Seeing as we just finished up a simple EIGRP lab, it seems a good opportunity to add one more simple thing in to the mix.
At the moment any one could in theory add a router in to the network, sniff for packets to determined the AS number we are running EIGRP on, and start advertising routes and forming neighbours. This is not something we want to happen, even if not a malicious attack a rogue router sending EIGRP hellos and updates could cause havoc with a network.
So like all good network administrators it is important to secure EIGRP against such happenings. This is achieved in EIGRP by means of md5 authentication and key-chains.
The theory works some thing like this. All routers must be in time with each other, if possible a time protocol such as NTP should be used, but you could also set the clocks manually (just remember to redo this after a reboot as the router will lose its time). One they are in sync we can set up the key-chains. Each key chain has a number, time frame in which it will be sent and a time frame in which it will be accepted, along with the actual key value its self.
For a key to be accepted as valid by a router, when it receives it, the key-chain number and the key value must match on both devices, and it must be revived with in the accepted time frame. Below is a generic template for setting up a key-chain.
The idea is that you may use one key each month for example, with the accept and send time of the next key in the chain over lapping with the last slightly (if you have NTP the over lap can be a matter of seconds due to the increased sync of the routers clocks), to insure the neighbours do not get dropped during the change over of keys.
Once the keys have been set up you apply them to the interface which is sending out EIGRP updates as below.
interface FastEthernet0/0 ip authentication mode eigrp <AS> md5 ip authentication key-chain eigrp <AS> <keychain name>
I have set this up in the GNS3 lab here.. to get it working you will need to set the time on router 1 to 00:00:00 24th october 2010 (#clock set 00:00:00 24 october 2010), and then on router 2 remove and re-add the NTP server. This will sync up the clocks to the correct time for the configured key chains. You should then see the neighbours come up. Running a #Debug eigrp packets, and you will see the hellos and updated getting sent with the md5 authentication.