Designing Subnets
By default, MapR automatically uses all available network interface cards (NICs) on each node in a network. MFS is going to try to use any interface that appears up to communicate with other machines and other machines will use any IP addresses that MFS used to register with the CLDB.
MAPR_SUBNETS variable can be set to restrict the set of interfaces that MFS advertise but that is only set/used when MFS is starting up, so if someone purposefully takes down link to an interface on an MFS node, other nodes will continue to try to contact it on that interface and MFS on that node would continue to try to use it. There is no way around this, there is no way to tell a running MFS process to stop using one of its interfaces. When MFS starts, it sends the list of its IP addresses to the CLDB. when something needs to talk to MFS, it retrieves the list of IP address from the CLDB. In some scenarios you might want MapR to use a restricted subnet of NICs. For example, if you use multiple NICs of mixed speeds (such as 1GbE and 10GbE) on each node, you might want to separate them into two subnets. That way, you can use the faster NICs for MapR and the slower NICs for other functions.
https://maprdocs.mapr.com/home/AdministratorGuide/Designating-Subnets-for-MapR.html?hl=mapr_subnets
Lets consider a host with multiple NIC's and part of different subnet .
[root@vm52 ~]# ip a
1: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 00:0c:29:bf:39:96 brd ff:ff:ff:ff:ff:ff
inet 10.10.80.91/24 brd 10.10.80.255 scope global eth1
inet6 fe80::20c:29ff:febf:3996/64 scope link
valid_lft forever preferred_lft forever
2: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
link/ether 00:0c:29:39:5c:31 brd ff:ff:ff:ff:ff:ff
inet 10.10.70.99/24 brd 10.10.70.255 scope global eth0
inet6 fe80::20c:29ff:fe39:5c31/64 scope link
valid_lft forever preferred_lft forever
We can use below tools/link to know CIDR range ( IP range ) which can be then be set at env.sh for MapR to use only particular subnet for communication.
[root@vm52 ~]# grep "SUBNETS=" /opt/mapr/conf/env.sh
#export MAPR_SUBNETS=
[root@vm52 ~]#
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