocol, or could the 
> forwarding tables be configured statically, e.g., with "route add ..."? 

In general you can't use commands like "route add" to install
multicast forwarding entries in the kernel.

Unlike unicast static routes, multicast static routes shouldn't be
used primarily for the following two reasons:

(a) multicast receiver membership is dynamic in nature and this
    can't be tracked by (multicast) static routes.
(b) If you create a multicast routing loop (which can easily happen
    with static routes), then you can easily bring down your network
    due to the exponential amplification of the looping
    packets.

I haven't used smcroute, but if you use it you need to understand
the above two issues.

Regards,
Pavlin

> I understand that it useful to use a routing protocol, but for a small 
> network with one or two routers it may be of interest to support 
> multicast routing without deploying, e.g., PIM-SM or DVMRP. When 
> googling I found smcroute (http://www.cschill.de/smcroute/) which seems 
> adequate for such situations, but I have not tried it and since the last 
> release is from 2002 I'm a bit suspicious ... Anyone out there with 
> experience of smcroute or setting up static multicast forward entries in 
> Linux in general?
> 
> Mvh J-O
> 
> 
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