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 describe as follows:

• 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.

Note Because there might be changes in the Master switch, all switches in the cluster need to be configured to handle SNMP configuration, SME roles, user credentials, and SSH. Switches in the cluster should directly communicate with KMC.

Two-Switch Cluster Scenarios

According to the cluster quorum logic “Cluster Quorum”, 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 switch 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. 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 non-operational.

2. 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 non-operational 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).

3. 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.

The next set of examples describe reboots of both switches (S1 with node ID 1 and S2 with node ID 2).

Caution If you perform any configuration change on a cluster, you must save the running configuration to the 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.

4. After a reboot, if both switches S1 and S2 come up about the same time, a two-switch cluster will be formed.

a. If the cluster configurations are the same, S1 (with the lower node ID) will become the master.

b. If the cluster configurations are different, the switch with the latest cluster configuration will become the master.

5. 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.

6. 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.

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. 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.

2. 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.

3. If two switches fail, the cluster will become nonoperational.

The examples below describe reboots on all switches in the cluster.

Caution If you perform any configuration change on a cluster, you must save the running configuration to the startup configuration by entering the copy running-config startup-config command on all switches before rebooting them. Otherwise, the cluster may not form correctly after the reboot.

4. After a reboot, if all switches come up at about the same time, first a 2-switch cluster will be formed and later the third switch will be added.

a. If the cluster configurations are the same, S1 (with the lower node ID) will become the master switch and form the 2-switch cluster first; and then add the third switch.

b. If the cluster configurations are different, the switch that is running the latest configuration will become the master switch and then form a 2-switch cluster; and then add the third switch.

5. After a reboot, if the switches come up one at a time, a 2-switch cluster will be formed after the first two switches are up. Later, when the third switch comes online, it will join the cluster.

If the third switch happens to be running the latest cluster configuration in the startup configuration (this can happen if you save the running configuration only on this switch but not on the other two), the third switch will not be able to join the cluster.

Caution It is critical that you save the running configuration on all switches before a reboot.

Four-Switch Cluster Scenarios

The four-switch cluster scenario is very similar to the examples above. The cluster will be operational if the cluster view has at least three switches (N/2 + 1), or if the cluster view has two switches including the switch with the lowest node ID (N/2 with lowest node ID).