How to configure EIGRP Authentication

Routing protocols can be configured to prevent receiving false routing updates and EIGRP is no exception. If you don’t use authentication and you are running EIGRP someone could try to form an EIGRP neighbor adjacency with one of your routers and try to mess with your network…we don’t want that to happen right?

EIGRP supports MD5 authentication and (since IOS 15.x) SHA authentication, there is no plaintext authentication.

What does authentication offer us?

Your router will authenticate the source of each routing update packet that it will receive.
Prevents false routing updates from sources that are not approved.
Ignore malicious routing updates.

A potential hacker could be sitting on your network with a laptop running GNS3 / Dynamips, boot up a Cisco router and try the following things:

Try to establish a neighbor adjacency with one of your routers and advertise junk routes.
Send malicious packets and see if you can drop the neighbor adjacency of one of your authorized routers.

In order to configure EIGRP authentication we need to do the following:

Configure a key-chain
- Configure a key ID under the key-chain.
  - Specify a password for the key ID.
  - Optional: specify accept and expire lifetime for the key.

Let’s use two routers and see if we can configure EIGRP MD5 authentication:

The configuration for both routers is very basic:

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 192.168.12.1 255.255.255.0

R1(config)#router eigrp 12
R1(config-router)#network 192.168.12.0

R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 192.168.12.2 255.255.255.0

R2(config)#router eigrp 12
R2(config-router)#network 192.168.12.0

The first thing we need to configure is a key-chain:

I called mine “MY_KEY_CHAIN” but it can be different on both routers, it doesn’t matter. The Key ID is a value that has to match on both routers and the key-string is the password which has to match of course.

R1(config)#key chain MY_KEY_CHAIN
R1(config-keychain)#key 1
R1(config-keychain-key)#key-string MY_KEY_STRING

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip authentication mode eigrp 12 md5 
R1(config-if)#ip authentication key-chain eigrp 12 MY_KEY_CHAIN

First you have to create the keychain and then you need to activate it on the interface. The “12” is the AS number of EIGRP. The configuration on R2 is exactly the same.

R2#debug eigrp packets 
EIGRP Packets debugging is on
    (UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY, SIAREPLY)

R2# EIGRP: FastEthernet0/0: ignored packet from 192.168.12.1, opcode = 5 (authentication off or key-chain missing)

You can check if your configuration is correct by using debug eigrp packets. You can see that we received a packet with MD5 authentication but I didn’t enable MD5 authentication yet on R2.

Let’s fix it:

R2(config)#key chain MY_KEY_CHAIN
R2(config-keychain)#key 1
R2(config-keychain-key)#key-string MY_KEY_STRING

R2(config)#interface fastEthernet 0/0
R2(config-if)#ip authentication mode eigrp 12 md5
R2(config-if)#ip authentication key-chain eigrp 12 MY_KEY_CHAIN

Right away I can see that the EIGRP neighbor adjacency is working: