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Conference 7.286::fddi

Title:	FDDI - The Next Generation

Moderator:	NETCAD::STEFANI

Created:	Thu Apr 27 1989
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	2259
Total number of notes:	8590

1601.0. "DAS failover on 2 separate LAN's." by CGOS01::DMARLOWE (Have you been HUBbed lately?) Fri Feb 24 1995 10:33

    I have a customer doing a redundant network for a SCADA application. 
    They would like the following config:
    
    DC900MX		ALPHA		DC900MX
          M-------------A   B-----------M
    			 DAS
    
    The DC900MX's are on separate LAN's.  I believe that the DAS adapter
    upon seeing 2 M Ports will shut down the M-A link and leave the M-B
    link active.  Is this true?
    
    Having 5 servers connected like this to the 2 concentrators will spell
    disaster.  The SCADA application controls which port is the primary
    path and will cause all servers to use the secondary when the primary
    to 1 server fails.  However, FDDI rules control the DAS failover, not 
    the application.  If one sever loses its M-B, the DAS will failover to
    the M-A but all other systems will still continue using the M-B.
    
    I believe in order to work they will need a pair of SAS adapters in
    order to do this.  Right?
    
    dave

T.R	Title	User	Personal Name	Date	Lines
1601.1		NETCAD::STEFANI	Welcome to the Revolution!	`Fri Feb 24 1995 14:06`	37
	>>The DC900MX's are on separate LAN's. I believe that the DAS adapter >>upon seeing 2 M Ports will shut down the M-A link and leave the M-B >>link active. Is this true? Yes. This configuration is called "dual homing" and the B port will become active with the A port in standby mode. >>Having 5 servers connected like this to the 2 concentrators will spell >>disaster. The SCADA application controls which port is the primary >>path and will cause all servers to use the secondary when the primary >>to 1 server fails. However, FDDI rules control the DAS failover, not >>the application. If one sever loses its M-B, the DAS will failover to >>the M-A but all other systems will still continue using the M-B. DAS adapters are generally not dual-homed to different networks. The purpose of dual homing is to provide redundancy to the same network. The normal configuration is that the two concentrators are on the dual ring, the DAS adapter is dual-homed between them and if the B port connection fails for whatever reason (B port fails, cable is cut, M port fails or entire concentrator fails) the A port will take over and the adapter will remain on that network. In your customer's configuration, all of the connections to the network on the right would get lost when the adapter failed over to the A port. That's probably not what they're looking for in terms of redundancy. >>I believe in order to work they will need a pair of SAS adapters in >>order to do this. Right? It's not as simple as just putting two SAS adapters in. Basically, the higher level software would need to accept two separate interfaces connected to two separate networks and provide the failover should one adapter fail. This may require rerouting at the protocol layer on the second interface. This is not normally a function of commercial operating systems. - Larry
1601.2	Thanks.	CGOS01::DMARLOWE	Have you been HUBbed lately?	`Fri Feb 24 1995 15:51`	15
	Thanks Larry for confirming my thoughts. The customer will have separate IP addresses on each SAS. Their application has been written to do failover to a second Ethernet interface so there will have to be a small change to do dual SAS adapters. They do not want FDDI between the 2 LAN's as a FDDI problem on one LAN cannot be allowed to appear on the redundant LAN. Ethernet is prone to these types of problems, ie. babbling, etc, but eventhough FDDI isn't, there must not be any chance of one FDDI affecting the other FDDI. This is pipeline control and therefore must be bullit proof. dave