Multicast and flooding issue

[Daniel Senie]

I've read through the archives and see people discussing an issue I've
seen, though my observance is just a little different. I'm working with
some code for a client which does a lot of multicasting (it's a new OSPF
implementation, I'm just testing it).

The target hardware I'm working with uses an Intel L440GX motherboard,
which has an Intel 82555 chip on the motherboard.

When I start the application I'm testing, it starts sending out
multicast frames, and quickly I get a lot of really odd results. These
include:

- Getting the same multicast packet out onto the wire huge numbers of
times.
- Getting odd packets with all-zeros source and dest. MAC addresses, and
some odd protocol type.

Two things seem to help:

- Killing the application, which turns off the multicast registrations.
- Turning on promiscuous mode (either with ifconfig or by running
tcpdump)

Also worth noting:

- most of the problem packets cannot be seen when looking from another
port on a 10/100 switch. My test environment has the device under test,
and a few other systems on a shared-media hub, so that I can analyze all
packets coming from the box.

Conclusion: It appears the Ethernet driver may get confused over
half/full duplex or be disabling or altering its collision logic when
these problems are occurring. I do note that the ifconfig statistics
show huge numbers for the received packets and collisions counters,
though a low number for the transmit packets.

Dan

-- 
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Daniel Senie                                        dts@senie.com
Amaranth Networks Inc.            http://www.amaranthnetworks.com