Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 673)

ATTENTION
This condition can cause packet duplication or unexpected packet loss.
Traffic protection during split-brain conditions
By default, Network OS has a capability to recover gracefully from the split-brain scenario. When all the
ISLs between the VDX cluster switches go down, the switch with the lower RBridge ID uses LACP to
inform the edge-switch partner that it has segmented out of the port-channel. It does this by changing its
advertised system ID. When the edge switch learns a different system ID on one of its members, it
removes this member from that port-channel, and continues to function with only one vLAG member —
the switch with the higher RBridge ID. The other vLAG member switch still has the link up, but remains
segmented out of the original port-channel (sync: 0). This capability prevents duplication of packets or
potential packet drops resulting from a split-brain scenario.
When a member switch is reloaded
Reloading the switch with the lower RBridge ID has no impact.
When the switch with the higher RBridge ID is reloaded, the other vLAG member perceives all of its
ISLs as down. Though this is not a real split-brain scenario, the switch with the lower RBridge ID may
not be able to differentiate, and thus would inform the partner about a changed system ID. The partner
edge switch would detect two events:
• The system ID on one link changes.
• The other interface goes down.
In such a case, LACP will renegotiate and reform the port-channel, which could flap the port-channel,
impacting traffic momentarily. The same effect could occur when the switch boots up and joins the
fabric again.
Thus, if the switch with the higher RBridge ID is reloaded, the potential impact could be a port-channel
flap that can momentarily disrupt traffic. Notice that this effect does not occur when the switch with the
lower RBridge ID is reloaded.
Avoiding traffic disruption during switch reload
Network OS switches offer flexibility to the user by providing a special vLAG ignore-split option that
you can configure for the logical port-channel. This option should be configured on both vLAG member
ports.
Configuring this option prevents the switch with the lower RBridge ID from changing its system ID, so
both switches will continue to advertise the same system ID. This action prevents the partner edge
switch from detecting a change when one of the member switches is reloaded and the traffic is handled
gracefully.
Using the vLAG ignore-split option
To use the vLAG ignore-split option, redundancy should be built around ISLs to prevent a situation in
which all ISLs are broken at the same time. Brocade recommends using multiple ISLs, and routing
those ISLs through different physical paths or conduits to eliminate the possibility of accidental damage
to all links at the same time.
Traffic protection during split-brain conditions
Network OS Administrator’s Guide 673
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