Indexing stops when too many unassigned shards

Hello,
I have a test cluster of 2 ES nodes (Elasticsearch 1.1.1-1).
I've noticed that whenever the number of unassigned shards increases past a
threshold, the cluster stops accepting Write operations.

Example1:
nodes: 2
primary shards: 6
replicas: 1

The cluster works as expected.

Example2:
nodes: 2
primary shards: 6
replicas: 2

Cluster state: 6 unassigned shards. The cluster works as expected.

Example3:
nodes: 2
primary shards: 6
replicas: 3

Cluster state: 12 unassigned shards. The cluster serves Read operations,
but not Write.
Error: {"error":"UnavailableShardsException[[myIndex][0] [4] shardIt, [2]
active : Timeout waiting for [59.9s], request: index
{[myIndex][myType][1G_yO1Y8TvGzaw9DH2Nbog], source[{ "name" : "Earl"
}]}]","status":503}
Indexing resumes if I reduce the number of replicas back to 2.

So, anything above 2 replicas seems to cause the 2-node cluster to stop
indexing.
Is this the expected behaviour?
Is there a max number of unassigned shards allowed?

Thank you,
Veselin

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