Conference irocz::common_brouters

828.0. "Vnswitch/vnbus connection" by TOSSUB::BRUSA () Wed Apr 02 1997 11:23

7

    	We are installing in an existing DEChub 900 configured as
    	shown below 2 VNswitch an EF and a EX both with software
    	version 1.5 installed.

	______  ______  ______  ______  ______  ______  ______  ______
       |      ||      ||      ||  vn  ||  vn  ||      ||      ||      |
       |900TM || 90T2 ||900CP ||900EF ||900EX ||900MX ||900MX ||900EF |
       |      ||      ||      ||      ||      ||      ||      ||      |
       |______||______||______||______||______||______||______||______|
          1|      1|    1| 7|     |E|     D|      1|      1|    1|2|3|    
           |       |        |     | |      |       |       |     | | |
           |       |        |     | |      |       |       |     | | |
   TW------*-------*--------*-----|-|------|-------|-------|-----*-|-|---
				  | |      |       |       |       | |
  ETH-----------------------------|-|------|-------|-------|-------*-|---
				  | |      |       |       |         |
VNBUS-----------------------------|-*------*-------|-------|---------|---
				  |                |       |         |
 FDDI=============================*================*=======*=========*===


    	The problem is the following: when from the LAN interconnect
    	screen we connect BOTH the VNswtches to the VNbus we lose
    	connectivity between the FDDI and the rest of the  hub (lans TW 
    	and ETH).
    	We still have connection among the users in the hub and we do not
    	break the ring but the users in the hub cannot see the FDDI
    	anymore.
    	If we connect just one VNswitch to the VNbus we do not see any 
    	problem but as soon we connect the second one, the problem shows 
    	up and from any module we cannot work with the FDDI anymore.
    	We tried also reverse the order to connect the VNswitches to the 
    	VNbus without solving the problem.

    	The problem do not shows up if we have one of the 2 VNswitches
    	connected OR to the FDDI OR to one Ethernet segment.

    	Basically it seems that we cannot connect the two VNswitches
    	using the VNbus without somehow having also a connection to 
    	someother lans in the hub.

    	Can anybody out there explain this please?

    	 

    I think what might be happening is that you've got your two
    VNswitch modules straddling the module slots where the last
    4 matrix backplane channels are split between slots 1-4 and
    the other two between slots 5-8. You might want to try and
    sawp the position of the VNswitch 900EF with the VNswitch 900EX
    so that you have all the FDDI capable modules within the
    same group of four sslots so as to take maximum advantage of
    configuring of the matrix backplane channels between the modules
    which use VNbus and more importantly FDDI.
    
    Bob

There is a problem with the scrambling code of MAM V5.0 if VNbus and
FDDI connections are used at the same time. The problem happens when you
first establish FDDI connections that span the hub, followed by VNbus
connections that also span the hub. When connecting the FDDI to the
backplane, the MAM will use either channels 8 or 9 or both. When
the VNbus connection is now attempted, the MAM has to scramble the
connections to re-attach the FDDI to different channels so a to free
up channels 8 and 9, which are needed for the VNbus. This operation will
almost always result in a broken or wrapped FDDI ring. The problem has
been fixed in a future release of the MAM (V5.1 or better). In the mean
time you can try swapping the modules, as Bob suggest, however you need
to do that in such a way that the VNbus doesn't have to cross the split
channels. Swapping the EX with the EF modules won't do. But swapping the EX
with the 900CP module (or any of the modules in the first three slots) will
do the trick. That way, the VNbus will use channels 11a and 12a (which goes
from slots 1 to 4), leaving channels 8, 9, 11b and 12b (and all others) to
be used by FDDI (and your Ethernet connection). Notice that your
configuration is using the backplane channels almost to maximum capacity.
Swapping the modules as I suggest guarantees that you will be using all the
split channels, leaving enough channels for all FDDI modules (8 channels
needed) and the Ethernet. 

Thanks,

Eladio Cortes
VNswitch System Test Group
[Posted by WWW Notes gateway]