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Impact of the Mac-persistence-timer in EX3300, EX4200 and EX4500 virtual-chassis on LACP enabled aggregated Ethernet interfaces



Article ID: KB23519 KB Last Updated: 04 Mar 2017Version: 2.0
This article describes the Impact of Mac-persistence-timer, which is present in EX3300, EX4200 and EX4500 virtual-chassis, on LACP enabled aggregated Ethernet interfaces
  • When a Virtual-chassis primary is removed from the Virtual-chassis and a new Virtual-chassis primary takes over, the default behavior is the MAC address of the new Virtual-chassis primary becomes active after 10 minutes, by using the new Virtual-chassis MAC address.

  • As a result, all LACP enabled aggregated interfaces flap on virtual-chassis; after 10 minutes of losing the original Virtual-chassis primary switch.
Due to the system based MAC address change, after the default 10 minutes of the persistence timer, LACP restarts on the virtual-chassis and all the aggregated Ethernet interfaces flap.
The Virtual Chassis (VC) MAC address (also referred as system base MAC address) is determined by the MAC address of the VC primary. When a VC  primary is removed from the VC and a new VC primary takes over, the default behavior is for the MAC address of the new VC primary to become active after 10 minutes; by using the new VC MAC address.

However, you can configure the persistent MAC address setting to allow a time delay; prior to the VC MAC address change. During this time period, if the previous VC primary rejoins the VC, it continues to use its original MAC address as the VC MAC address; even though the switch is only a Virtual-chassis member and not a VC primary. If the previous VC primary does not rejoin the VC during this period, the VC takes the MAC address of the new VC primary as the VC MAC address.

The idea of this feature is to remove the MAC addresses that are no longer valid in the system. For example, in a 2 member VC, both the members constitute one logical system. So the MAC address of the primary can be used for the whole system. Assume that both the members have to be independently used; then each member will constitute a separate system. So, the former backup should not be using the resources (in this case MAC address) of the former primary.

Configuring the timer for the Backup Member to start using Its own MAC address as the VC primary (CLI procedure):

When a backup member takes control of a VC configuration, due to a reset or some other temporary failure, the backup uses the MAC address of the old primary. This helps to ensure a smooth transition of mastership with no disruption to network connectivity. The MAC persistence timer is used in situations, in which the primary is no longer a member of the Virtual Chassis configuration; as it has been physically disconnected or removed.

The default timer value is 10 minutes. There are no minimum or maximum limits; this timer can be modified as per the network requirement.

Prior to configuring the timer, ensure that at least two member switches are present in the Virtual Chassis configuration. To configure or modify the MAC persistence timer, use the following command:
[edit virtual-chassis]
user@switch# set mac-persistence-timer 30
This command modifies the MAC persistence timer value to specify a timer value of 30 minutes; rather than the default timer value of 10 minutes.

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