eigrp passive interface...

won't send eigrp messages or establish eigrp neighborship, but it is an active interface...

r2(config-router)#do sh ip eigrp int
EIGRP-IPv4 Interfaces for AS(1)
                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Se1/0                    1        0/0       0/0           7       5/195        231           0
Se1/1                    1        0/0       0/0          18       2/97         169           0
Lo0                      0        0/0       0/0           0       0/0            0           0
Se2/1                    1        0/0       0/0          15       0/23          91           0
Se2/2                    1        0/0       0/0          13       1/48          88           0
Se2/3                    1        0/0       0/0          12       5/190        246           0
Se1/2                    1        0/0       0/0          23       0/15         119           0

changing an interface to passive drops it out of eigrp...

r2(config-if)#router eigrp 1
r2(config-router)#passive-inter s1/2
r2(config-router)#
*Sep 13 05:44:04: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 172.20.15.6 (Serial1/2) is down: interface passive
r2(config-router)#do sh ip eigrp int
EIGRP-IPv4 Interfaces for AS(1)
                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Se1/0                    1        0/0       0/0           8       5/195        219           0
Se1/1                    1        0/0       0/0          18       2/97         161           0
Lo0                      0        0/0       0/0           0       0/0            0           0
Se2/1                    1        0/0       0/0          17       0/23          79           0
Se2/2                    1        0/0       0/0           9       1/48          96           0
Se2/3                    1        0/0       0/0          15       5/190        246           0

but it is still up up...

r2(config-router)#do sh int s1/2
Serial1/2 is up, line protocol is up
  Hardware is M4T
  Internet address is 172.20.15.5/30

and can be communicated with...

r3#ping 172.20.15.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.20.15.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/12 ms


and ip proto will display it as passive...

r2(config-router)#do sh ip proto    
*** IP Routing is NSF aware ***

Routing Protocol is "eigrp 1"
  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-IPv4 Protocol for AS(1)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
    NSF-aware route hold timer is 240
    Router-ID: 172.20.10.1
    Topology : 0 (base)
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Maximum hopcount 100
      Maximum metric variance 7

  Automatic Summarization: disabled
  Maximum path: 4
  Routing for Networks:
    172.20.0.0
  Passive Interface(s):
    Serial1/2
  Routing Information Sources:
    Gateway         Distance      Last Update
    172.20.15.22          90      00:03:29
    172.20.15.18          90      00:03:29
    172.20.15.6           90      00:07:34
    172.20.15.2           90      00:03:29
    172.20.15.14          90      00:03:29
    172.20.15.10          90      00:03:29
  Distance: internal 90 external 170