Why did the Summary Null Route disappear from the ABR's Routing Table

[jwangCisco]

Hi Experts, 

Just testing out the simple theory...

I could not figure out why a route was missing from the ABR's routing table :

Could anybody please shed light for me:

I understood the theory that said : [ when you place the

Area Range

command on the ABR, then a Summary route to null0 was supposed to be installed into this ABR’s routing table ].

It did not happen on my ABR router.

I had labbed it up using Packet Tracer , with the following configurations, and a Summary Null route is currently missing from Router B’s table :

Please see my simple topology diagram :
Router B is the NSSA ABR , and is connecting to two areas : the backbone internal Router A, as well as connecting to Area 1 NSSA Router C (NSSA ASBR) .

**** Below is the configuration for NSSA ABR (Router B)

hostname Router

no ip cef

no ipv6 cef

spanning-tree mode pvst




interface FastEthernet0/0

 ip address 10.1.2.2 255.255.255.0




interface FastEthernet0/1

 ip address 1.1.1.1 255.255.255.0




router ospf 24

 log-adjacency-changes

 area 0 range 10.0.0.0 255.0.0.0

 area 1 nssa

 network 10.0.0.0 0.255.255.255 area 0

 network 1.0.0.0 0.255.255.255 area 1

!

ip classless

!

ip flow-export version 9

!

no cdp run

!

end







**** Below is the configuration for the Backbone internal Router A:

hostname Router

no ip cef

no ipv6 cef

spanning-tree mode pvst

!

interface FastEthernet0/0

 ip address 10.1.1.1 255.255.255.0

!

interface FastEthernet0/1

 ip address 10.1.2.1 255.255.255.0

!

router ospf 24

 log-adjacency-changes

 network 10.0.0.0 0.255.255.255 area 0

!

ip classless

ip flow-export version 9

end







**** Below is the configuration for NSSA ASBR (Router C 

hostname Router

no ip cef

no ipv6 cef

spanning-tree mode pvst

!

interface FastEthernet0/0

 ip address 1.1.1.2 255.255.255.0

!

interface FastEthernet0/1

 ip address 172.16.1.1 255.255.255.0

 !

router ospf 24

 log-adjacency-changes

 area 1 nssa

 redistribute rip subnets

 network 1.0.0.0 0.255.255.255 area 1

!

router rip

 redistribute ospf 24

 network 172.16.0.0

ip classless

ip flow-export version 9

end

!







**** Below is the configuration for the RIP Router 

hostname Router

no ip cef

no ipv6 cef

spanning-tree mode pvst

!

interface FastEthernet0/0

 ip address 172.16.1.2 255.255.255.0

 !

interface FastEthernet0/1

 ip address 172.16.2.1 255.255.255.0

 !

router rip

 network 172.16.0.0

!

ip classless

ip flow-export version 9

end










**** Here is the output “show ip route” for Router  ***********




Gateway of last resort is not set

     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

C       172.16.1.0/24 is directly connected, FastEthernet0/0

L       172.16.1.2/32 is directly connected, FastEthernet0/0
















***** Here is the output “show ip route” for Router C: ***********




Gateway of last resort is not set

    1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       1.1.1.0/24 is directly connected, FastEthernet0/0

L       1.1.1.2/32 is directly connected, FastEthernet0/0

O IA 10.0.0.0/8 [110/2] via 1.1.1.1, 02:13:08, FastEthernet0/0

     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

C       172.16.1.0/24 is directly connected, FastEthernet0/1

L       172.16.1.1/32 is directly connected, FastEthernet0/1













****** Here is the output “show ip route” for Router B: ***********




Gateway of last resort is not set

     1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       1.1.1.0/24 is directly connected, FastEthernet0/1

L       1.1.1.1/32 is directly connected, FastEthernet0/1

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       10.1.2.0/24 is directly connected, FastEthernet0/0

L       10.1.2.2/32 is directly connected, FastEthernet0/0

     172.16.0.0/24 is subnetted, 1 subnets

O N2    172.16.1.0/24 [110/20] via 1.1.1.2, 01:56:15, FastEthernet0/1







**** Here is the output “show ip route” for Router A: ***********




Gateway of last resort is not set

     1.0.0.0/24 is subnetted, 1 subnets

O IA    1.1.1.0/24 [110/2] via 10.1.2.2, 02:11:36, FastEthernet0/1

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C       10.1.2.0/24 is directly connected, FastEthernet0/1

L       10.1.2.1/32 is directly connected, FastEthernet0/1

     172.16.0.0/24 is subnetted, 1 subnets

O E2    172.16.1.0/24 [110/20] via 10.1.2.2, 02:11:36, FastEthernet0/1

Much Appreciated for your good trouble shooting knowledge

[paul driver]

Hello

---

@jwangCisco wrote:

Thank you Paul , you had good suggestions there as well, but I personally prefer to let simple theory work on its own , without extra bells and whistles ,

---

TBH -The above isn’t bell and whistles - it just a fact of running rip ver 1, which by default is classful and has auto-summarization enabled also your are using PT which has very limited in network simulation features., the added discard command was just in case PT didn’t have it enabled, because in real world hardware and much better network simulation software it will be enabled by default

[jwangCisco]

I like the brilliant answer from Laz as well....Let me share with you all here :

Hello Jenny

I understand more clearly now what you were referring to and I think that I will be revising my previous answer. Although it is true that the summary route appears in the neighboring router, you are correct that when you create the summary route using the area range command, the Null0 summary route should appear in the router on which the area range was configured.

This is done to protect routers from routing loops. When you use summaries it’s possible that other routers will send traffic to you for networks that you have no clue about where they are. You may forward them to a default route, and that traffic may return to you, causing a loop. For this reason, the Null0 summary route is installed locally so that the traffic will be forwarded to the null0 interface and dropped.

Now having said that, to be honest, I don’t know why the Null0 summary route does not appear in Router B as shown in your post. It should indeed appear as you originally suspected.

At first I thought it may have to do with the NSSA area 1 or the fact that you are summarizing a network that includes some directly connected networks on router B. For this reason, I labbed this up in CML and attempted to recreate your issue, and found in all cases that the Null0 summary route appears in the router that has been configured with the summary route. So to answer your original question, I don’t know why your network does not show a Null0 route in Router B.

My suggestion would be to attempt to apply the summary-address command instead and see if there is any change. Can you let us know the following:

1. how are you emulating this, in GNS3?
2. What are your results with the summary-address command?

Let us know so that we can help you further.

I hope this has been helpful!

Laz

[paul driver]

Hello

---

@jwangCisco wrote:

It creates a summary route for inter-area routes (LSA type 3 routes), but not to Null0

---

FYI-
Its a discard route to null, bit it internal or external and it is created.