On this post which is again an online lecture notes for my CCIE preparations,  I will be going through some details of OSPF Summarizations in particuilar TYPE 3 LSA Prefix Summarization.

OBJECTIVE:

1. To understand that prefix  summarizations of OSPF only applies for LSA Type 3 (Summary LSA) and LSA Type 5 ( AS External LSA).

2. Understand Prefix Summarization by looking into details of the LSDB.

3. Verify the results of the routing table and test the connectivity of the whole network.

4.  To proved that the interface with the lowest cost will be carried over by the summarized prefix.

5. To check that the ABR or ASBR which did the summarizations will create a Null Interface.

CONCEPTS:

 
 

Here is the topology for my laboratory about this topic,

 

Here are my router configurations

 

MELBOURNE Router:

R2#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#hostname MELBOURNE
MELBOURNE(config)#int s3/0
MELBOURNE(config-if)#ip address 192.168.12.2 255.255.255.0
MELBOURNE(config-if)#no shut
MELBOURNE(config-if)#exit
MELBOURNE(config)#int lo2
MELBOURNE(config-if)#ip address 20.20.20.1 255.255.255.0
MELBOURNE(config-if)#exit
MELBOURNE(config)#int s3/1
MELBOURNE(config-if)#ip address 192.168.23.2 255.255.255.0
MELBOURNE(config-if)#no shut
MELBOURNE(config-if)# exit

MELBOURNE(config)#router ospf 1
MELBOURNE(config-router)#network 192.168.12.0 0.0.0.255 area 0
MELBOURNE(config-router)#network 20.20.20.0 0.0.0.255 area 0
MELBOURNE(config-router)#network 192.168.23.0 0.0.0.255 area 1
MELBOURNE(config-router)#^Z

SINGAPORE Router:


R3#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#hostname SINGAPORE
SINGAPORE(config)#int s3/1
SINGAPORE(config-if)#ip address 192.168.23.3 255.255.255.0
SINGAPORE(config-if)#no shut
SINGAPORE(config-if)#exit
SINGAPORE(config)#int lo3
SINGAPORE(config-if)#ip address 30.30.30.1 255.255.255.0
SINGAPORE(config-if)#exit

SINGAPORE(config)#router ospf 1
SINGAPORE(config-router)#network 192.168.23.0 0.0.0.255 area 1
SINGAPORE(config-router)#network 30.30.30.0 0.0.0.255 area 1
SINGAPORE(config-router)#^Z
SINGAPORE#

Now, I want to show what does the router’s LSDB have at the moment,

 

<<<  first let's start with MELBOURNE router.

 

 

 

 

>>> As seen above, although, I have only advertise 10.10.0.0/16  from MANILA router, we can see that MELBOURNE router shows 4 LSDB entries for the summarized subnets 10.10.0.0/16.

 

 

>>> We should be observing the same results on SINGAPORE router as shown below,

 

 

Let’s check the routing table entries. As expected, we should also be seeing  4 prefixes for the 10.10.0.0/16 on the routing table of both MELBOURNE & SINGAPORE Router as shown below.

 

 

 >>> Take note that MELBOURNE router has an OSPF interface in Area 0 and I have advertised the 10.10.x.x subnets on the same Area 0 from MANILA router so we can see them as a normal OSPF Routtes.

>>> As observed above, we can see the 4 prefixes but by this time they are considered as TYPE 3 LSA prefixes or Inter-Area Routes ( or so called Summary LSA ).

 

 Now, before I will do the SUMMARIZATION,  I would like to tweak a bit the Interface COST of the Loopbacks on MANILA router.

 

Let’s do this for each of the Loopback with the following COST Metrics:

Now, let’s check the OSPF interface COST on the MANILA Router with the “show ip ospf interface” command:

Now, my objective is to apply Summarization on SINGAPORE Router (ABR) under the OSPF process through the command:  Area

range

I will be using a class A subnet, 10.0.0.0/8 to summarized the Loopback Prefixes of MANILA Router.

 

VERIFICATIONS:

 

A.  Now, let’s check the LSDB  of SINGAPORE router. I am expecting that the LSDB will have the summarized address for the 10.10.x.x prefixes which is 10.0.0.0/8 as I have applied under OSPF process from the MELBOURNE router.

 

<< So as seen above, I have a single entry for the 10.10.x.x  prefixes  on the LSDB of SINGAPORE router. This is true because MELBOURNE router (20.20.20.1 ) below have updated SINGAPORE router with its LSDB entries with the LINK ID of 10.0.0.0.

 

  

  B. Let us check what does the routing table of SINGAPORE router tell us,

>> As seen above, we have  just a single routes for the 4 prefixes which is 10.0.0.0/8 and it does have a COST of 228. How did the SINGAPORE router arrived with a 228 cost here? Let’s check the interface metrics of the path from SINGAPORE towards MANILA router loopback 0.

 

 Adding the Loopback interface from MANILA router with the lowest COST (Loopback 0 with a cost of 100).  

 

 

 But what does the routing table of MELBOURNE tells us about the Summarized prefix?

 

 

 >> It does create a NULL interface for 10.0.0.0/8 to avoid routing loops. But as notice, we can still see the individual prefixes. This should be a normal behavior as we did the summarization on MELBOURNE router.

 

 C. Test the connectivity