Default Passive Interfaces

The Default Passive Interfaces feature simplifies the configuration of distribution devices by allowing all interfaces to be set as passive by default. In ISPs and large enterprise networks, many distribution devices have more than 200 interfaces. Obtaining routing information from these interfaces requires configuration of the routing protocol on all interfaces and manual configuration of the passive-interface command on interfaces where adjacencies were not desired.

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Information About Default Passive Interfaces

Default Passive Interfaces

In large enterprise networks, many distribution devices have more than 200 interfaces. Before the introduction of the Default Passive Interfaces feature, routing information could be obtained from these interfaces in these ways:

  • Configure a routing protocol such as Open Shortest Path First (OSPF) on the backbone interfaces and redistribute connected interfaces.

  • Configure a routing protocol on all interfaces and manually set most of them as passive.

Network operators might not always be able to summarize type 5 link-state advertisements (LSAs) at the device level where redistribution occurs, as in the first possibility. Thus, a large number of type 5 LSAs can be flooded over the domain.

In the second possibility, large type 1 LSAs might be flooded over the domain. The Area Border Router (ABR) creates type 3 LSAs, one for each type 1 LSA, and floods them to the backbone. You can, however, have unique summarization at the ABR level, which injects only one summary route into the backbone, thereby reducing the processing overhead.

Before the introduction of the Default Passive Interfaces feature, you could configure the routing protocol on all interfaces and manually set the passive-interface router configuration command on interfaces where adjacencies were not desired. But in some networks, this solution meant configuring 200 or more passive interfaces. The Default Passive Interfaces feature solved this problem by allowing all interfaces to be set as passive by default. You can set all interfaces as passive by default by using the passive-interface default command and then configure individual interfaces where adjacencies are desired using the no passive-interface command.

The Default Passive Interfaces feature simplifies the configuration of distribution devices and allows the network administrator to obtain routing information from interfaces in ISPs and large enterprise networks.

Preventing Routing Updates Through an Interface

To prevent other devices on a local network from learning about routes dynamically, you can keep routing update messages from being sent through a device interface. This feature applies to all IP-based routing protocols except the Border Gateway Protocol (BGP).

Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) behave somewhat differently. In OSPF, the interface address that you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified device interface. In IS-IS, the specified IP addresses are advertised without actually running IS-IS on those interfaces.

To prevent routing updates through a specified interface, use the passive-interface type number command in router configuration mode.

How to Configure Default Passive Interfaces

Configuring Default Passive Interfaces

Perform this task to set all interfaces on a device, in an Enhanced Interior Gateway Routing Protocol (EIGRP) environment, as passive by default, and then activate only those interfaces where adjacencies are desired.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router eigrp {autonomous-system-number | virtual-instance-number}
  4. passive-interface [default ] [type number]
  5. no passive-interface [default ] [type number]
  6. network network-address [options]
  7. end
  8. show ip eigrp interfaces
  9. show ip interface

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

router eigrp {autonomous-system-number | virtual-instance-number}

Example:


Device(config)# router eigrp 1

Configures an EIGRP process and enters router configuration mode.

  • autonomous-system-number —Autonomous system number that identifies the services to the other EIGRP address-family devices. It is also used to tag routing information. The range is 1 to 65535.

  • virtual-instance-number —EIGRP virtual instance name. This name must be unique among all address-family router processes on a single device, but need not be unique among devices

Step 4

passive-interface [default ] [type number]

Example:


Device(config-router)# passive-interface default

Sets all interfaces as passive by default.

Step 5

no passive-interface [default ] [type number]

Example:


Device(config-router)# no passive-interface gigabitethernet 0/0/0

Activates only those interfaces that need adjacencies.

Step 6

network network-address [options]

Example:


Device(config-router)# network 192.0.2.0

Specifies the list of networks to be advertised by routing protocols.

Step 7

end

Example:


Device(config-router)# end

Exits router configuration mode and returns to privileged EXEC mode.

Step 8

show ip eigrp interfaces

Example:

Device# show ip eigrp interfaces

Verifies whether interfaces on your network have been set to passive.

Step 9

show ip interface

Example:

Device# show ip interface

Verifies whether interfaces you enabled are active.

Configuration Examples for Default Passive Interfaces

Examples: Passive Interfaces Configuration for OSPF

In Open Shortest Path First (OSPF), hello packets are not sent on an interface that is specified as passive. Hence, the device is not able to discover any neighbors, and none of the OSPF neighbors are able to see the device on that network. In effect, this interface appears as a stub network to the OSPF domain. This configuration is useful if you want to import routes associated with a connected network into the OSPF domain without any OSPF activity on that interface.

The passive-interface router configuration command is typically used when the wildcard specification on the network router configuration command configures more interfaces than is desirable. The following configuration causes OSPF to run on all subnets of 172.18.0.0: 


Device(config)# interface GigabitEthernet 0/0/0
Device(config-if)# ip address 172.18.1.1 255.255.255.0 
Device(config-if)# exit 
Device(config)# interface GigabitEthernet 1/0/0
Device(config-if)# ip address 172.18.2.1 255.255.255.0 
Device(config-if)# exit 
Device(config)# interface GigabitEthernet 2/0/0
Device(config-if)# ip address 172.18.3.1 255.255.255.0 
Device(config-if)# exit 
Device(config)# router ospf 1 
Device(config-router)# network 172.18.0.0 0.0.255.255 area 0 
Device(config-router)# exit

If you do not want OSPF to run on 172.18.3.0, enter the following commands: 


Device(config)# router ospf 1 
Device(config-router)# network 172.18.0.0 0.0.255.255 area 0 
Device(config-router)# no passive-interface GigabitEthernet 2/0/0
Device(config-router)# exit

Example: Default Passive Interfaces Configuration for OSPF

The following example configures the network interfaces, sets all interfaces that are running Open Shortest Path First (OSPF) as passive, and then enables serial interface 0/0/0: 


Device(config)# interface GigabitEthernet 0/0/0
Device(config-if)# ip address 172.19.64.38 255.255.255.0 secondary 
Device(config-if)# ip address 172.19.232.70 255.255.255.240 
Device(config-if)# no ip directed-broadcast 
Device(config-if)# exit 
Device(config)# interface Serial 0/0/0 
Device(config-if)# ip address 172.24.101.14 255.255.255.252 
Device(config-if)# no ip directed-broadcast 
Device(config-if)# no ip mroute-cache 
Device(config-if)# exit 
Device(config)# interface TokenRing 0/0/0
Device(config-if)# ip address 172.20.10.4 255.255.255.0 
Device(config-if)# no ip directed-broadcast 
Device(config-if)# no ip mroute-cache 
Device(config-if)# ring-speed 16 
Device(config-if)# exit 
Device(config)# router ospf 1 
Device(config-router)# passive-interface default 
Device(config-router)# no passive-interface Serial 0/0/0
Device(config-router)# network 172.16.10.0 0.0.0.255 area 0 
Device(config-router)# network 172.19.232.0 0.0.0.255 area 4 
Device(config-router)# network 172.24.101.0 0.0.0.255 area 4 
Device(config-router)# end

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Command List, All Releases

IP routing protocol-independent commands

Cisco IOS IP Routing: Protocol-Independent Command Reference

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Feature Information for Default Passive Interfaces

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Default Passive Interfaces

Feature Name

Releases

Feature Information

Default Passive Interfaces

In ISP and large enterprise networks, many of the distribution devices have more than 200 interfaces. Obtaining routing information from these interfaces required configuration of the routing protocol on all interfaces and manual configuration of the passive-interface command on the interfaces where adjacency was not desired. The Default Passive Interface feature simplifies the configuration of distribution devices by allowing all interfaces to be set as passive by default using a single passive-interface default command, and then by configuring individual interfaces where adjacencies are desired using the no passive-interface command.