Dell Networking 2024 Reference Manual page 613

User Guidelines
MLAG will not become operational until the peer keepalive protocol detects
a peer and syncs the peer information. Peer keepalive timeout state
transitions are suppressed if the Dual Control Plan Detection (DCPDP) is
enabled and detects that the peer is still alive.
Two failure situations cause state transitions:
• The peer device fails: A peer does not receive any more advertisements and
the timeout timer expires.
• Secondary device fails: All MLAG members' port information
regarding the secondary device that the primary switch maintains are
removed from the primary switch. Forwarding and control processing
continues on the local MLAG ports on the primary switch. Once the
secondary comes back up again, it starts the keepalive protocol and, if
successful in contacting the primary device, moves to the secondary
state. It then initiates an FDB sync and becomes operational again.
• Primary device fails: The secondary device transitions to primary state
and continues forwarding traffic on its local MLAG ports. It also starts
processing control messages. The MLAG connected devices see a
change in the source MAC address. Once the peer device comes up
again, it starts the keepalive protocol and transitions to the secondary
state.
• The peer-link fails: This occurs when either switch cannot contact the peer
through the peer keepalive protocol and the DCPDP protocol. The
secondary switch transitions to a primary role which results in two primary
switches. Both primaries continue forwarding traffic. Each primary also
processes control traffic and sends LACP and BPDU packets with a unique
source MAC address (the system MAC of the local switch). The MLAG
connected devices become aware that they are connected to two devices
and, if LACP is enabled, block the links to one of the peers as a new actor
id is received. STP re-convergence may also occur in this scenario.
Example
console(config)#vpc domain 1
console(config-vpc 1)#peer-keepalive enable
console(config-vpc 1)#peer-keepalive destination 192.168.0.2 source
192.168.0.1
console(config-vpc 1)#peer detection enable
console(config-vpc 1)#exit
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MLAG