As stated earlier in this chapter, when EIGRP is enabled for a network, the router begins to send out Hello packets on all interfaces that fall within the specified network range. This allows EIGRP to dynamically discover neighbors and establish network relationships. This is desired on interfaces that are actually connected to physical media, e.g. Ethernet and Serial interfaces. However, this default behavior also results in an unnecessary waste of route resources on logical interfaces, such as Loopback interfaces, that will never have any other device connected to them that the router could ever establish an EIGRP neighbor relationship with.
Cisco IOS software allows administrators to use the passive-interface [name|default] router configuration command to specify the named interface as passive, or all interfaces as passive. EIGRP packets are not sent out on passive interfaces. No neighbor relationship will ever be established between passive interfaces. The following example illustrates how to configure two EIGRP-enabled interfaces as passive on a router:
| R1(config)#interface loopback 0 R1(config-if)#ip address 10.0.0.1 255.255.255.0 R1(config-if)#exit R1(config)#interface loopback 1 R1(config-if)#ip address 10.1.1.1 255.255.255.0 R1(config-if)#exit R1(config)#interface serial 0/0 R1(config-if)#ip address 150.1.1.1 255.255.255.0 R1(config-if)#exit R1(config)#router eigrp 150 R1(config-router)#no auto-summary R1(config-router)#network 150.1.1.0 0.0.0.255 R1(config-router)#network 10.0.0.0 0.0.0.255 R1(config-router)#network 10.1.1.0 0.0.0.255 R1(config-router)#passive-interface loopback 0 R1(config-router)#passive-interface loopback 1 R1(config-router)#exit |
Based on this configuration, Loopback 0 and Loopback 1 are enabled for EIGRP routing and the directly connected networks will be advertised to EIGRP neighbors. However, no EIGRP packets will be sent by R1 out of these interfaces. Serial0/0, on the other hand, is also configured for EIGRP routing but EIGRP is allowed to send packets on this interface because it is not a passive interface. All three network entries are installed in the EIGRP Topology Table:
| R1#show ip eigrp topology IP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply Status, s – sia Status P 10.1.1.0/24, 1 successors, FD is 128256 |
However, the output of the show ip eigrp interfaces command shows that EIGRP routing is only enabled for the Serial0/0 interface:
| R1#show ip eigrp interfaces IP-EIGRP interfaces for process 150Xmit Queue Mean Pacing Time Multicast Pending Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes Se0/0 1 0/0 0 0/15 0 0 |
You can also view the interfaces configured as passive in the output of the show ip protocols command as illustrated below:
| R1#show ip protocols Routing Protocol is “eigrp 150” Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 150 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is not in effect Maximum path: 4 Routing for Networks: 10.0.0.0/24 10.1.1.0/24 150.1.1.0/24 Passive Interface(s): Loopback0 Loopback1 Routing Information Sources: Gateway Distance Last Update Distance: internal 90 external 170 |
The default keyword makes all interfaces passive. Let us assume that a router is configured with 50 Loopback interfaces configured. If you wanted to make each Loopback interface passive, you would need to add 50 lines of code. The passive-interface default command can be used to make all interfaces passive. Those interface which you do want to send EIGRP packets can then be configured with the no passive-interface [name] command. The following illustrates the use of the passive-interface default command:
| R1(config)#interface loopback 0 R1(config-if)#ip address 10.0.0.1 255.255.255.0 R1(config-if)#exit R1(config)#interface loopback 1 R1(config-if)#ip address 10.1.1.1 255.255.255.0 R1(config-if)#exit R1(config)#interface loopback 3 R1(config-if)#ip address 10.3.3.1 255.255.255.0 R1(config-if)#exit R1(config)#interface loopback 2 R1(config-if)#ip address 10.2.2.1 255.255.255.0 R1(config-if)#exit R1(config)#interface serial 0/0 R1(config-if)#ip address 150.1.1.1 255.255.255.0 R1(config-if)#exit R1(config)#router eigrp 150 R1(config-router)#network 10.0.0.1 255.255.255.0 R1(config-router)#network 10.1.1.1 255.255.255.0 R1(config-router)#network 10.3.3.1 255.255.255.0 R1(config-router)#network 10.2.2.1 255.255.255.0 R1(config-router)#network 150.1.1.1 255.255.255.0 R1(config-router)#passive-interface default R1(config-router)#no passive-interface serial 0/0 R1(config-router)#exit |
The show ip protocols can be used to view which interfaces are passive under EIGRP:
| R1#show ip protocols Routing Protocol is “eigrp 150” Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 150 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is not in effect Maximum path: 4 Routing for Networks: 10.0.0.0/24 10.1.1.0/24 10.2.2.0/24 10.3.3.0/24 150.1.1.0/24 Passive Interface(s): Loopback1 Loopback2 Loopback3 Loopback4 Routing Information Sources: Gateway Distance Last Update (this router) 90 00:02:52 Distance: internal 90 external 170 |
By using the passive-interface default command, the configuration of multiple passive interfaces is simplified and reduced. Used in conjunction with the no passive-interface serial 0/0 command, EIGRP packets are still sent out on Serial0/0, allowing EIGRP neighbor relationships to be established across that interface as illustrated below:
| R1#show ip eigrp neighbors IP-EIGRP neighbors for process 150 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 150.1.1.2 Se0/0 12 00:02:47 1 3000 0 69 |