How Mac Retention Works - Alcatel-Lucent OmniSwitch AOS Release 6 Manual

MAC Retention Overview
had previously associated Stack 1 with the stack address M1, now has to change its ARP tables to associ-
ate Stack 1 with the new stack address M2.
Similarly, in IPv6 routing, Switch 1 has to change its Neighbor Discovery tables to associate Stack 1 with
the new stack address M2.
Another aspect that may be impacted is the recalculation of the Spanning Tree in accordance with the
Spanning Tree Protocol (STP). If the stack address is changed due to the election of a new primary
element, a new Spanning Tree has to be recalculated to account for this change. This becomes even more
difficult when the newly elected primary element becomes the new root bridge.
Link Aggregation Control Protocol (LACP) is another application that is influenced by the takeover. This
application uses the base MAC address of the switch as the system ID while exchanging the LACP PDUs
in the network. After takeover, the aggregate ports will administratively go down and then come up again
due to the change in the system ID.
Therefore, to avoid these recalculations, when a primary element fails in a stack, the secondary element,
which takes over as the new primary element uses the MAC address of the former primary element.This
feature of retaining the base MAC address of the former primary element for a fixed or indefinite period of
time is called MAC Address Retention. In this way, recalculation of protocols, such as Spanning Tree and
Link Aggregation and updation of tables, such as the Address Resolution Protocol (ARP) table for IPv4
routing and the Neighbor Discovery table for IPv6 routing is minimized.
Note. The MAC Retention feature is only supported on the switch that operates in the single MAC mode.

How MAC Retention Works

During a full system startup, all the elements in the stack receive the base MAC address read from the
EEPROM of the primary element. When the primary element of the stack fails, the secondary element
takes over as the new primary element.
This new primary element and all the idle elements of the stack retain this base MAC address. Therefore,
this address is called the retained base MAC address.
The ability of the elements to retain this address can be configured, i.e., the MAC Retention feature can be
enabled or disabled on the stack. By default, it is disabled.
After a takeover, if the element still uses a retained base MAC address, you can disable the retention
process manually. Thereafter, the element will start using the base MAC address from the EEPROM of the
currently active primary
When the element retains the base MAC address during a takeover, it continues to use this base MAC
address irrespective of the return of the former primary element to the stack. This can lead to the duplica-
tion of the MAC address.
The duplication of MAC addresses may arise in the following scenarios:
•
Failure of non-adjacent elements
•
Failure of non-adjacent primary and secondary elements
•
Failure of non-adjacent primary and idle elements
•
Failure of non-adjacent secondary and idle elements
page 9-4
element.
OmniSwitch AOS Release 6 Network Configuration Guide
Configuring MAC Retention
July 2010