Conference netcad::hub_mgnt

4275.0. "how to avoid switch loops when using 612EX in MS600" by NETCAD::MOWER () Thu Mar 13 1997 18:02

 +----------------+
 | QUESTION       |
 +----------------+

   Since the 612EX does not support spanning tree, how do I avoid making 
   "switch loops" when using the 612EX in a MS600?



 +----------------+
 | ANSWER         |
 +----------------+

   (Assuming no "cross-wiring" of front panel ports of devices in a MS600...)
   1. Any two 612EXs must have no more than ONE lan in common between them.
   2. Only connect one device in the MS600 to the backplane Thinwire.



 +----------------+
 | EXPLANATION    |
 +----------------+

 Terms:
   "MS600" = DIGITAL MultiSwitch 600 System
   "612EX" = DIGITAL MultiSwitch 612EX
   "624T"  = DIGITAL MultiSwitch Hub 624T
   "MCM"   = clearVISN MultiChassis Manager


 Overview:
   This issue only affects those customers that use the 612EX in their MS600.

   The spanning tree protocol automatically detects loops in a network's
   topology, and automatically de-activates the switch ports forming the loop 
   in order to eliminate such loops.  ("De-activated" Switch ports are more
   formally said to be in "blocking", "standby", or "backup", mode).

   The 612EX does not support the spanning tree protocol, and thus it is 
   possible to create switch loops using the 612EX.



 How do I know if I have a switch loop, and if so, how do I break it?

   A switch loop will evidence itself as two or more LANs with extremely high 
   utilization, which may be observed on the LAN activity LEDs.  Devices on 
   those LANs will likely become un-reachable (due to the very heavy load).
   If you complete a switch loop by making a backplane connection in MCM's 
   LAN interconnect or port grouping windows, the last connection may result 
   in an error, with no further operations succeeding after that point, (if 
   the path from your NMS to the MS600 stack director involves one of the LANs
   in the switch loop).

   If you have a switch loop, follow these steps to eliminate it:
     1. remove one of the modules involved in the loop
     2. use MCM to disable "backplane auto-healing", (if enabled)
     3. re-insert the module removed in step #1
   Alternatively, a factory reset of the MS600 will always clear a switch loop.




 Switch-Only switch loops

   A "switch-only" switch loop, (a switch loop that consists only of 612EXs),
   is formed when two or more 612EX ports are connected in loop.  For example:

           +---------+    +---------+
           | 612EX   |    | 612EX   |
           +---------+    +---------+
             |  |           |  |
         ====|==*===========|==*======== any backplane LAN
             |              |
         ====*==============*=========== any other backplane LAN

   Switch loops can also involve 3 or more switches, and may involve front
   panel wiring.  This paper does not discuss how to avoid switch loops when
   front panel ports are "cross-wired";  that is, when the front panel ports
   of one or more modules in the same MS600 are wired together.

   Avoiding "switch-only" switch loops is as simple as ensuring that no two 
   612EXs have more than one LAN in common.  (MCM prohibits the 
   simplistic case of connecting two ports from the same 612EX to the same 
   backplane LAN).




 A brief description of the 624T

   A 624T has 2 or 3 "port groups", depending on hardware configuration:

               port-group #1   port-group #2   port-group #3, (if any)
               -------------   -------------   -----------------------
     24 ports:   1-12            13-24
     25 ports:   1-12 & mm-A     13-24
     25 ports:   1-12            13-24 & mm-B
     26 ports:   1-12 & mm-A     13-24 & mm-B
     36 ports:   1-12            13-24           25-36

   The 624T has the same number of Ethernet "internal LANs", (also labeled as 
   "groups"), as it does port-groups.  Each internal LAN results in a green 
   backplane port pull-down in the MCM LAN interconnect view.  All 624Ts also 
   have one blue Thinwire pull-down which can be connected to one, (and only
   one), of that 624T's internal LANs.  Each port-group can be connected to
   one, (and only one), internal LAN.

   For example, a 24-port 624T is illustrated below.
     - It has 2 port-groups, and 2 internal LANs.
     - The first port-group consists of ports 1-12, and is connected to 
          internal LAN #2.
     - The second port-group consists of ports 13-24, and is connected to
          internal LAN #1.
     - The Thinwire backplane pull-down is connected to internal LAN #1.  

   Note that in this example, the 624T is repeating traffic from backplane 
   pull-down #3, (the Thinwire), to backplane pull-down #1.  If pull-down #3 
   is connected to the backplane Thinwire, and if pull-down #1 is connected to
   another backplane LAN, the 624T will be repeating between those two 
   backplane LANs.

                    front panel ports
                  1 2   12    13 14  24      
         +--------|-|...|------|-|...|-------------+
         |        | |   |      | |   |       624T  |
         | int-   `-+-+-'      `-+-+-'             |
         | LAN/       |            |               |
         | grp:       |            |               |
         |  1 --X-----O------------X------------.  |
         |      |     |            |            |  |
         |  2 --O-----X------------O--------.   |  |
         |      |                           |   |  |
         |      |                           |   |  |
         +------|---------------------------|---|--+
                |                           |   |
               [3]                         [2] [1]
             Thinwire                      Ethernet (IMB)
             (blue)                        (green)
             backplane                     backplane
             pull-down                     pull-downs




 Switch + Repeater loops

   Switch loops can also be formed in conjunction with repeaters:

           +---------+    +---------+
           | 624T    |    | 612EX   |
           +---------+    +---------+
             |  |           |  |
         ====|==*===========|==*======== backplane Thinwire LAN
             |              |
         ====*==============*=========== another backplane LAN

   (  These switch loops that involve repeaters should not be confused with  )
   (  "repeater loops", which consist solely of repeaters, and are fully     )
   (  detected and prevented/prohibited by the clearVISN/MCM software.       )
   (  MCM does not detect nor prevent switch loops - except that MCM will    )
   (  not permit you to connect two ports from the same switch to the same   )
   (  backplane LAN.                                                         )


   Assuming the same internal 624T configuration as shown above, the full
   picture of the switch+repeater loop would be:


                  1 2   12    13 14  24      
         +--------|-|...|------|-|...|-------------+
         |        | |   |      | |   |       624T  |
         | int-   `-+-+-'      `-+-+-'             |
         | LAN/       |            |               |
         | grp:       |            |               |
         |  1 --X-----O------------X------------.  |
         |      |     |            |            |  |
         |  2 --O-----X------------O--------.   |  |
         |      |                           |   |  |    +---------+
         |      |                           |   |  |    | 612EX   |
         +------|---------------------------|---|--+    +---------+
                |                           |   |         |  |
                |                          [1]  |         |  |
                |                               |         |  |
         =======*===============================|=========*==|=== bp Thinwire
                                                |            |
         =======================================*============*=== other bp LAN


   Note that the only way a 624T can contribute to a switch loop is if it is 
   repeating traffic between two MS600 backplane LANs.  The only way that a 
   624T can repeat traffic between two backplane LANs is if one of the LANs is
   the backplane Thinwire.




   You will avoid switch+repeater switch loops if you:

     - connect only one device in your MS600 to the backplane Thinwire

          Since all switch+repeater loops involve the backplane Thinwire,
          if only one device in the MS600 is connected to the Thinwire, there
          can be no loop.

          This is the simplest and easiest-to-remember rule to follow,
          but if you do choose to connect more than one device in your
          MS600 to the backplane Thinwire, you will avoid switch+repeater
          switch loops if you:

     - connect **EITHER** the blue (Thinwire) pull-down, **OR** the green (IMB)
       pull-down(s), but not both from the same 624T

          If none of the 624Ts in the MS600 have both their green and blue
          pull-downs connected, no 624T will be repeating packets from the
          Thinwire to another backplane LAN.

          If you do choose to connect more than one device in your
          MS600 to the backplane Thinwire, and you choose to connect both
          blue and green pull-downs from the same 624T, you will avoid 
          this type of switch loop if you:

     - guarantee that for those 624Ts with both blue and green pull-downs
       connected, the connected green pull-down(s) are NOT for the internal 
       LAN that the Thinwire is connected to.

          A 624T only repeats packets from the backplane Thinwire to the 
          internal LAN to which it is connected;  if the pull-down for that
          internal LAN is not connected, the 624T will not repeat between the
          backplane Thinwire and another backplane LAN.


   (It is possible to break all of the above guidelines and still not form
   a loop;  if you understand what has been said to this point, you are well
   equipped to avoid switch loops without the above simple rules-of-thumb).



   <A Microsoft Word version of this document is available>.