[ClusterLabs] corosync cannot acquire quorum

[cys]

We have a cluster containing 3 nodes(nodeA, nodeB, nodeC). 
After nodeA is taken offline(by ifdown, this may be not right?), nodeC cannot acquire quorum while nodeB can.
NodeC: corosync-quorumtool -s
Quorum information
------------------
Date:             Sat Mar 11 10:42:22 2017
Quorum provider:  corosync_votequorum
Nodes            2
Node ID:          2
Ring ID:          2/24
Quorate:          No


Votequorum information
----------------------
Expected votes:   3
Highest expected: 3
Total votes:      2
Quorum:          2 Activity blocked
Flags:            WaitForAll 


Membership information
----------------------
    Nodeid      Votes Name


         2          1 68.68.68.15 (local)
         3          1 68.68.68.16


NodeB: corosync-quorumtools -s
Quorum information
------------------
Date:             Sat Mar 11 10:45:44 2017
Quorum provider:  corosync_votequorum
Nodes:            2
Node ID:          3
Ring ID:          2/24
Quorate:          Yes


Votequorum information
----------------------
Expected votes:   3
Highest expected: 3
Total votes:      2
Quorum:          2  
Flags:            Quorate 


Membership information
----------------------
    Nodeid      Votes Name
         2          1 68.68.68.15
         3          1 68.68.68.16 (local)


So what's the problem?
Thanks.
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