This is all about IOS Mode of Operation

1. Global Configuration Mode

2. Global configuration commands apply to features that affect the device as a whole.
Use the configure privileged EXEC command to enter global configuration mode. The
default is to enter commands from the management console.

3. Related terms:
Authentication Internet Protocol Authentication Protocol Routing Protocol Local Area
Networks Point-to-Point Protocol

4. MD5 Authentication
The type of authentication that EIGRP supports is MD5 authentication. MD5
authentication adds keyed information into each EIGRP packet that is sent. The
receiving router uses the keyed information to confirm that the source of the packet
is really who they say they are. To configure MD5 authentication, you configure an
authenticating key (sometimes referred to as a password) and a key ID on the source
and destination routers.

5. Here are the steps to configure MD5
authentication for EIGRP:
1. Enter Global Configuration mode.
2. Enter interface configuration mode.
3. Enable MD5 authentication in EIGRP packets.
4. Enable authentication of EIGRP packets (this identifies the key to be used on this interface).
5. Exit to Global Configuration mode

6. Figure 8.4 shows the diagram of a network
with two routers that will use MD5
authentication for EIGRP.

7. Configuring EIGRP MD5 Authentication
In this exercise, we’re going to configure MD5 authentication on the two routers in
1. Enter Global Configuration mode:
Router1# configure terminal

8. 2. Enter interface configuration mode:
Router1(config)# interface Fastethernet 0/0
3. Enable MD5 authentication in EIGRP packets:
Router1(config-if)# ip authentication mode eigrp 100 md5
! The 100 is the EIGRP AS.

9. 4. Enable authentication of EIGRP packets:
Router1(config-if)# ip authentication key-chain eigrp 100 ourkey1
! Ourkey1 is the name of the key
5. Exit to Global Configuration mode:
Router1(config-if)# exit

10. Configuring Router2:
1.Enter Global Configuration mode:
Router1# configure terminal
2. Enter interface configuration mode:
Router1(config)# interface Fastethernet 0/0
3. Enable MD5 authentication in EIGRP packets:
Router1(config-if)# ip authentication mode eigrp 100 md5

11. 4. Enable authentication of EIGRP packets:
Router1(config-if)# ip authentication key-chain eigrp 100 anotherkey2
5. Exit to Global Configuration mode:
Router1(config-if)# exit