[Bridge] Re: tg3 bridge problems

Neil Horman nhorman at redhat.com
Mon Jan 10 08:04:55 PST 2005

Gergely Madarasz wrote:
> On Mon, Jan 10, 2005 at 10:05:56AM -0500, Neil Horman wrote:
>>Gergely Madarasz wrote:
>>>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
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           ...
> # 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
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           ....
> # 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
Strange.  My concern was that the tg3 interface has its hardware reset 
whenever its set to be up, and part of that is a resetting of its 
receive mode.  If for some reason IFF_PROMISC was cleared after you set 
it using brctl, the interface might be taken out of promisc mode.  Do 
you have any iptables rules running that might drop bpdus?

> 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:
Thats not entirely true.  ethereal/tcpdump/etc use the PF_PACKET 
interface, which talks directly to the driver to set the receive list, 
and doesn't always go through the device layer, which is where 
IFF_PROMISC gets asserted.  Agruably it should set promisc, but not 
doing so doesn't prevent ethereal and friends from capturing in 
promiscuous mode.
>   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
> Greg

  *Neil Horman
  *Software Engineer
  *Red Hat, Inc.
  *nhorman at redhat.com
  *gpg keyid: 1024D / 0x92A74FA1

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