Re: Spanning-tree detected flag

Check out the following link:
http://www.freepatentsonline.com/EP1330078.html

"More specifically, upon detection of a change in the Spanning Tree,
switches transmit Topology Change Notification Protocol Data Unit (TCN-PDU)
frames toward the root. The TCN-PDU is propagated hop-by-hop until it
reaches the root which confirms receipt of the TCN-PDU by setting a Topology
Change Flag; additionally a TC flag can be set in BPDUs subsequently
transmitted by the root for a period of time. Other switches, receiving
these BPDUs, note that the Topology Change Flag has been set, thereby
alerting them to the change in the active topology."

The way I read that is when a change happens, the TCN is sent up to the root
bridge. When the root bridge receives the TCN it acknowledges it by setting
the Topology Change Flag. When other bridges receive these frames, they set
their TC detected.

On Wed, Sep 2, 2009 at 11:58 AM, Anantha Subramanian Natarajan <
anantha.natarajan_at_gravitant.com> wrote:

> Hi All,
>
> When I was going through the "show spanning-tree active detail " command
> ,I noticed about the detected flag as pasted below (output truncated)
>
> VLAN0001 is executing the ieee compatible Spanning Tree protocol
> Bridge Identifier has priority 24576, sysid 1, address 0015.2b2c.f680
> Configured hello time 2, max age 20, forward delay 15
> We are the root of the spanning tree
> Topology change flag not set, *detected flag not set*
> **
> I was trying to understand,whether the detected flag is a seperate flag in
> 801.2d BPDU frame format or it just the switch sets for his purpose to
> understand that the TC notification has received from another switch with
> TC
> flag set.
>
> The way in which I understand the spanning-tree flags are
>
> 1) Its 1 byte in length
> 2) Only 2 bits are used for the flag
> 3) If the lower order bit is set (00000001),then its TC flag is set or if
> the high order bit is set (10000000),then it is TCA flag set.
>
> For some reason I couldn't understand the detected flag,Kindly help me to
> understand the same.I am sure missing some basic.
>
> Thanks for the help
>
> Regards
> Anantha Subramanian Natarajan
>
>
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