[Bridge] Invalid BPDU transmission?

Rodolfo Giometti giometti at enneenne.com
Wed Oct 6 08:32:32 PDT 2010


On Tue, Oct 05, 2010 at 06:10:32PM +0530, Srinivas M.A. wrote:
> Both CONFIG and TCN BPDUs are used only in STP compatibility mode. In
> RSTP mode, the BPDUs sent in both directions are RSTP BPDUs. So they
> can be sent on the root port.
> 
> For a description of how topology changes are handled, see the section
> titled "New Topology Change Mechanisms" in
> http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
> 
> Is this causing any particular issue?

Yes. By using the following connection:

  +---------------+        +----------------------+
  | EXTRA bridge  O <----> O(eth5) bridge on test |
  +---------------+        |      (eth0) (eth1)   |
                           +--------O------O------+
                                    ^      ^
                                    |      |
                                    +------+

where «EXTRA bridge» is a rstp bridge acting as root bridge and NOT
running rstpd (but a proprietary rstp implementation), while «bridge
on test» is my bridge running rstpd.

«EXTRA bridge» sends to «bridge on test» the following BDPU:

   01 80 c2 00 00 00 00 02 3e 99 ba 1c 00 27 42 42
   03 00 00 02 02 7e 80 00 00 02 3e 99 ba 1c 00 00
   00 00 80 00 00 02 3e 99 ba 1c 80 01 00 00 14 00
   02 00 0f 00 00

and «bridge on test» replies on «eth5», which is its root port.

Without my modification «bridge on test» is unstable, in fact the
states of eth0 and eth1 ports doesn't fit to Fwd end Blk but they
change in a strange manner and don't stabilize... also opening and
closing the loop cause more instability and the bridge takes very long
time before changing its ports states!

By adding my modification everything works well.

Ciao,

Rodolfo

-- 

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Linux Device Driver                          giometti at linux.it
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