What is purpose of Null0?

Unanswered Question
Nov 8th, 2007

Hi guys.

Please see below output,

What is the exact purpose of null0? is it something like dead-end?

Any specific purpose in routing decision?

RTR>sh ip ro 128.x.108.249

Routing entry for 128.x.0.0/16

Known via "eigrp 2x55", distance 5, metric 28160, type internal

Redistributing via eigrp 2x55

Routing Descriptor Blocks:

* directly connected, via Null0

Route metric is 28160, traffic share count is 1

Total delay is 100 microseconds, minimum bandwidth is 100000 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 0

RTR>sh int Null0

Null0 is up, line protocol is up

Hardware is Unknown

MTU 1500 bytes, BW 10000000 Kbit, 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

19637604 packets output, 1396931714 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out


Thanks in advance.

I have this problem too.
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Jon Marshall Thu, 11/08/2007 - 04:43


This entry is created by having an EIGRP summary route on your router

Routing entry for 128.x.0.0/16

Known via "eigrp 2x55", distance 5, metric 28160, type internal

Redistributing via eigrp 2x55

Routing Descriptor Blocks:

* directly connected, via Null0

So under an interface on your router you should see something along the lines of

ip summary-address eigrp "AS number" 128.x.0.0

This will create a Null0 entry in the routing table. This is to ensure that there are no routing loops created. This summary route is advertised out to other routers.

Any traffic destined for a subnet within the 128.x.0.0/16 network will end up at this router.

This router should then have more specific routes to individual subnets with the summary range 128.x.0.0/16.

If the router does not have a route to the more specific subnet then the packet is directed to Null0 ie the traffic is blackholed on this router and not forwarded on which is what you want.



chuckholley Wed, 12/12/2007 - 08:18

Hi, I would like to open this back up if I could. I am a little confused in my enviorment. let me show you some configs, and then if you could, please let me know if I need this route, because it is causing me some issues.

I have two WAN routers, one routing MPLS on and one is my frame router. The routing protocols setup is as follows:

WANRTR-2 (MPLS router)

router ospf 123



auto-cost reference-bandwidth 10000

redistribute bgp 65100 subnets route-map bgp_filter

network area 0


router bgp 65100

no synchronization

bgp log-neighbor-changes


redistribute connected

redistribute static

neighbor remote-as 65000

no auto-summary


ip classless

ip route Null0 name BGP

WANRTR-1 (Frame and OSPF ABR)

router ospf 123



auto-cost reference-bandwidth 10000

area 5 stub no-summary

area 5 range

area 6 stub no-summary

area 6 range

area 7 stub no-summary

area 7 range

area 8 stub no-summary

area 8 range

redistribute static subnets

network area 5

network area 6

network area 7

network area 8

network area 9

network area 0

As you can see I have that default route to NULL0. And I have a network statement for 10/8 for both routers. All of my WAN sites are 10. addressing on the LAN sides of course.

If additional info is needed please let me know.


Kevin Dorrell Wed, 12/12/2007 - 08:25

The static route should not cause you any problems because the routes you are getting from your OSPF are more specific, and therefore they take precedence.

That is, your OSPF tells you how to route, etc.etc. The static route then says "any traffic for any other 10 network that is not specified should be discarded."

Kevin Dorrell


chuckholley Wed, 12/12/2007 - 08:41

Thanks for the response, and I agree!

The issue that I have had is that legitimate routes have been advertised to my downstream core router(directly connected to WAN-1 and WAN-2 and also in OSPF area 0)going to NULL0.

So my question is, do I even need that route there since I am redistributing BGP into OSPF so all the 10. addressing should be in the downstream routers table going to the respective WAN router?

long sentence :)

Thanks again!

Richard Burts Wed, 12/12/2007 - 10:38


Here are a couple of points about your question:

- your post says: As you can see I have that default route to NULL0.

but I do not see anything in your post about default route. Did you mean your static route for

- you also comment that all the 10. addressing should be in the downstream routers table, but that really has little to do with the static route on your MPLS router.

There are 2 things that the static route for to null0 is doing for you:

- the static route for is making sure that there is always a route for in the routing table. This will assure that your BGP process will always see that network and will advertise it (assuming that this is the desired behavior).

- the static route for will assure that you do not forward packets to your external peer for network unknown addresses. I see that the neighbor in the BGP process is an external peer and I would assume that it might be advertising a default route to you. Lets assume that some device in your network generates a packet with destination address and lets assume that subnet does not exist in your network. The default route will have that packet forwarded to your MPLS router. At the MPLS router it looks in the routing table and realizes that the subnet does not exist. Since the router does have a default route should it forward the packet using the default route (to your external peer)? It should not - because the 10 network is internal to your network and the external peer can not get to it. The static route to null0 will assure that the MPLS router discards the packet rather than forwarding to the external peer.

So I suggest that you should leave the static route to null 0 on the MPLS router.



chuckholley Wed, 12/12/2007 - 11:23

Thanks very much! One more question for sanity sake. The network statement in BGP for, is that necessary?

I am not completely sure if I need that.

Richard Burts Wed, 12/12/2007 - 12:02


Without knowing more about your network, about your environment, and about your requirements it is difficult for us to know whether you need that statement or not.

Lets review what it does and then perhaps you can decide better whether you need the network statement or not. The network statement in BGP tells the BGP process to look in the local routing table and if it finds that network in the routing table then it should advertise that network to its BGP neighbors. So if you have the statement network then your BGP process will look in the local routing table and if it finds an entry for then it will advertise the network to its BGP peers.

So do you want to advertise to your BGP peer? If so then you need the network statement. If not then you do not need the network statement.

Note: I can not tell whether the values in your post are the actual values (network addresses, AS numbers, etc) or are disguised. If your BGP neighbor is really an external BGP peer and if the network is really network then I would comment that it is unusual to advertise "private" addresses like 10. to external peers.




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