When you use the EIGRP network command, two things will happen:
- All interfaces that have a network that falls within the range of your network command will be advertised in EIGRP.
- EIGRP hello packets will be sent on these interfaces.
Sometimes however you might want to advertise a network in EIGRP but you don’t want to send hello packets everywhere. Take a look at the topology below for an example:
Above we have two routers, R1 and R2. On the left side there’s the 192.168.10.0 /24 network with a switch and some computers. R1 wants to advertise this network to R2 but since there are no other EIGRP routers in the 192.168.10.0 /24 network, it’s pointless to send EIGRP hello packets on the FastEthernet 0/1 interface.
It’s also a security risk, when someone connects a router in the 192.168.10.0 /24 network (or starts a virtual router on their computer) they will be able to become EIGRP neighbors with R1.
To prevent this from happening, we will use the passive-interface command. This will ensure that the network is advertised in EIGRP but it will disable hello packets on the interface.
Let me show you how to configure this.
Here’s the EIGRP configuration of R1 and R2:
R1(config)#router eigrp 12 R1(config-router)#network 192.168.12.0 R1(config-router)#network 192.168.10.0
R2(config)#router eigrp 12 R2(config-router)#network 192.168.12.0
As a result, R2 will learn network 192.168.10.0 /24:
R2#show ip route eigrp D 192.168.10.0/24 [90/307200] via 192.168.12.1, 00:00:59, FastEthernet0/0
The problem however is that R1 is sending hello packets towards our computers. You can verify this by enabling a debug:
R1#debug eigrp packets hello EIGRP Packets debugging is on (HELLO) EIGRP: Sending HELLO on FastEthernet0/0 AS 12, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 EIGRP: Sending HELLO on FastEthernet0/1 AS 12, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
Above you can see that the hello packets are going in both directions.
Let’s use the passive interface command to disable the hello packets towards the switch: