Solved: Re: Why did the Summary Null Route disappear from the ABR's Routing Ta

jwangCisco · ‎07-04-2022

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

David Ruess · ‎07-04-2022

Hello,

According to your output it should work. To test it I also labbed up your example (minus extra configs and RIP router since it was irrelevant). I was able to produce a summary route of Null0. It may be just a buggy Packet tracer. Try configuring from scratch again (just the OSPF routers) or maybe do a reload.

Another indicator you at least know summarization is working by the output of RouterC. It would see both 10.1.0.0 networks from Router A. The fact it sees only one as a /8 means your summary is getting downstream.

Hope that helps

-David

View solution in original post

Georg Pauwen · ‎07-04-2022

Hello,

I might be wrong, but I seem to recall that you cannot use the

area range

command in area 0. Try area 1 for that range, the null route will show up....

jwangCisco · ‎07-05-2022

Thank you George.

In the past, I had read it somewhere, published by Cisco, that it was acceptable practice to summarise routes from the backbone direction, into other non-backbone areas. If I should come across it again, you bet, I am going to copy and paste that valuable document here. ( My memory might be wrong though, so please feel free to point me to the relevant Cisco article).....thanking you for your awesome speedy ray of light

MHM Cisco World · ‎07-04-2022

I think I get issue here

in router B and all router the wildcard is config wrong

interface FastEthernet0/0

 ip address 10.1.2.2 255.255.255.0<-mask 

!

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<<--here wildcard must us not mask so it must be 0.0.0.255, this i think make area range not work.

jwangCisco · ‎07-05-2022

I like your thinking MHM, so I had labbed it up, but the Summary Null route was still missing in action from the ABR's table... thank you with gratitude

MHM Cisco World · ‎07-05-2022

sorry I don't have PKT in my Mac,

I TEST it in GNS3 even wrong wildcard the summary appear.

so as @David Ruess test it is bug.

Georg Pauwen · ‎07-05-2022

Odd. I just recreated the topology, and now it works as predicted:

RouterB

router ospf 1
router-id 1.1.1.1
area 0 range 10.0.0.0 255.0.0.0
network 1.1.1.1 0.0.0.0 area 1
network 10.0.0.0 0.255.255.255 area 0

RouterB#sh ip route

Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
O 10.0.0.0/8 is a summary, 00:00:58, Null0
C 10.1.2.0/30 is directly connected, GigabitEthernet0/0
L 10.1.2.1/32 is directly connected, GigabitEthernet0/0

David Ruess · ‎07-05-2022

Great. I was just about to upload my files as well. At least I got some practice configuring OSPF summary

I was able to re-create it in packet tracer and GNS3. I'm sure you're good but if you still want the files let me know.

I will say in packet tracer it was a bit buggy. I had to reload the routers and wait another minute or 2 before it showed the summary route which I thought was weird. And another minute before it showed up on the router C downstream.

-David

MHM Cisco World · ‎07-05-2022

same as Me, still this issue from my side need more deep study wildcard and area range and NSSA and area 0.
later if I get any update I will share it here.

David Ruess · ‎07-04-2022

Hello,

According to your output it should work. To test it I also labbed up your example (minus extra configs and RIP router since it was irrelevant). I was able to produce a summary route of Null0. It may be just a buggy Packet tracer. Try configuring from scratch again (just the OSPF routers) or maybe do a reload.

Another indicator you at least know summarization is working by the output of RouterC. It would see both 10.1.0.0 networks from Router A. The fact it sees only one as a /8 means your summary is getting downstream.

Hope that helps

-David

Georg Pauwen · ‎07-05-2022

@David Ruess Do you care posting your configs ? For some weird reason, in GNS3, I could not get a null route for area 0, as soon as I changed it to any other area, the null route appeared right away...

MHM Cisco World · ‎07-05-2022

yes please also @Georg Pauwen can you share your config

David Ruess · ‎07-05-2022

When I get home this afternoon I will.

paul driver · ‎07-04-2022

Hello
Your topology doesn’t support the configuration you have posted in relation to the addressing, Also given that the rip process looks like it has auto summarization enabled and is running ver 1 which is classful so doesn’t understand cidr’d address space, plus I would suggest to append

rip metric

to the ospf redistributed routes.

Another possibility is that PT doesn’t by default have discard route enabled, which on proper software the discard (internal/external) is enabled.

Try the following:

rtrb
router ospf 1
discard-route internal



rtrc
router rip
version 2
no auto-summary
redistribute ospf x metric 5

rtrd
router rip
version 2
no auto-summary

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

jwangCisco · ‎07-05-2022

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 , if my network must get too complicated, just to make a simple theory work, then something in my own network design need to improve ..... I need to strive to make Cisco router my soul mate... Thank you so much for lending a hand

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