Why Cisco Type 7 is not secure
Cisco IOS Type 7 passwords use a fixed XOR key string called the Vigenère cipher key. The encoded string starts with a two-digit seed that selects the starting offset in the key; the remaining bytes are the XOR of the key with the plaintext, expressed as hex. Because the key is constant and public, decoding is instantaneous.
Cisco's own documentation warns against relying on Type 7 for security; it is intended only to prevent shoulder-surfing of configuration files. For actual protection, use the Type 5 (MD5-based) or Type 8/9 (PBKDF2/scrypt-based) hash forms.