A passive interface is a feature used across routing protocols that stops routing updates from being sent and received on a specific interface, though the exact mechanics vary from one protocol to another. In EIGRP specifically, configuring the passive-interface command stops the router from sending outgoing hello packets on that interface. Because EIGRP neighbor adjacencies can only form after hello packets are exchanged, the router cannot form any neighbor relationship through a passive interface, and as a direct result, no EIGRP updates are sent or received through that specific interface.
This is the single most important, and most commonly misunderstood, fact about EIGRP passive-interface: while no EIGRP protocol traffic passes through the passive interface itself, the subnet directly connected to that passive interface is still advertised to EIGRP neighbors through all of the router’s other active interfaces. The passive interface stops EIGRP from communicating through that specific connection, but it does not hide the existence of that network from the rest of the EIGRP domain.
The passive-interface command can be used in router configuration mode. It is commonly enabled to suppress unnecessary update traffic, for example on a LAN interface with no other routers connected, and it also increases security by preventing an unknown or untrusted device from receiving EIGRP updates on that interface.
The command syntax is:
Router(config-router)# passive-interface [interface-type interface-number]Example Topology
Consider three routers, R1, R2, and R3, each with a GigabitEthernet 0/2 interface that connects to a local LAN segment with no other EIGRP routers attached, for example a segment with host address 192.168.0.0. Because no EIGRP neighbor exists on the other end of these interfaces, forming EIGRP adjacencies there would be pointless, and leaving hello packets active on them is unnecessary overhead as well as an unneeded security exposure.
Configuring the Passive Interface
The passive-interface command prevents EIGRP hello packets and route exchange traffic on the specified interface, but as established above, EIGRP still includes that interface’s connected subnet in the updates it sends out its other active interfaces.
Router R1:
R1(config)# router eigrp 1
R1(config-router)# passive-interface GigabitEthernet0/2Router R2:
R2(config)# router eigrp 1
R2(config-router)# passive-interface GigabitEthernet0/2Router R3:
R3(config)# router eigrp 1
R3(config-router)# passive-interface GigabitEthernet0/2Making All Interfaces Passive by Default
Using the passive-interface default command configures every interface on the router as passive at once, a common baseline security practice on routers with many interfaces. To then selectively re-enable EIGRP on a specific interface, use no passive-interface [interface-type interface-number] in router configuration mode:
R1(config-router)# passive-interface default
R1(config-router)# no passive-interface GigabitEthernet0/1This pattern, default-deny with explicit exceptions, is generally considered better security practice than manually marking each individual interface as passive one at a time, since any new interface added later is passive by default rather than accidentally left active.
Passive-Interface as a Security Control
Beyond simply suppressing unnecessary update traffic, passive-interface is a genuine security control. Consider a network that connects to a third-party organization the local administrator does not control, such as an internet service provider’s network, over a directly connected interface. If that interface is left as a normal active EIGRP interface, the local router will form an EIGRP neighbor adjacency with anything on the other end of that link that also speaks EIGRP, and will both send and receive routing updates across it.
This creates a real security risk: a compromised or misconfigured device on the ISP’s side could inject false routing information into the local network, or could receive detailed routing information about the local network’s internal topology that it has no legitimate need to see.
Configuring the interface connected to the ISP as passive closes this exposure. The local router no longer forms an EIGRP adjacency across that link at all, so no routing information is sent to or accepted from anything on the other side of it, while the local network’s own internal EIGRP domain continues operating normally across all of its other active interfaces.
Verifying the Passive Interface
Use the show ip protocols command in privileged EXEC mode to confirm which interfaces are configured as passive:
R1# show ip protocols
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
Automatic Summarization: disabled
Maximum path: 4
Routing for Networks:
192.168.0.0
10.0.0.0
Passive Interface(s):
GigabitEthernet0/2
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170Note two things in this output. First, the “Passive Interface(s):” field explicitly lists GigabitEthernet0/2, confirming it is configured as passive. Second, under “Routing for Networks:”, the 192.168.0.0 network, the subnet directly connected to that same passive interface, is still listed. This directly demonstrates the core behavior explained above: the interface itself is passive and forms no EIGRP adjacency, but its connected network is still part of what EIGRP advertises out the router’s other active interfaces.
Troubleshooting Passive-Interface Misconfigurations
Symptom: An expected EIGRP neighbor relationship never forms on a given interface. Check whether that interface has been accidentally left passive, especially if passive-interface default was used elsewhere in the configuration without the corresponding no passive-interface exception for this interface. Confirm with show ip protocols and look for the interface under “Passive Interface(s).”
Symptom: A directly connected subnet is missing from a neighbor’s routing table entirely. This is not expected passive-interface behavior; a passive interface’s subnet should still appear in updates sent from other active interfaces. If the subnet is missing entirely, check that the network is correctly included under the EIGRP process with the appropriate network statement, since passive-interface alone does not add or remove a network from the EIGRP process, it only controls whether hello packets and adjacencies are permitted on that specific interface.
CCNA Exam Pointers
- Passive-interface stops hello packets and adjacency formation on the specified interface, not the advertisement of its connected network
- The subnet attached to a passive interface is still advertised via the router’s other active EIGRP interfaces
passive-interface [interface]— makes a single interface passivepassive-interface default— makes all interfaces passive; combine withno passive-interface [interface]for a default-deny security posture- Configuring an interface as passive tears down any existing neighbor adjacency on that interface immediately
show ip protocolsis the primary verification command; look for the interface listed under “Passive Interface(s):”- This behavior applies conceptually across RIP, EIGRP, and OSPF, though the exact mechanism differs: in OSPF specifically, a passive interface’s network is still advertised as a stub network via other active interfaces, similar in spirit to EIGRP’s behavior
- Common real-world use case: mark the WAN interface facing an ISP or untrusted third party as passive to prevent unauthorized EIGRP adjacencies
Conclusion
The EIGRP passive-interface command is a precise tool: it stops hello packets and neighbor adjacency formation on a specific interface, closing off both unnecessary overhead and a real security exposure, without removing that interface’s connected network from the routing domain entirely. Understanding this distinction, that the interface itself goes silent while its subnet remains advertised elsewhere, is the single most important and most exam-relevant fact about this feature. Combined with the passive-interface default plus selective no passive-interface pattern for a stronger default security posture, and verified with show ip protocols, passive-interface is a small but genuinely useful configuration tool for both network efficiency and security.
FAQs
What is EIGRP?
EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco-developed advanced distance-vector routing protocol that automates route discovery, selection, and updates between routers within an autonomous system. It uses a composite metric based on bandwidth and delay by default, forms neighbor adjacencies via hello packets, and is widely covered in CCNA and CCNP routing and switching curricula alongside OSPF as one of the two primary interior gateway protocols students are expected to master.
What is a passive-interface in EIGRP?
A passive interface in EIGRP is an interface on which the router has stopped sending EIGRP hello packets, which in turn prevents any EIGRP neighbor relationship from forming through that interface. Because no neighbor adjacency exists there, no EIGRP updates are sent or received through that specific interface. Importantly, the subnet directly connected to that passive interface is not hidden from the EIGRP domain; it continues to be advertised in updates sent out the router’s other active EIGRP interfaces.
Why would I use a passive-interface in EIGRP?
Passive-interface serves two practical purposes. First, it improves efficiency by suppressing unnecessary hello and update traffic on interfaces where no EIGRP neighbor will ever exist, such as a LAN segment connecting only to end-user devices. Second, and often more importantly, it improves security by preventing an EIGRP adjacency, and the routing information that comes with one, from forming across an interface connected to an untrusted or externally managed network, such as an ISP connection, where an unauthorized or compromised device could otherwise inject false routes or learn internal network topology.
How do I configure a passive-interface in EIGRP?
Enter router configuration mode for the EIGRP process with router eigrp [process-id], then use passive-interface [interface-type interface-number] to mark a specific interface as passive. This immediately stops EIGRP hello packets from being sent on that interface and tears down any existing neighbor adjacency there. To make every interface passive at once as a security baseline, use passive-interface default, then selectively restore EIGRP functionality on specific interfaces with no passive-interface [interface-type interface-number].
Can I use the passive-interface command on multiple interfaces?
Yes. Each interface must be specified with its own separate passive-interface [interface-type interface-number] command line within EIGRP router configuration mode; there is no single command that marks an arbitrary list of specific interfaces as passive in one line. For routers with many interfaces that should mostly be passive, using passive-interface default followed by individual no passive-interface exceptions for the interfaces that do need active EIGRP is typically more efficient and safer than listing every passive interface individually.
What happens to existing EIGRP neighbors when I configure an interface as passive?
Any existing EIGRP neighbor relationship on that interface is torn down immediately, since the interface stops sending the hello packets required to maintain that adjacency. This is an important operational consideration: configuring passive-interface on a live production interface with an active, working neighbor relationship will disrupt that relationship right away, so this change should be made deliberately and, in a production network, during a planned maintenance window rather than applied casually.
Is there a way to make all interfaces passive by default in EIGRP?
Yes, using the passive-interface default command within EIGRP router configuration mode makes every interface on the router passive simultaneously. You then selectively enable EIGRP on the specific interfaces that actually need to form neighbor adjacencies using no passive-interface [interface-type interface-number] for each one. This default-deny approach is generally considered a stronger security posture than the reverse, since any new interface added to the router later is passive by default rather than accidentally left open to EIGRP adjacency formation.
How can I verify the configuration of passive interfaces in EIGRP?
Use the show ip protocols command in privileged EXEC mode. The output includes a dedicated “Passive Interface(s):” section listing every interface currently configured as passive for that EIGRP process, alongside a separate “Routing for Networks:” section that will still list the subnet connected to a passive interface, directly demonstrating that the network remains advertised even though the interface itself forms no adjacency. This is the correct and most direct command for this specific verification task.
