Priscilla Oppenheimer wrote:
>
> At 04:18 PM 6/11/01, Jianfeng Wang wrote:
> >Actually, the switch diameter rule is to keep spanning-tree protocol
> >working. Cisco switches sets their echo time to 2s and forward-delay to
> >15s. In which case, diameter larger than 7 makes spantree failed to work.
>
> Please do not send messages to me personally. If you are confident in your
> comment, you will send it to the whole group and everyone can benefit. If
> you aren't confident, then I don't want to hear it.
>
Sometimes that happens accidentally. If one does a "reply all" in the
typical email client, [EMAIL PROTECTED] gets a "cc" instead of a "to".
That bounces due to Paul's spam filter accepting only messages directly
sent "to" the list.
> I checked IEEE 802.1D more carefully and it does say the recommended
> maximum diameter of a bridged network is 7. There's no such thing as an
> echo timer, but the hello timer is 2 seconds by default. The forward delay
> is 15 seconds by default, as you say.
[snip]
Back when there was only one STP -- the DEC one -- the seven-bridge
guideline existed. DEC engineers said it allowed for conversative
BPDU propogation, but also for serialization delay and WAN latency.
Their spec allowed one (or two?) bridge hops to have a slow serial link,
as low as 56kb. (Hey, back then 56kb was a common WAN speed!)
With more than seven bridge hops, a frame might take longer than one
second to travel end-to-end. That would cause DEC's LAT protocol
(which can only be bridged) to time out and retransmit. If the max
retry count was exceeded, the session was dropped.
Someplace in my paper archives I believe I have the slides from a
DECUS presentation that broke out the timing budget.
Marty Adkins Email: [EMAIL PROTECTED]
Mentor Technologies Phone: 240-568-6526
133 National Business Pkwy WWW: http://www.mentortech.com
Annapolis Junction, MD 20701 Cisco CCIE #1289
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8078&t=7578
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