Chapter 3. Planning 59
The port designation patterns that are shown in 3.6, “SAN configuration planning” on page 50 
provide the required traffic isolation and simplify migrations to configurations with greater 
number of ports. More complicated port mapping configurations that spread the port traffic 
across the adapters are supported and can be considered. However, these approaches do 
not appreciably increase availability of the solution. 
Alternative port mappings that spread traffic across HBAs might allow adapters to come back 
online following a failure. However, they do not prevent a node from going offline temporarily 
to restart and attempt to isolate the failed adapter and then rejoin the cluster. Also, the mean 
time between failures (MTBF) of the adapter is not significantly shorter than that of the 
non-redundant node components. The presented approach takes all of these considerations 
into account with a view that increased complexity can lead to migration challenges in the 
future, and simpler approach is usually better.
3.6.8  Port masking
You can use a port mask to control the node target ports that a host can access. By using 
local FC port masking, you can set which ports can be used for node-to-node/intracluster 
communication. By using remote FC port masking, you can set which ports can be used for 
replication communication. Port masking, combined with zoning, enables you to dedicate 
ports to particular type of traffic. Setting up Fibre Channel port masks is particularly useful 
when you have more than four Fibre Channel ports on any node in the system, as it saves 
setting up many SAN zones.
Two Fibre Channel port masks are on a system. The local port mask controls connectivity to 
other nodes in the same system; the partner port mask controls connectivity to nodes in 
remote, partnered systems. By default, all ports are enabled for both local and partner 
connectivity.
The port masks apply to all nodes on a system; a different port mask cannot be set on nodes 
in the same system. You do not have to have the same port mask on partnered systems.
A mixed traffic of host, back-end, intracluster, and replication might cause congestion and 
buffer to buffer credit exhaustion. This type of traffic can result in heavy degradation of 
performance in your storage environment.
Fibre Channel I/O ports are logical ports, which can be on Fibre Channel platform ports or 
FCoE platform ports.
The port mask is a 64-bit field that applies to all nodes in the cluster. In the local FC port 
masking, you can set a port to be dedicated to node-to-node/intracluster traffic by setting a 1 
to that port. Remote FC port masking allows you to set which ports can be used for replication 
traffic by setting 1 to that port. 
If a port has a 0 in the specific mask, it means no traffic of that type is allowed. Therefore, in a 
local FC port map, a 0 means no node-to-node traffic occurs. A 0 on the remote FC port 
masking means that no replication traffic occurs on that port. Therefore, if a port has a 0 on 
local and remote FC port masking, only host and back-end storage traffic is allowed on it. 
Note: With 12 or more ports per node, four ports should be dedicated for node-to-node 
traffic. Doing so is especially important when high write data rates are expected because 
all writes are mirrored between I/O Group nodes over these ports.