Subject: Re: multicast and load balancing

From: Tony Ballardie <[email-masked]>
Date: Fri, 1 Jun 2001 10:42:00 +0100

Theo,

> IGMPvX and MLD mention that the router has to be a Querier in the
> particular interface so that multicast packets can be forwarded. When
> one has two multicast enabled routers and would like to load balance
> traffic between the two routers it seems to me that there is problem
> with load balancing since only the interface in any of the 2 IGMP-enable
> routers, with the lowest IPv6 addr will win the Querier election. So in
> that case the second router cannot load balance multicast forwarding on
> that link.

there's usually a distinction made between the IGMP Querier and
the multicast routing protocol's "designated router" (DR), which is
responsible for forwarding mcast traffic to/from the LAN. The explicit join
protocols, including PIM, have a DR and a DR election mechanism
(usually via priority config'd Hellos) so the DR need not be the Querier.
But, the elected DR is DR for *all* groups, so it's not really load
balancing.

To enable group based load balancing there'd need to be one DR per
group (or better... group range). But that makes DR election more complex
and ramps up the DR election traffic on the LAN. Not done at present,
though it would be the ideal.

cheers

Tony

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From: Theo Pagtzis <[email-masked]>
Date: Fri, 01 Jun 2001 06:49:12 -0700

Hi Tony,

   thanks for that. If, however, the DR router is empowered to fwd multicast
traffic to/fro the LAN, regardless of being the Querier or not, then what's
the real benefit of having the Querier election and even more why should the
router be constraint by the rule that if the router is not a querier in that
interface it cannot forward traffic to it?

It seems to me that the DR overrides that and the IGMP protocol makes no
mention about it..that is that the IGMP querier can be overriden if the
multicast enabled routers is a DR one..

Theo

UCL/ Mobile Systems

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From: Bill Fenner <[email-masked]>
Date: Fri, 1 Jun 2001 09:48:31 -0700

>why should the

It's not.  This is a constraint imposted by IGMP proxying, but is not
a general rule.

  Bill