Stack Split Detection (Ssp); Stack Split Key Components And Terms - Alcatel-Lucent OmniSwitch 6350 Hardware User's Manual

Managing OmniSwitch 6350 Stacks

Stack Split Detection (SSP)

In the case of a stack split due to a stacking link failure or the failure of one of the stack elements, both of
the resulting stacks could end up having the same system MAC and IP addresses. Since there is no
communication between these individual stacks due to the stacking link failure they end up
communicating with the rest of the network devices using the same MAC and IP addresses. This stack
split scenario is disruptive to the network as the conflicting MAC and IP addresses can lead to layer 2
loops and L3 traffic disruption.
Stack Split Detection provides the following benefits:
Avoid network disruptions by preventing duplicate MAC and IP addresses on the network when MAC
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retention is enabled and a stack split occurs.
The sub-stack that forms out of the stack split is able to detect that a stack split has occurred.
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• Once the stack split condition has been determined, the sub-stack will put its front-panel ports into an
operationally down state preventing traffic forwarding and avoiding loops and possible traffic
disruption. The SSP link will be shut down after the protection mode SSP PDU is sent to the helper and
the acknowledgment is received from the helper by the protection mode unit.
A trap can be sent by the active-stack indicating the stack split state. The trap indicates that the stack
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split has occurred and which elements are in the operationally down sub-stack.
A mechanism is available to recover the non-operational sub-stack.
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• A method of detecting a stack split in a remote stacking topology where the stack may consist of
elements located in different physical locations such as a remote site, or multiple floors of a building.
A redundant stacking cable connection should be used for proper SSP functionality. For more information
on helping to avoid stack splits with redundant stacking cable connections, refer to

Stack Split Key Components and Terms

SSP PDU - A proprietary packet forwarded between stack elements to help determine that state of the
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stack.
SSP Helper - A neighboring OmniSwitch, not an element of the stack, responsible for forwarding SSP
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PDUs between the stack elements. The SSP feature and the SSP Helper functionality cannot be enabled
on the same switch for the same link aggregate ID.
SSP Link Aggregate - A dedicated link aggregate configured between elements of a stack and a helper
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switch to be used for forwarding SSP PDUs.
Active-stack - An element, or multiple elements, that results when a split stack occurs. The active-
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stack will keep its front panel ports enabled and continue to forward traffic on the network.
Sub-stack - An element, or multiple elements, that results when a split stack occurs. The sub-stack will
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disable its front panel ports to prevent traffic disruption caused by duplicate MAC/IP addresses with
the active stack.
Protection State - A state an element will transition to after determining a stack split has occurred. Its
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ports will be operationally disabled to to prevent duplicate MAC and IP addresses and network
disruption.
• Guard Timer - A configurable timer determining how long a unit will wait before beginning to send
SSP PDUs after a stack recovery.
OmniSwitch 6450 Hardware Users Guide October 2016
Stack Split Detection (SSP)
page 7-17.
page 7-37