Subject: IGMP Proxy - Upstream interface

From: Narayanan Vidya-CVN065 <[email-masked]>
Date: Tue, 25 Jul 2000 16:32:54 -0500

Hi,
Quoting from the IGMP Proxy draft (section 3), "A router performing
IGMP-based forwarding has a single upstream interface and one or more
downstream interfaces.". Is there any downside to having multiple upstream
interfaces? If not, why wasn't this considered? 

If IGMP Proxy is enabled on multiple interfaces, why can't some kind of
weighting (which could be configurable) be used to determine on which
interface the IGMP messages are forwarded? Often, there are multiple routes
to the same destination for redundancy purposes. Allowing this method of
configuration would make the use of IGMP proxy feasible in these cases. By
restricting the number of upstream interfaces to one, there is a complete
loss of connectivity in case of link failures. For interfaces that need to
remain simple but redundant, IGMP proxy could prove as a valuable solution
if multiple upstream interfaces are allowed. 

Thanks,
Vidya

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From: Bill Fenner <[email-masked]>
Date: Tue, 25 Jul 2000 14:42:59 -0700

I considered the following rules to allow "backup" upstream interfaces:

- Only be the Querier downstream if you hear a Querier upstream.
- If you don't hear a Querier on your primary upstream interface,
  you may switch to using a "backup" upstream interface if you
  are not the Querier on that interface.

The first rule causes failure information to propogate downwards.
The second rule allows failover on failure.

However, IGMP proxying is already really susceptible to loops.  I was
very wary of adding transient behaviors that might end up putting a
router downstream of itself (especially since the propogation of
failure information takes so long).  IGMP proxying already requires
careful configuration to prevent loops in the steady state, but it
requires even more thought to prevent loops in failure conditions.

  Bill