How to add an IP alias with UCX much the way you would

[David Webb]

> The UCX command 'set interface/clus' allows multiple VMS cluster nodes
> to share a 'cluster' ip adress, however it does not work the same as a
> DECnet alias where a semblance of load balancing is involved. Instead a
> cluster alias results in one of the named servers dealing with all IP
> traffic directed to the alias adress. The machine which 'holds' the
> adress is called the 'impersonator' in UCX speak. If the current
> 'impersonator' is shutdown or crashes then one of the other configured
> nodes takes over as impersonator automatically without intervention from
> support folks.

> Very fault tolerant but unfortunately no concept of load sharing.
> ¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤

To do better you need to involve a DNS server. This can be running on one of
the VMS systems or on any other system.

The DNS server can be setup to round-robin the address which provides a
limited type of load balancing but NO fault tolerance. If one of the systems
goes down the address of that system will still have been given out in the
DNS response if it was that systems turn.

The better solution is to setup metric servers and a load broker on the VMS 
systems which update the DNS according to their load. This requires either to 
use the DNS server which comes with VMS TCPIP services or a fairly modern 
BIND server.
This provides both load balancing and fault tolerance since if the metric
server does not update the load broker then it is removed from the list
passed to the DNS server.
 

How to setup this type of DNS based load balancing is described in the DEC
TCPIP manuals see http://www.openvms.compaq.com:8000/tcpip51/6526/6526pro.HTML
in particular Chapter 6


David Webb
VMS and Unix team leader
CCSS
Middlesex University



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