Three-Switch Cluster Scenarios - Cisco MDS 9000 series Configuration Manual
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Appendix B
Cluster Management and Recovery Scenarios
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
2.
3.
The next set of examples describe reboots of both switches (S1 with node ID 1 and S2 with node ID 2):
If you perform any configuration change on a cluster, you must save the running configuration to the
Caution
startup configuration by entering the copy running-config startup-config CLI command on all switches
before rebooting them. Otherwise, the cluster may not form correctly after the reboot (see example 3.).
1.
2.
3.
Caution
It is critical that you save the running configuration on all switches before a reboot.
Three-Switch Cluster Scenarios
In a three-switch cluster, the quorum requires two switches to be in the cluster view (N/2 + 1). The
examples below explain three scenarios in a three-switch cluster with switches S1 (node ID 1), S2 (node
ID 2) and S3 (node ID 3). S1 is the master switch.
1.
2.
3.
These examples describe reboots on all switches in the cluster:
OL-20708-01, Cisco MDS NX-OS Release 5.0(1a)
Assume that in a two-switch cluster with switches S1 (node ID 1) and S2 (node ID 2), S2 is the
master (note that the master has the higher node ID because it has the latest configuration when both
the switches came online).
When the switches lose connectivity between them, switch S2 becomes nonoperational and S1 takes
over as the master to form a 1-switch cluster. This is consistent with the quorum logic in a
two-switch cluster (N/2 with lowest node ID).
Assume that in a two-switch cluster with switches S1 (node ID 1) and S2 (node ID 2). If S1 fails
(regardless of which switch was the master), S2 will also become non-operational as long as S1 is
down.
When S1 comes up, S1 and S2 will form a two-switch cluster.
After a reboot, if both switches S1 and S2 come up about the same time, a two-switch cluster will
be formed.
If the cluster configurations are the same, S1 (with the lower node ID) will become the master.
a.
b.
If the cluster configurations are different, the switch with the latest cluster configuration will
become the master.
After a reboot, if switch S2 comes up first, it will not be able to form a cluster until S1 also comes
up. After that, the algorithm explained in the previous case will be used.
After a reboot, if switch S1 comes up first, it will form a one-switch cluster (N/2 with lowest node
ID). When S2 comes up, it will join the cluster to form a two-switch cluster.
When S2 comes up and if it happens to have the latest cluster configuration in the startup
configuration (this can happen if you did not save the running configuration to the startup
configuration on S1 but did so on S2), it will not be able to join the cluster formed by S1.
In a three-switch operational cluster, if switch S3 fails or loses connectivity with the other two
switches, then S3 becomes nonoperational. Switches S1 and S2 will form an operational cluster.
When S3 comes up again, it will rejoin the cluster.
In a three-switch operational cluster, if the master switch S1 fails or loses connectivity with the other
two switches, then S1 becomes nonoperational. Switches S2 and S3 will form an operational cluster
and S2 will be the master. When S1 comes up again, it will rejoin the cluster. Note that S2 will
continue to be the master.
If two switches fail, the cluster will become nonoperational.
Cluster Quorum and Master Switch Election
Cisco MDS 9000 Family I/O Accelerator Configuration Guide
B-3
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