Configure a Crypto ISAKMP Key

In order to configure a preshared authentication key, enter the crypto isakmp key command in global configuration mode:

crypto isakmp key cisco123 address 172.16.1.1

https://community.cisco.com/t5/vpn/isakmp-with-0-0-0-0-dmvpn/td-p/4312380

It is a bad practice but it is valid. 172.16.0.0/16 the full range will be accepted as possible PEER

https://www.examtopics.com/discussions/cisco/view/46191-exam-350-701-topic-1-question-71-discussion/#:~:text=Command%20reference%20is%20not%20decisive,172.16.1.128%20cisco123%0ACSR%2D1(config)%23

Testing without a netmask shows that command interpretation has a preference for /16 and /24. CSR-1(config)#crypto isakmp key cisco123 address 172.16.0.0

CSR-1(config)#do show crypto isakmp key | i cisco

default 172.16.0.0 [255.255.0.0] cisco123

CSR-1(config)#no crypto isakmp key cisco123 address 172.16.0.0

CSR-1(config)#crypto isakmp key cisco123 address 172.16.1.0

CSR-1(config)#do show crypto isakmp key | i cisco

default 172.16.1.0 [255.255.255.0] cisco123

CSR-1(config)#no crypto isakmp key cisco123 address 172.16.1.0

CSR-1(config)#crypto isakmp key cisco123 address 172.16.1.128

CSR-1(config)#do show crypto isakmp key | i cisco default 172.16.1.128 cisco123

CSR-1(config)#