[Bridge] Re: tg3 bridge problems

Gergely Madarasz gorgo at broadband.hu
Mon Jan 10 07:45:06 PST 2005

On Mon, Jan 10, 2005 at 10:05:56AM -0500, Neil Horman wrote:
> Gergely Madarasz wrote:
> >Hello,
> >
> >I've got a very strange problem. Lately I've been setting up my linux
> >servers for network (layer2) redundancy with a bridge interface containing
> >two ethernet interfaces connecting to two switches. So far I didn't have
> >any problems with it, but now a very strange thing happens with a new
> >server I'm installing. The server is an ibm x346 having two onboard
> >BCM5721 cards, the switches are cisco 3550, and I've tested with kernel
> >versions 2.6.10 and 2.4.28.
> >
> >The bpdu's from the cisco switches simply cannot be seen on the server,
> >causing loops in l2 traffic. I've tested with sticking a hub between the
> >c3550 and the server, the switch sends out the bpdu's, but they are not
> >seen by linux (running tethereal). This happens only on eth0, on eth1
> >everything seems fine. Any IP traffic on eth0 goes through, no packet
> >loss, no errors.
> >
> >And something even more strange: if I do an 
> >ifconfig eth0 0 up; brctl addif br0 eth0; 
> >it seems to be working fine, if I do it the other way
> >round, then the bpdu's sent by the switches are lost somewhere.
> >
> >Considering all these, the problem seems to me a strange interaction
> >between the bridge driver, the tg3 driver and the hardware in question. 
> It looks to me like either order should work just fine, as long as the 
> IFF_PROMISC flag isn't cleared when you bring up the interface.  Is 
> IF_PROMISC clear in ifconfig after you issue your ifconfig eth0 up command?

ifconfig has never showed PROMISC on either of my bridged servers. 
ip shows it though. I noticed this before but couldn't find the reason and
didn't seem important.

This is on a machine with working bridge:

# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:09:6B:49:89:80
# ip link list eth0
2: eth0: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1500 qdisc pfifo_fast qlen 1000
    link/ether 00:09:6b:49:89:80 brd ff:ff:ff:ff:ff:ff

And this is on the problematic machine:

# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:0D:60:55:3B:02
# ip addr list eth0
2: eth0: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1500 qdisc pfifo_fast qlen 1000
    link/ether 00:0d:60:55:3b:02 brd ff:ff:ff:ff:ff:ff

And even if didn't get promisc, tcpdump or tethereal would have made it
so, when I was looking for the bpdu packets. Tethereal shows only this:

  0.000000 00:0d:60:55:3b:02 -> 01:80:c2:00:00:00 STP Conf. Root = 65535/00:0d:60:55:3b:02  Cost = 0  Port = 0x8001


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