EIGRP authentication

When configuring EIGRP it is possible to enable authentication between EIGRP neighbors to ensure that only valid and accepted EIGRP routers become neighbors. The authentication process exchanges an MD5 hash that is generated from the configured key, the key ID as well as the contents of the packet itself.

This means that the hash exchanged for each individual packet will be different even though the same key and key IDs are used, simply because the contents of the packet change.

Links

https://networklessons.com/eigrp/how-to-configure-eigrp-authentication