You can configure LAN interfaces as
passive interfaces when enabling a routing protocol. A passive
interface receives updates, but does not send them. The
passive-interface command can be used with all IP interior gateway
protocols (that is, RIP, IGRP, EIGRP, OSPF, and IS-IS). The syntax
of this command is as follows:
Router(config-router)# passive-interface
type number
You can configure RTA's E0 as a passive interface, as shown in
Figure .
You can also configure WAN interfaces as passive interfaces, to
prevent the sending of updates to link partners. You can use the passive-interface
command on WAN interfaces to prevent routers from sending updates to
link partners.
There may be several reasons to prevent updates on the WAN. If
RTA and RTX are connected by a dial-on-demand ISDN link ,
regular RIP updates will keep the link up constantly and will result
in an unnecessarily large bill from the provider. Instead, a static
route can be configured on both routers with RTA's Bri0 configured
as a passive interface.
Note that for RTA to update the other routers (RTY and RTZ) about
the route to 172.16.1.0, RTA must be configured to redistribute
static routes into RIP. The redistribute
static command tells RIP to
import the static routes into RIP and advertise them as part of a
RIP update. Route redistribution is covered in more detail later in
this chapter.
The passive-interface
command works differently with the different IP routing protocols
that support it. In OSPF, the network address of the passive
interface appears as a stub network. OSPF routing information is
neither sent nor received via a passive interface. In EIGRP and
OSPF, the router stops sending hello packets on passive interfaces.
When this happens, the router can not form neighbor adjacencies, and
thus can not send and receive routing updates on the interface. You
will see later in this chapter that the passive effect can be
achieved for an EIGRP interface (without preventing adjacency
relationships) by using the distribute-list command.
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