[c-nsp] Need help with setting up ip multicastrouting

[Gert Doering]

Hi,

On Tue, Oct 12, 2010 at 09:18:04PM +0200, Peter Rathlev wrote:
> On Tue, 2010-10-12 at 21:07 +0200, Gert Doering wrote:
> > But what if you have 20 routers in a subnet and want the multicast
> > traffic to be heard by only those 5 that are interested in it?
> 
> Ah, of course. Does the switch (e.g. some Catalyst 2k/3k) just learn
> many mrouter ports and forward correctly? Or would one need to add some
> specific configuration for that?

As far as I understand, in the "many-router" scenario, things get tricky.

- if the switch is dumb enough, all routers will see all traffic, and
  routers not interested will drop it (things will work, but you waste
  bandwidth, and potentially overload ports if there are many routers
  and each router only wants a small subset of the groups on the LAN)

- if the switch is doing IGMP snooping, it might cause black holing,
  as it won't actually *see* the routers - the PIM routers elect a single
  PIM/DR, and if I remember correctly, only the DR will send IGMP queries
  (my memory is a bit fuzzy here, we disabled all our multicast routing 
  some two years ago).

  But even if IGMP snooping does not harm anything, it won't bring any
  benefits either, as the routers won't use IGMP to send each outer 
  group joins/leaves...

- thus: if the switch is really smart, it can do *PIM* snooping, to really 
  understand which router wants to receive which groups.

gert
-- 
USENET is *not* the non-clickable part of WWW!
                                                           //www.muc.de/~gert/
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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