[Security Concepts] What is the result of running the crypto isakmp key ciscXXXXXXXX address 172.16.0.0 command?

The command crypto isakmp key is used to configure a pre-shared key for Internet Key Exchange version 1 (IKEv1) authentication. The keyword isakmp specifically refers to the IKEv1 framework in Cisco IOS. The syntax address 172.16.0.0 specifies the peer to be authenticated. According to the Cisco IOS command reference, when a peer address is provided in this command without an explicit subnet mask, the system assumes a default mask of 255.255.255.255. This means the key applies to a single, specific host with the IP address 172.16.0.0, which is equivalent to a /32 prefix.

A. The crypto isakmp command is used for IKEv1 configuration. IKEv2 is configured using the crypto ikev2 command set.
C. To authenticate an entire range of peers, such as the 172.16.0.0/16 network, an explicit subnet mask (e.g., 255.255.0.0) must be included in the command.
D. This command configures pre-shared key (PSK) authentication. It does not involve or secure digital certificates, which are used in a different IKE authentication method.

1. Cisco IOS Security Command Reference

A-C: In the documentation for the crypto isakmp key command

the usage section for the address peer-address argument states: "IP address of the remote peer. If you do not specify a mask

the default mask 255.255.255.255 is assumed." This confirms the command targets a single /32 host.

2. IKEv1 Configuration Guide

Cisco IOS XE Gibraltar 16.12.x: Under the chapter "Configuring Internet Key Exchange Version 1

" the section on "Configuring IKEv1 Authentication" details the crypto isakmp key command. The syntax is shown as crypto isakmp key keystring {address peer-address [ mask ] | hostname peer-hostname}

indicating the mask is an optional parameter that

if omitted

results in matching a specific peer address.

3. Cisco IOS Security Configuration Guide

Release 12.4

"Configuring Internet Key Exchange for IPsec VPNs": This guide explains that crypto isakmp commands are part of the IKE (IKEv1) framework. It also clarifies that to use a single key for a subnet

you must specify the subnet mask with the IP address

reinforcing that its absence implies a single host.