Re: [Etherboot-developers] [RFC] Multicast reception....

[Donald J C. <dj...@ci...>]

"Eric W. Biederman" wrote:
...
> My understanding of the guts of NIC is limited, so please check me.  I
> believe NICs have a hardware filter that allows them to receive just
> broadcast packets, and packets to their own mac address.  So to
> receive multicast packets I need open up or disable the filter all
> together.  The simplest solution appears to be disabling the hardware
> filter and go into promiscous mode, and then replace the hardware
> filter with a software filter in await_reply.

I thought at least some NICs have support for monitoring some number
of multicast addresses, in addition to broadcast and the local MAC.
I'm not an expert on NICs, but I think it might be worthwhile to
check the Linux drivers for some of the newer NICs and see if there
is anything obvious there as far as multicast support.

> Does anyone know if there is any communication between NICs and
> switches about what a NIC is listening for that would make promiscous
> mode a bad state to put NICs into?

Other than IGMP, I am not aware of any communication along these lines.
I think the main problem with promiscuous mode is the additional CPU
overhead if there is a lot of unwanted traffic.  Since you are doing
this at boot time, it probably is not a concern, especially if all of
the clients are booting at the same time, in which case there will be
very little "unwanted" traffic.

-Don


-- 
Don Christensen       Senior Software Development Engineer
dj...@ci...         Cisco Systems, Santa Cruz, CA
  "It was a new day yesterday, but it's an old day now."