Advanced Traffic Management Guide K/KA/KB.15.15

After switchover/failover of control from the Commander to the Standby, the OOBM port IP address
of the new Commander is the Global IP address. This change in address causes some undesirable
behavior (after failover):
• When using DHCP or DHCPv6, the new Commander requests a new lease and typically
receives a new network address (IPv4 or IPv6). With OOBM high availability (HA), it will
seem as if a new link has come up requesting a network address.
• IPV6 link-local or auto-config addresses will change.
Using a static IP address avoids these issues. During failover, it will be as if the IP address is
reconfigured. All ARP entries are updated automatically.
For more information on OOBM operation, see the Management and Configuration Guide for
your switch.
Understanding stacking election
Electing a Commander
In those cases in which the Commander of the stack is not identified, the stack undergoes a
Commander Election process. This occurs when the entire stack is rebooted simultaneously, such
as during a building power failure recovery, or when the stack becomes split and the Commander
is isolated in the Inactive fragment, requiring the Active fragment to elect a new Commander.
All of the switches go through discovery and election at the same time. There is an election timer
that is set for 60 seconds, and if there are no new switches discovered during that timeout period,
the switches in the stack enter the election phase.
During the election process, for each group of switches that has the same STACK-ID (they should
all be the same), these steps occur:
1. The switches with the highest Stack Revision are discovered.
2. The switch with the highest configured priority is selected as the Commander.
3. If there are switches with the same “highest” priority, the switch that was the previous
Commander is selected.
4. If no switches were previous commanders, the switch that was the previous Standby is selected.
5. If none of the above conditions apply, the switch with the lowest MAC Address is selected as
the Commander.
Electing a Standby
The Standby switch is selected by the Commander following the same rules used to elect the
Commander. Like the Commander, the Standby switch is not changed unless a failure occurs (for
example, the Standby switch fails, or the Commander fails and the Standby becomes the
Commander).
NOTE: Since the Commander will update the revision software and set the stack IDs of all the
switches, this information will be the same for all Standby switch contenders.
The criteria used by the Commander to select the Standby is in this priority order:
1. A switch with the same system revision software as the commander is available. This speeds
up the initial boot since the stack will not have to wait for the standby to be updated. (If this
were not the case, then the selected Standby would need to be reloaded with the new system
and rebooted, resulting in the selection of a new Standby. This process would continue until
either the original Standby was rebooted or a Standby was chosen that already had the
correct system revision).
2. For all switches with the Commander's revision software, the switch with the highest priority
that is not the current Commander will become the Standby switch.
320 Stack management for the 3800 switches