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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:
- Select the router that requires the passive interface.
- Determine which interface(s) you do not want routing update
traffic to be sent through.
- 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.
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