cisco support community - null interface - 2012-12-15

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Null InterfaceDocumentSat, 12/15/2012 - 10:06

Paul Varghese Jul 2nd, 2009

Table of Contents

Introduction:

Description:

Configuration Example:

Verification:

Related Information:

Introduction: This document describe about Null interface. Null interface is not physical interface; it’s a

virtual interface and is always up. Null interface never forward or receive traffic but packet

route to null interface are dropped.

Description:

1) Only interface configuration command that you can specify for the null interface is the “ip

unreachable" or " ipv6 unreachable” command. Directing packets to the null0 interface will

prevent the router from sending an Internet Control Message Protocol (ICMP) network

unreachable message when it receives a packet with an unknown destination. A situation inwhich you might want to use the null0 interface is when you have configured a default

network, but wants packets destined for a particular network to be dropped.

2) If a static route is configured for the network to the null0 interface, the packets will be

dropped without the ICMP message or without having to configure an access list. This interface

is always up and can never forward or receive traffic; encapsulation always fails. The null

interface provides an alternative method of filtering traffic. You can avoid the overhead

involved with using access lists by directing undesired network traffic to the null interface.

3) The Null interface is typically used for preventing routing loops. Enhanced Interior Gateway

Routing Protocol (EIGRP), for instance, always creates a route to the Null0 interface when it

summarizes a group of routes. Whenever a routing protocol summarizes, this means that the

router might receive traffic for any IP address within that summary. Because not all IP

addresses are always in use, there is a risk of looping packets in case default routes are used

on the router which receives the traffic for the summary.

4) When you configure summarization in OSPF ABR and ASBR generate the summary routes,

but they aren't real routes, so it simply points them to its Null0 interface called discard routes.

To know more about Discard route check document on OSPF Inter-area route summarization

http://supportforums.cisco.com/printpdf/29191#Introduction:http://supportforums.cisco.com/printpdf/29191#Introduction:http://supportforums.cisco.com/printpdf/29191#Description:https://supportforums.cisco.com/document/116011/ospf-inter-area-route-summarizationhttps://supportforums.cisco.com/users/paul-varghesehttps://supportforums.cisco.com/document/116011/ospf-inter-area-route-summarizationhttp://supportforums.cisco.com/printpdf/29191#Related_Information:http://supportforums.cisco.com/printpdf/29191#Verification:_http://supportforums.cisco.com/printpdf/29191#Configuration_Example:http://supportforums.cisco.com/printpdf/29191#Description:http://supportforums.cisco.com/printpdf/29191#Introduction:https://supportforums.cisco.com/users/paul-varghese


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Configuration Example:

1) Configure a basic Null interface:

R1#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#interface null0R1(config-if)#ip unreachable

R1(config-if)#end

R1#

2) Configure default static route point to the null0 interface:

R1#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#ip route 0.0.0.0 0.0.0.0 null 0

R1(config)#end

Base Initial configuration:

Verification:

To Verifies the configuration of the null interface use "show interface null0" command.

R1#show interface null0

Null0 is up, line protocol is up

Hardware is Unknown

MTU 1500 bytes, BW 10000000 Kbit/sec, DLY 0 usec,

reliability 0/255, txload 0/255, rxload 0/255

Encapsulation ARPA, loopback not set

Last input never, output never, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

45 packets output, 4500 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 unknown protocol drops

0 output buffer failures, 0 output buffers swapped out

Related Information:


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Configure a Null Interface

Configuring Virtual Loopback and Null Interfaces on Cisco IOS XR Software

Use a Static Route to the Null0 Interface for Loop Prevention

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cisco support community - null interface - 2012-12-15

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