// Чтобы подключиться с 22 ubuntu
// marley - пользователь под которым работаю

$ vi ~/.ssh/config
Host cisco-router
  User marley
  Hostname 192.168.1.1
  HostKeyAlgorithms +ssh-rsa


$ ssh \
    -oKexAlgorithms=+diffie-hellman-group1-sha1 \
    -c aes256-cbc \
    cisco-router

// Чтобы подключиться с 20 ubuntu
$ ssh \
    -oKexAlgorithms=+diffie-hellman-group1-sha1 \
    -c aes256-cbc \
    192.168.1.1

// Так можно подключиться с 18 ubuntu, с 20 уже нет.
$ ssh -c aes256-cbc 192.168.1.1

cisco-router-1941> en

# show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
Embedded-Service-Engine0/0 unassigned      YES NVRAM  administratively down down
GigabitEthernet0/0         unassigned      YES DHCP   up                    up
GigabitEthernet0/1         192.168.1.1     YES NVRAM  up                    up
NVI0                       unassigned      YES unset  administratively down down
Virtual-PPP1               unassigned      YES NVRAM  administratively down down

# show dhcp lease
Temp IP addr: 0.0.0.0  for peer on Interface: GigabitEthernet0/0
Temp  sub net mask: 0.0.0.0
    DHCP Lease server: 0.0.0.0, state: 3 Selecting
    DHCP transaction id: 78
    Lease: 0 secs,  Renewal: 0 secs,  Rebind: 0 secs
    Next timer fires after: 00:00:01
    Retry count: 1   Client-ID: cisco-a493.4cba.00a0-Gi0/0
    Client-ID hex dump: 636973636F2D613439332E346362612E
                        303061302D4769302F30
    Hostname: cisco-router-1941

# conf t

# interface GigabitEthernet0/0
# shutdown

# no shutdown

$ debug ip dhcp server events
$ debug ip dhcp server packet

$ undebug ip dhcp server events
$ undebug ip dhcp server packet