IBM Computer Hardware User Manual page 36

Working with two HMCs eases the planning of HMC downtimes for software
maintenance, as there is no downtime needed. While doing the HMC code
update on one HMC, the other one continues to manage the environment. This
situation allows one HMC to run at the new fix level, while the other HMC can
continue to run the previous one. You should take care to move both HMCs to
the same level to provide an identical user interface.
High availability HMCs in various network environments
Using DHCP-servers or working with a fixed IP-address for the CEC are
considerations going along with your network structure. Here are some examples
of how you can set up your network:
Two HMCs on different private networks are shown in Figure 22.
DHCP Server
eth0
P5 Managed
Figure 22 HMCs connected to the SFP using 2 networks
Figure 22 describes two HMCs in different networks both running DHCP servers.
The CEC uses two LAN-adapters, one gets the IP-address from HMC1 and the
second one from HMC2.
If you use your HMC as a DHCP server for the CEC, be sure to have the HMC up
and running before powering on the CEC; otherwise the CEC will get its default
IP-address and will not work in your network.
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Hardware Management Console (HMC) Case Configuration Study for LPAR Management
HMC1
T1 T2
System
DHCP Server
HMC2
eth0
T1 T2
P5 Managed
System