To authenticate routing updates, all EIGRP-enabled interfaces must be configured to support authentication. Figure 1 shows the IPv4 topology and which interfaces are configured with authentication.
Figure 2 shows the configuration for router R1 using the EIGRP_KEY key chain and the cisco123 key string. After R1 is configured, the other routers receive authenticated routing updates. Adjacencies are lost until the neighbors are configured with routing protocol authentication.
Figure 3 shows a similar configuration for router R2. Notice that the same key string, cisco123, is used to authenticate information with R1 and ultimately R3.
Use the Syntax Checker in Figure 4 to configure EIGRP authentication for R3.
Configuring EIGRP for IPv6 Authentication
The algorithms and the configuration to authenticate EIGRP for IPv6 messages are the same as EIGRP for IPv4. The only difference is the interface configuration mode commands use ipv6, instead of ip.
Router(config-if)# ipv6 authentication mode eigrp as-number md5
Router(config-if)# ipv6 authentication key-chain eigrp as-number name-of-chain
Figure 5 shows the commands to configure EIGRP for IPv6 authentication on router R1 using the EIGRP_IPV6_KEY key chain and the cisco123 key string. Similar configurations would be entered on R2 and R3.