Administrator Guide

Stacking 205
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 layer-2 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
redundant 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 removing the stack master from a stack for use in a different place in the
network, make sure to power down the whole stack before redeploying the
stack master so that the stack members do not continue to use the MAC
address of the redeployed master switch.
NSF Network Design Considerations
A network can be designed 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.
If the switch stack performs VLAN routing, another way to take advantage of
NSF is to configure multiple “best paths” to the same destination on
different stack members. If a unit fails, the forwarding plane removes Equal
Cost Multipath (ECMP) next hops on the failed unit from all unicast
NOTE: Each switch is assigned four consecutive MAC addresses. A stack of
switches uses the MAC addresses assigned to the stack master.