IP Routing
How Does the Passive Interface Feature Work in
EIGRP?
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How Does the Passive Interface Feature Work in
EIGRP?
Document ID: 13675
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Contents
Introduction
Prerequisites
Requirements
Components Used
Conventions
The passive interface
Command
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Introduction
You can use the
passive-interface
command to control the advertisement of routing information. The
command enables the suppression of routing updates over some interfaces while it
allows updates to be exchanged normally over other interfaces.
With most routing protocols, the
passive-interface command restricts
outgoing advertisements only. However, when used with Enhanced Interior Gateway
Routing Protocol (EIGRP), the effect is slightly different. This document
demonstrates that use of the
passive-interface command in EIGRP
suppresses the exchange of hello packets between two routers, which results in
the loss of their neighbor relationship. This stops not only routing updates
from being advertised, but it also suppresses incoming routing updates. This
document also discusses the configuration required in order to allow the
suppression of outgoing routing updates, while it also allows incoming routing
updates to be learned normally from the neighbor.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
The information in this document is based on these software and hardware
versions:
Cisco IOS? Software Release 12.2(10b)
Cisco 2600 series routers
The information in this document was created from the devices in a specific
lab environment. All of the devices used in this document started with a cleared
(default) configuration. If your network is live, make sure that you understand
the potential impact of any command.
Conventions
For more information on document conventions, refer to the
Cisco Technical Tips Conventions.
The passive interface
Command
With EIGRP running on a network, the
passive-interface command stops
both outgoing and incoming routing updates, since the effect of the command
causes the router to stop sending and receiving hello packets over an interface.
Below is a sample
debug output with the
passive-interface
command not configured for Serial0:
R1#
debug eigrp packet hello
EIGRP Packets debugging is on
(HELLO)
R1#
Nov 20 08:07:33.131: EIGRP:
Sending HELLO on Serial0
Nov 20 08:07:33.135: AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
Nov 20 08:07:35.327: EIGRP:
Received HELLO on Serial0 nbr 192.168.1.1
Nov 20 08:07:35.331: AS 1, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
You can see that hello packets are being exchanged in both directions. The
output of the
show ip eigrp neighbor is shown below.
R1#
show ip eigrp neighbor
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 192.168.1.1 Se0 13 00:24:47 1 3000 0 1
Note: The interface is both sending and receiving hellos, and the
two routers are neighbors.
Below is a sample
debug output after the
passive-interface
command has been configured for Serial0.
R1(config)#
router eigrp 1
R1(config-router)#
passive-interface serial 0
R1#
debug eigrp packet hello
EIGRP Packets debugging is on
(HELLO)
Note: Since no output is displayed, EIGRP is not only suppressing
outbound hellos, but ignores inbound ones as well. The two routers are not
neighbors any more. The output of the
show ip eigrp neighbor after
entering the
passive-interface command is as shown below.
R1#
show ip eigrp neighbor
IP-EIGRP neighbors for process 1
When the
passive-interface command is used in EIGRP, the router cannot
form neighbor adjacencies on the interface, or send or receive routing updates.
However, if you want the outgoing routing updates alone be suppressed but the
inbound updates continue to be received (and the routers still continue to be
neighbors), then use the
distribute-list
command as follows:
R1(config)#
access-list 20 deny any
R1(config)#
router eigrp 1
R1(config-router)#
no passive-interface serial 0
R1(config-router)#
distribute-list 20 out serial 0
The output of the
show ip eigrp neighbor commands after using the
distribute-list command, is as shown below.
R1#
show ip eigrp neighbor
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 192.168.1.1 Se0 14 00:01:31 1 3000 0 3R1#
You can see that the routers are now neighbors. This example allows a
neighbor adjacency to form between R1 and its neighbor on serial 0. R1 continues
receiving routing updates from its neighbor, but the
distribute-list
prevents routes from being advertised out of serial
0.
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