Managing a Switch Stack 153
Switch Stack MAC Addressing and Stack Design Considerations
The switch stack uses the MAC addresses assigned to the stack master.
If the backup unit assumes control due to a stack master failure or warm
restart, the backup unit continues to use the original stack master’s MAC
addresses. This reduces the amount of disruption to the network because
ARP and other L2 entries in neighbor tables remain valid after the failover to
the backup unit.
Stack units should always be connected with a ring topology (or other
biconnected topology), so that the loss of a single stack link does not divide
the stack into multiple stacks. If a stack is partitioned such that some units
lose all connectivity to other units, then both parts of the stack start using the
same MAC addresses. This can cause severe problems in the network.
If you move the stack master to a different place in the network, make sure
you power down the whole stack before you redeploy the stack master so that
the stack members do not continue to use the MAC address of the redeployed
switch.
NSF Network Design Considerations
You can design your network to take maximum advantage of NSF. For
example, by distributing a LAG's member ports across multiple units, the
stack can quickly switch traffic from a port on a failed unit to a port on a
surviving unit. When a unit fails, the forwarding plane of surviving units
removes LAG members on the failed unit so that it only forwards traffic onto
LAG members that remain up. If a LAG is left with no active members, the
LAG goes down. To prevent a LAG from going down, configure LAGs with
members on multiple units within the stack, when possible. If a stack unit
fails, the system can continue to forward on the remaining members of the
stack.
NOTE: Each switch is assigned three consecutive MAC addresses. The switch
uses the MAC addresses for the service port, network port, and routing
interfaces. A stack of switches uses the MAC addresses assigned to the stack
master.
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