7.1 Controlling Routing Update Traffic
7.1.2 Using and configuring the passive-interface command

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

To configure a passive interface, regardless of the routing protocol, perform the following steps:

  1. Select the router that requires the passive interface.
  2. Determine which interface(s) you do not want routing update traffic to be sent through.
  3. Configure the passive interface as follows:

router(config-router) #passive-interface type number

type  refers to the type of interface, such as serial or Ethernet.

number  refers to the interface number.

The passive interface feature behaves differently with different protocols. On most protocols, passive interface stops the router from sending updates to a particular neighbor, but continues to listen and use routing updates from that neighbor. However, passive interfaces in OSPF behave somewhat differently. In OSPF, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface. In EIGRP, passive interface causes the router to stop sending hello packets. When this happens, the router can't form neighbor adjacencies on the interface or send or receive routing updates. To achieve the common effect of passive interface with EIGRP, use the distribute-list command as follows:

Router A
router eigrp 1
distribute-list 20 out interface serial 0
access-list 20 deny any

This example allows a neighbor adjacency to form between router A and its neighbor on serial 0. Router A continues receiving routing updates from its neighbor, but the distribute-list prevents routes from being advertised out of serial 0.

The passive-interface command is typically used in conjunction with other capabilities, as you will see in this chapter.

Use this command to prevent all route updates from being sent. If you want to be selective about which route updates not to send, you must use one of the other capabilities, such as route filtering and distribution filtering.