| Shoichi, the value range you mention (0-65535), is a 802.1d
*recommended* range, *not* a mandated range. Our implementation of
values for parameters is one based on the practical nature of extended
LANs, (so I am told). I posed your question to one of our Firmware
developers and he confirmed that the range chosen for the DECswitch
900EF is indeed 0-255 for Bridge Priority which is also the range
used in most other Digital bridges/switches.
Bob
|
| I'm not sure I understand your question. Are you asking whether
the DECswitch was designed to become a designated root bridge in
a normal LAN, the answer is an afirmative. Of course it can.
As far as spanning tree, the DECswitch 900EF doesn't behave any
differently basically than any other DEC bridge/switch.
The default value for the the Bridge Priority parameter in a DECswitch
is 128, but could be as high (but no higher), as 255.
Normally, all bridges with the default of 128 determine between
themselves by knowing what each others MAC address, who is going to
be elected as the root. If one wants to make sure that the
bridge/switch that is as close to the center (recommended)
(physically), of the LAN topology becomes the root, one changes the
bridge priority to a lower value than 128 to guarantee it specifically
becomes the root.
Bob
|
| Hi,
A late response on this but if you add the DECswitch 900EF into a
network with an existing Spanning Tree config. where all the existing
kit supports the range 1-65535, it is quite likely that the highest
priority of the existing kit is a number between 255 and 65535, which
does mean that the DECswitch 900EF will become the root bridge. This
has just nearly happened to me, we spotted up front what was going to
happen and the customer is going to have to reset all
the 3-Com kit that they have and want to keep as root bridges to
different priorities less than 255. It's not a big deal but it is a bit
of a pain and doesn't leave a good impression. If they had just
connected the DECswitch without checking, we would have had a spanning tree
adjustment on a large Extended LAN, and not been popular :->
Euan
|
| The DECswitch supports both the LB100 (aka "dec stp" in some other
vendors boxes) as well as 802.1D spanning tree. Since the two
algorithms are basically the same, if you configure a set of
parameters, they would apply equally to both regardless of which mode
the switch was operating in.
One of the minor differences is that the LB100 STP (spanning tree
protocol) allows only for a byte's worth of priority, which is a max
of 255, while the 802.1D allows for up to 2 bytes or 65535.
As you can see, if the parameter is configured for a value between
0 and 255 it could be used regardless of which mode the switch was
operating in, but if it was configured for a value greater than 255
then it could not be applied to the LB100 STP. In order to alleviate
this we limited the max value to 255. It's really not an issue because
0 to 255 is a big enough range for whatever you're trying to do. For
example, in the case you described, had the priority of the 3Com root been
configured to say 10 or something low like that, then the root would
have been unchanged. If you didn't configure the priority to a low value
then the root could change at any time regardless of whether the new
bridge introduced was Digital, 3Com or any other. This is because once
the priorities are all the same it is the hardware address which is
used to decide the root, hence no predictability. So the
thing to do, if you care about reconfigurations when adding switches,
is to fix the root by configuring it with a low priority. (In fact,
if you want the network to be really robust, you should set a "deputy
root" physically close to the root, with a priority a little higher than
the root's but lower than all others, eg 20, so that if something were
to happen to the root it could take over with minimal reconfiguration.)
Anil
|