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Conference netcad::hub_mgnt

Title:	DEChub/HUBwatch/PROBEwatch CONFERENCE
Notice:	Firmware -2, Doc -3, Power -4, HW kits -5, firm load -6&7
Moderator:	NETCAD::COLELLADT

Created:	Wed Nov 13 1991
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	4455
Total number of notes:	16761

3757.0. "Spanning-Tree 900EF and GIGAswitch questions" by UTRTSC::VDCRUIJSEN () Wed Jul 31 1996 05:52

Hello,

Some spanning-tree configuration questions.

Configuration:

               +-----------+                +------------+
               |Gigaswitch |                |Gigaswitch  |Connected to the 
               |           |----------------|            |Giga swichtes are 
               |           |FDDI Hunt group |            |3 large BRS clusters
               |           |4 lines         |            |
               +-----------+STP cost 10     +------------+
         STP cost 10|                 STP cost 10  |
                    |                              |
                    |                              |      
              FDDI  |                        FDDI  |
                    |                              |
                    |                              |
        STP cost 4  |*                  STP cost 4 |
               +------------+                +------------+
               |900EF switch|                |900MX switch| Root Bridge
               +------------+                +------------+
               *|*| |*|*|                     | | | | |
                | | | | |                     | | | | |
                | | | | |_____________________| | | | | 
                | | | |_________________________| | | |    5 ethernet segments
                | | |_____________________________| | |    used for connecting
                | |_________________________________| |    stations to the 
                |_____________________________________|    network
                              STP cost Ethernet 10

The lines marked with an *  are put into BACKUP.

Questions:
	1) Why is the default STP cost for FDDI on a Gigaswitch 10  and the
           4 on the DECswitch 900EF/DECbridge 900MX

        2) What makes the 900EF in above configuration determine what
           ethernet line to put in  forwarding because the cost to the root is
           equal 10 on all ethernet lines.(24 on FDDI).

Remark: We advised customer to change root to one of the Gigaswitches 
        and backup root the other gigaswitch.




Regards Gerrit

T.R Title User Personal
Name Date Lines

3757.1 NETCAD::ANIL Wed Jul 31 1996 13:33 30

T.R	Title	User	Personal Name	Date	Lines
3757.1		NETCAD::ANIL		`Wed Jul 31 1996 13:33`	30
	> 1) Why is the default STP cost for FDDI on a Gigaswitch 10 and the > 4 on the DECswitch 900EF/DECbridge 900MX On the 900EF, we wanted a difference between the cost of the Ethernet and FDDI -- actually wanted the FDDI to be lower -- so that whenever there's a tie between an FDDI path and an Ethernet path, the FDDI would be the one that ends up forwarding. This helps ensure that the highest speed links available get used wherever possible. > 2) What makes the 900EF in above configuration determine what > ethernet line to put in forwarding because the cost to the root is > equal 10 on all ethernet lines.(24 on FDDI). The tiebreaker goes as follows: Root, Cost, Bridge-ID, Port-ID -- in that order. Since both bridges have the same Root and Cost, the Bridge-ID is used to determine which port goes into Forwarding. The Bridge-ID is an 8-byte quantity: the first two, or most significant, bytes being the priority of the bridge (which is settable), and the following 6 bytes being the MAC address of the bridge. (In this case where there are 7 MAC addresses, it is the MAC address of the lowest-numbered port, also known as "Node ID" of the bridge, the one labelled on the box and reported in the console.) Given that the switch on the right in this picture is the one that put all its ports into forwarding, it must have a numerically lower address than the other (assuming you left all spanning tree settings at their default values). Regards, Anil

>	1) Why is the default STP cost for FDDI on a Gigaswitch 10  and the
>           4 on the DECswitch 900EF/DECbridge 900MX
    
    On the 900EF, we wanted a difference between the cost of the Ethernet
    and FDDI -- actually wanted the FDDI to be lower -- so that whenever
    there's a tie between an FDDI path and an Ethernet path, the FDDI would
    be the one that ends up forwarding.  This helps ensure that the
    highest speed links available get used wherever possible.
    
>        2) What makes the 900EF in above configuration determine what
>           ethernet line to put in  forwarding because the cost to the root is
>           equal 10 on all ethernet lines.(24 on FDDI).
    
    The tiebreaker goes as follows: Root, Cost, Bridge-ID, Port-ID -- in
    that order.  Since both bridges have the same Root and Cost, the
    Bridge-ID is used to determine which port goes into Forwarding.
    The Bridge-ID is an 8-byte quantity: the first two, or most significant,
    bytes being the priority of the bridge (which is settable), and the
    following 6 bytes being the MAC address of the bridge.  (In this case
    where there are 7 MAC addresses, it is the MAC address of the
    lowest-numbered port, also known as "Node ID" of the bridge,
    the one labelled on the box and reported in the console.)
    
    Given that the switch on the right in this picture is the one that
    put all its ports into forwarding, it must have a numerically lower
    address than the other (assuming you left all spanning tree settings
    at their default values).
    
    Regards,
    Anil