On Thu, Jun 14, 2012 at 5:53 PM, Sujata Verma <sujataverma3@xxxxxxxxx> wrote:
Hi ,
I am going through spanning tree protocol and was testing it on Linux. My observation is there is no validation of timers for configuration BPDU. Lets say Root bridge received another BPDU from new bridge with invalid timer values but less priority, the existing bridge is becoming non-root bridge and is advertising the invalid timer values.
As i have gone through 802.1D-1998 standard, i understand that 2004 is current one but i was looking into STP not RSTP, i preferred to read this standard. I find these lines:
===============================================
9.3.3 Validation of received BPDUs
A Bridge Protocol Entity shall process a received BPDU as specified in 8.7 if and only if the BPDU contains at least four octets and the Protocol Identifier has the value specified for BPDUs (9.3.2), and
a) The BPDU Type denotes a Configuration BPDU and the BPDU contains at least 35 octets, and the
value of the BPDUs Message Age parameter is less than that of its Max Age parameter; or
b) The BPDU Type denotes a Topology Change Notification BPDU.
In case a), any octets that are present beyond Octet 35 are ignored, as far as processing according to this
standard is concerned. Similarly, in case b), any octets beyond Octet 4 are ignored.
============================================
Does this implies that any value timer values present within octet 35 is valid value and there is no validation done. Even if range for hello timer, max age and forward delay is defined and is limited. Is it an issue or fine within the standard?
Not all STP implementation do BPDU validations i.e validates all BPDU parameters present within 35 octet. The validation checks for invalid values present in the bpdu,
if the BPDU validation fails it drops the BPDU. The have seen this validations in proprietary software.
if the BPDU validation fails it drops the BPDU. The have seen this validations in proprietary software.
Please help me understand this issue and thanks for any comments.
Regards,
Sujata
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