One reason that route tables may not reflect the correct routes is due to the passive-interface command. With EIGRP running on a network, the passive-interface command stops both outgoing and incoming routing updates. For this reason, routers do not become neighbors.
To verify whether any interface on a router is configured as passive, use the show ip protocols command in privileged EXEC mode. Figure 1 shows that R2’s GigabitEthernet 0/0 interface is configured as a passive interface, because there are no neighbors on that link.
In addition to being configured on interfaces that have no neighbors, a passive interface can be enabled on interfaces for security purposes. In Figure 2, notice that the shading for the EIGRP routing domain is different from previous topologies. The 184.108.40.206/27 network is now included in the R2’s EIGRP updates. However, for security reasons, the network administrator does not want R2 to form an EIGRP neighbor adjacency with the ISP router.
Figure 3 shows the addition of the 220.127.116.11/27 network command on R2. R2 now advertises this network to the other routers in the EIGRP routing domain.
The passive-interface router configuration mode command is configured on Serial 0/1/0 to prevent R2’s EIGRP updates from being sent to the ISP router. The show ip eigrp neighbors command on R2 verifies that R2 has not established a neighbor adjacency with ISP.
Figure 4 shows that R1 has an EIGRP route to the 18.104.22.168/27 network in its IPv4 routing table (R3 will also have an EIGRP route to that network in its IPv4 routing table). However, R2 does not have a neighbor adjacency with the ISP router.
EIGRP for IPv6
Similar commands and troubleshooting criteria also apply to EIGRP for IPv6.
The following are the equivalent commands used with EIGRP for IPv6:
- Router# show ipv6 protocols
- Router(config-rtr)# passive-interface type number