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EIGRP Static route pointing to NULL 0


Hello Everyone,

I was just going through one of the labs in EIGRP in which I created two static routes both pointing to Null0.

However I observed that one of them got advertised and the other didn't.

R3#sh run | sec ip rou      ##router on which static route was created
ip route Null0
ip route Null0


R2#sh ip route    ###the neighbor router on which one of the static route was advertied but not the other.
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

C is directly connected, FastEthernet0/0 is subnetted, 1 subnets
D [90/409600] via, 00:05:37, FastEthernet0/0 is subnetted, 1 subnets
C is directly connected, Loopback0 is subnetted, 1 subnets
D [90/409600] via, 00:05:29, FastEthernet0/1
C is directly connected, FastEthernet0/1
D [90/281600] via, 00:05:29, FastEthernet0/1



Can anyone plz have a look and throw some light.





2 Accepted Solutions

Accepted Solutions

Thank you for posting the configuration of both routers. It does make clear that there is not any filtering of routes on either router.


I believe that this behavior reflects the use of the statement network under router EIGRP. There are two networks associated with interface Null 0 and EIGRP is picking up only one of them. I am not clear whether it is matching on the first one in the configuration or on the lowest numerical value. I do not believe that it is a question of classful vs classless behavior. I believe that if you were to change the static routes to this that the behavior would be the same

ip route Null0
ip route Null0


It would also be interesting to see what happens if you add network statements for both networks under router EIGRP

router eigrp 100



note that I am suggesting adding these to the existing network and am not suggesting changing or eliminating it. Though it would also be interesting to see the results if you remove network and put in network statements for the specific networks that you want to be advertised.







View solution in original post


That is (probably) because of bug in 12.4 IOS, that is not putting "classless" prefixes into EIGRP topology, unless you have any other classless prefix already available (in RIB). is a classful prefix; is "classless" prefix.


View solution in original post

13 Replies 13

syed kazim abbas


Plz show your Eigrp  config on R3# most probably here is the problem.

Actually two possible reasons:

1) advertise route with wrong mask

2) Distribute-list  is filtering the route



Kazim Abbas


"Don't forget to select correct answer and rate for helpful posts"


Thanks Kazim for the reply.

The Eigrp config on R3 is as:

R3#sh run | section ei
router eigrp 1
 no auto-summary

Actually this problem created some more confusions  is it possible that only classful addresses get redistributed automatically in Eigrp and not classless,and,

what will be  the case if these addresses don't exist on any physical interface or loopback.

Hope u got my point.


There is a difference in behavior on 12.4 and 15.x IOSs.

In 15.2 all the networks (in you case) would be put into eigrp toppology.

But when (on 12.4(15)T13) I created loopback with on the router, it started advertising prefix.

I would say it looks like a bug (or a feature) on old IOSs.