If max age isn't changed from the default of 20 seconds, it shouldn't be a
problem? The seven hops came from an assumption that each bridge spends a
few seconds taking in and propagating configuration BPDUs, but high-speed
switches don't take that much time. However, they also don't track how much
time they take and simply add one second, per Clark and Kennedy. It's like
a reverse TTL.
Interestingly, Radia Perlman's description of message age and propagating
BPDUs in her book "Interconnections: Bridges, Routers, Switches, and
Internetworking Protocols" doesn't agree with IEEE. See page 63 of the
second edition. She says the Root Bridge sends BPDUs with the message age
set to 0. When bridges receive the message they transmit a message on each
of their ports for which they are the Designated Bridge with the message
age set to 0.
IEEE says the recipient bridges must send the BPDU with the message age
greater than the value received.
IEEE says: "If the Bridge has been selected as the Root, i.e., if the
values of the Designated Root and Bridge Identifier parameters held by the
Bridge are the same, the Message Age shall be set to zero.
f) Otherwise, the value of the Message Age shall be set such that the
transmitted Configuration BPDU does not convey an underestimate of the age
of the Protocol Message received on the Root Port; i.e., the value
transmitted shall be no less than that recorded by the Message Age Timer
for that Port, shall be greater than the value received, and will
incorporate any transmission delay. The value of
the parameter shall not exceed its true value by more than the maximum
Message Age increment overestimate as specified in 8.10.2."
By the way, I have captured some BPDUs where the message age is more than
zero, so I think switches are following IEEE.
Interesting, eh?
Priscilla
At 11:38 PM 8/27/01, Leigh Anne Chisholm wrote:
>Here's something funky I've just started researching. Thought many of you
>might not be aware of this...
>
>Awkward STP Parameter Tuning and Diameter Issues
>
>We already saw that an aggressive value for the max-age parameter and the
>forward-delay could lead to a very unstable STP. The loss of some BPDUs can
>then cause a loop to appear. Another issue, not very known, is related to
>the diameter of the bridged network. The conservative default values for the
>STP impose a maximum network diameter of seven. This means that two distinct
>bridges in the network should not be more than seven hops away the one to
>the other. Part of this restriction is coming from the age field BPDU carry:
>when a BPDU is propagated from the root bridge towards the leaves of the
>tree, the age field is incremented each time it goes though a bridge.
>Eventually, when the age field of a BPDU goes beyond max age, it is
>discarded. Typically, this will occur if the root is too far away from some
>bridges of the network. This issue will impact convergence of the spanning
>tree.
>
>
>This came from: http://www.cisco.com/warp/public/473/16.html#2f
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=17542&t=17489
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
