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[...] Ubuntu 14.04 servers [...]

That's probably the problem. The version of OpenSSH in Ubuntu 14.04 doesn't seem to be compatible with the version of Dropbear (v2012.55) installed on CirrOS 0.3.2. Try connecting from an older version of OpenSSH instead; the version in Ubuntu 12.04 works fine for me.

From Ubuntu 14.04:

root@trusty:~# lsb_release -d 
Description:    Ubuntu 14.04 LTS
root@trusty:~# ssh -V
OpenSSH_6.6.1p1 Ubuntu-2ubuntu2, OpenSSL 1.0.1f 6 Jan 2014
root@trusty:~# ssh cirros@10.1.1.51
(hangs)

From Ubuntu 12.04:

root@precise:~# lsb_release -d 
Description:    Ubuntu 12.04.4 LTS
root@precise:~# ssh -V
OpenSSH_5.9p1 Debian-5ubuntu1.4, OpenSSL 1.0.1 14 Mar 2012
root@precise:~# ssh cirros@10.1.1.51
cirros@10.1.1.51's password:
$ # now ssh'd in

[...] Ubuntu 14.04 servers [...]

That's probably Try lowering the problem. The version of OpenSSH in Ubuntu 14.04 doesn't seem to be compatible with the version of Dropbear (v2012.55) installed on MTU. On CirrOS 0.3.2. Try connecting from an older version of OpenSSH instead; the version in Ubuntu 12.04 works fine for me.

From Ubuntu 14.04:that would be

root@trusty:~# lsb_release -d 
Description:    Ubuntu 14.04 LTS
root@trusty:~# ssh -V
OpenSSH_6.6.1p1 Ubuntu-2ubuntu2, OpenSSL 1.0.1f 6 Jan 2014
root@trusty:~# ssh cirros@10.1.1.51
(hangs)
sudo ip link set eth0 mtu 1400

From Ubuntu 12.04:After you run that, try SSHing into the machine again. If it works, you can make it permanent by making the following changes (source):

In /etc/neutron/dhcp_agent.ini:

root@precise:~# lsb_release -d 
Description:    Ubuntu 12.04.4 LTS
root@precise:~# ssh -V
OpenSSH_5.9p1 Debian-5ubuntu1.4, OpenSSL 1.0.1 14 Mar 2012
root@precise:~# ssh cirros@10.1.1.51
cirros@10.1.1.51's password:
$ # now ssh'd in
[DEFAULT]
...
dnsmasq_config_file = /etc/neutron/dnsmasq-neutron.conf

In /etc/neutron/dnsmasq-neutron.conf:

dhcp-option-force=26,1400

Now

killall dnsmasq
service neutron-dhcp-agent restart

Next time you boot up an instance other than CirrOS ≤0.3.21, the MTU should be set correctly and you should be able to SSH in. (To view the MTU, you can run ip link.)

1 CirrOS ≤0.3.2 doesn't respect the DHCP MTU option, so SSH will still be broken on CirrOS. More information here.