Why is my z/OS IP address changing when using zPDT, and routing does not work?

I was looking into configuring IP V6 on my z/OS running on zPDT running on Linux. I could not understand why configuring the IP V6 link between Linux and z/OS was so difficult.

IP V6 address for use within a connection are like fe80::b0b6:daff:fe64:77f5 where b0b6:daff:fe64:77f5 is based on the MAC(hwaddr). On many systems, this value does not change across IPLs – and so most of the documentation uses the “constant” value.

The connection between Linux and z/OS is a “tap” interface (a kernel virtual device) which looks like an OSA adapter to z/OS.

I found a comment

Each TAP device has a random MAC address that is used as source address.

This explains why the connection was getting a different IP address every time I ipled.

On z/OS you defined a route using this IP address, for example

BEGINRoutes 
ROUTE 2001:db8::7/128 fe80::3f:67ff:fe08:51dc   IFPORTCP6   MTU 5000 
ENDRoutes 

To get round this problem you need to explicitly define an address on Linux

sudo ip -6 addr  add fec0::cccc/64 dev tap1

where cccc is for my initials!

You then put this address into the z/OS routing statements.

BEGINRoutes 
ROUTE 2001:db8::7/128 fe80::cccc   IFPORTCP6   MTU 5000 
ENDRoutes 

and it works first time!