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Null Interface

 

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 in which 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

Configuration Example:

 

1) Configure a basic Null interface:

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface null0
R1(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:

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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