Cluster Quorum; Master Switch Election; Two-Switch Cluster Scenarios - Cisco MDS 9000 series Configuration Manual

Cluster Quorum and Master Switch Election
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m

Cluster Quorum

For a cluster to be operational, it must include more than half the number of configured switches in the
cluster view. In an N-node cluster, N/2 + 1 nodes form a cluster quorum.
If N is even, the cluster quorum requires N/2 nodes and also, the presence of the switch with the lowest
node ID.
The quorum logic ensures that in the event of cluster partitions at least one partition can be operational.
All other switches are nonoperational. This guarantees the consistency of the cluster.

Master Switch Election

When a cluster is created, the switch on which the cluster is created becomes the cluster master switch.
When the master switch fails or is rebooted, another switch takes over as the master switch. The master
election logic uses the node ID and the latest cluster configuration to determine which switch in the
cluster will become the master switch. The master election logic is described as follows:
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Two-Switch Cluster Scenarios

According to the cluster quorum logic, a cluster with two configured switches can be operational if both
switches are operational or the switch with the lowest node ID is operational.
In the latter case, the switch with the lowest node ID is the master of the one-switch cluster. The other
switch could have failed or simply lost connectivity to the operational switch. In either case, the switch
with the higher node ID would become nonoperational. If the node with the lower node ID failed, the
other switch cannot form an operational cluster.
The examples that follow describe these scenarios. The first three examples consider single switch
failures.
1.
Cisco MDS 9000 Family I/O Accelerator Configuration Guide
B-2
If the master switch fails in an operational cluster, the switch with the next lowest node ID takes over
as the master switch. Note that in an operational cluster, all the switches run the same cluster
configuration.
– When the previous master switch comes back online and joins the cluster, it does not
immediately become the master.
When all the switches of a cluster are coming up, the switch that has the latest cluster configuration
becomes the master switch. If there are multiple switches with the same configuration, the switch
with the lowest node ID is chosen to be the master switch.
Once a master switch is chosen and the cluster is operational (there is a quorum), even if a
–
switch with a lower node ID joins the cluster at a later time, the master switch does not change.
For example, there are three switches S1, S2, and S3 with node IDs 1, 2, and 3, respectively. If
switches S2 and S3 form a quorum then switch S2 becomes the master switch. Even if switch
S1 with the node ID of 1 comes up and joins the cluster at a later time, switch S2 continues to
be the master. However, if switch S2 goes down for any reason, switch S1 will become the
master switch.
Assume that in a two-switch cluster with switches S1 (node ID 1) and S2 (node ID 2), S1 is the
master (the master has the lower node ID).
When the switches lose connectivity between them, the master switch S1 continues to be operational
since it has the lower node ID and can form an (N/2) switch cluster. Switch S2 becomes
nonoperational.
Appendix B Cluster Management and Recovery Scenarios
OL-20708-01, Cisco MDS NX-OS Release 5.0(1a)