First: I recommend scanning only specific management IP addresses of devices rather than network ranges. The reason for that is that someone could set up a rogue SSH server and intercept the credential you use for scanning. You can export to the list of IPs from CiscoWorks or use NMAP scan and import result to Nessus.
Second: Nessus supports only SSH authentication for Cisco devices.
Third: our policy will include checks for IOS, CatOS and Linksys devices.
Fourth: Probably the most important one. You may be running version of IOS that has known vulnerabilities but your device may not be vulnerable. For example if there is a vulnerability in http server but your device doesn't have it enabled you are not vulnerable. Furthermore there are different feature sets of the same IOS version. You may be running "IP Base" set which doesn't support MPLS but Nessus will show MPLS vulnerability.
To perform this scan an IOS user with privilege 1 is sufficient. Nessus uses output of "show version".
There are a number of approaches to creating Nessus scanning policies.
1. enable whole "Cisco" family. This policy would however include checks specific to ASA, Aironet etc - This is Tenable recommended approach.
2. create a filter (plugin name - contains - cisco) and enable all plug-ins. This policy would include things like CSA agent detection on Windows, VPN client software, etc. Lot of checks not applicable to switches or routers.
3.use Nessus plugin filtering feature and selectively enable only switch/router applicable plugins - this is my proffered method as it reduces the risk of potential adverse effects the scan could have. Even though this should not happen (Nessus should not fire a plugin that's not applicable), last thing we'd like would be to bring a core switch down.
Here we go:
1. Configure General settings:
- disable both Netstat SSH & WMI
- disable SNMP Scan
- enable UDP Scan
- set port range to 1-65535
- leave remaining settings on defaults
2. Configure Credentials:
- Specify SSH User name
3. Select Plugins:
- All plugins are enabled by default so click "Disable All"
- Enable Cisco plugin family
- Enable Settings family