Mac Retention Overview - Alcatel-Lucent OmniSwitch 6850-48 Network Configuration Manual

Configuring MAC Retention

MAC Retention Overview

A "stack element" or simply "element" is a switch that has designated stacking ports. The switches are
operatively interconnected via these ports to form a virtual chassis referred to as a stack. Each element in a
stack can be elected as the primary or the secondary element. The primary element is elected based on the
highest uptime or the lowest slot number or the lowest base MAC address. The secondary element is
elected based on the lowest slot number or the lowest base MAC address of the remaining elements in the
stack.The system of stackable switches is generally coupled in a series and the topology of the system is
generally characterized by a closed loop called a ring. A stackable switch is adapted to perform switching
between its own data ports and between the data ports of other stackable switches by transmitting packets
via the stacking ports.
Each stack element has a unique base MAC address. Generally, the stack address is the MAC address of
the current primary element. When a primary element fails, a secondary element starts functioning as the
new primary element. This is known as takeover. During takeover, the stack address is also accordingly
changed to reflect the base MAC address of the new primary element.
Whenever a takeover occurs, it impacts not only the stack, but also the devices that communicate with that
stack.
The following diagram shows a stack connected to a stand-alone switch:
1
2
3
Stack 1
In the above diagram, Stack 1 has the stack address M1. When a takeover occurs, the secondary element
starts functioning as the new primary element and the stack address is also changed, for example, to M2,
the new primary element's MAC address. Stack 1 advertises its new stack address M2. Switch 1, which
OmniSwitch AOS Release 6 Network Configuration Guide
a
Primary Element
b
a
Secondary Element
b
a
Idle Element
b
Initial State of Stack with 3 Stack Elements
September 2009
MAC Retention Overview
Switch 1
page 18-3