OSPF Route Summarization on ABR and ASBR [GNS3 Lab]

Introduction

Every time that a route disappears in one area, routers in other areas also get involved in the shortest-path calculation. To reduce the size of the area database, you can configure summarization on an area boundary or autonomous system boundary.

Normally, type 1 and type 2 LSAs are generated inside each area and translated into type 3 LSAs in other areas. With route summarization, the ABRs or ASBRs consolidate multiple routes into a single advertisement. ABRs summarize type 3 LSAs, and ASBRs summarize type 5 LSAs. Instead of advertising many specific prefixes, advertise only one summary prefix.


Topology

I- Summarization of LSA-3 at the ABR

R1 OSPF Configuration:

router ospf 10
 network 10.0.10.0 0.0.0.255 area 0
 network 10.0.20.0 0.0.0.255 area 0
 network 10.0.30.0 0.0.0.255 area 0

let’s first check the routing table on R3 before the summarization:

we can see that there 3 entries:

  • 10.0.10.0/24
  • 10.0.20.0/24
  • 10.0.30.0/24
R3#show ip route

      1.0.0.0/32 is subnetted, 1 subnets
O E2     1.1.1.1 [110/20] via 172.16.0.2, 00:09:39, FastEthernet0/0
      5.0.0.0/32 is subnetted, 1 subnets
O E2     5.5.5.5 [110/20] via 172.16.0.2, 00:09:39, FastEthernet0/0
      6.0.0.0/32 is subnetted, 1 subnets
O IA     6.6.6.6 [110/3] via 172.16.0.2, 00:09:44, FastEthernet0/0

      10.0.0.0/24 is subnetted, 3 subnets
O IA     10.0.10.0 [110/3] via 172.16.0.2, 00:00:19, FastEthernet0/0
O IA     10.0.20.0 [110/12] via 172.16.0.2, 00:00:19, FastEthernet0/0
O IA     10.0.30.0 [110/12] via 172.16.0.2, 00:00:19, FastEthernet0/0

      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.16.0.0/30 is directly connected, FastEthernet0/0
L        172.16.0.1/32 is directly connected, FastEthernet0/0
      172.17.0.0/30 is subnetted, 1 subnets
O IA     172.17.0.0 [110/2] via 172.16.0.2, 00:12:59, FastEthernet0/0


Summarization configuration on Router R2:

R2(config)#router ospf 10
R2(config-router)# area 0 range 10.0.0.0 255.255.0.0

The previous command will tel the ABR (R2) to send LSA-3 summarized subnet 10.0.0.0/16 to other OSPF Areas instead of all included subnets coming from Area 0.

in simple words, In this command, we mention the area from which the subnets (LSA) are coming from, in our case, they are received from Area 0, then “range” the summarized subnet, we want to send to other OSPF Areas.



Verification:

We can verify on router R3

R3# show ip route
      1.0.0.0/32 is subnetted, 1 subnets
O E2     1.1.1.1 [110/20] via 172.16.0.2, 00:17:48, FastEthernet0/0
      5.0.0.0/32 is subnetted, 1 subnets
O E2     5.5.5.5 [110/20] via 172.16.0.2, 00:17:48, FastEthernet0/0
      6.0.0.0/32 is subnetted, 1 subnets
O IA     6.6.6.6 [110/3] via 172.16.0.2, 00:17:53, FastEthernet0/0

      10.0.0.0/16 is subnetted, 1 subnets
O IA     10.0.0.0 [110/3] via 172.16.0.2, 00:04:33, FastEthernet0/0

      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C        172.16.0.0/30 is directly connected, FastEthernet0/0
L        172.16.0.1/32 is directly connected, FastEthernet0/0
      172.17.0.0/30 is subnetted, 1 subnets
O IA     172.17.0.0 [110/2] via 172.16.0.2, 00:21:08, FastEthernet0/0


II- Summarization of LSA-5 at the ASBR

R1 configuration:

router ospf 10
 redistribute connected subnets


first, let’s check the routing table of router R2:

R2# show ip route

O E2     10.0.10.0 [110/20] via 172.17.0.2, 00:00:16, FastEthernet0/1
O E2     10.0.20.0 [110/20] via 172.17.0.2, 00:00:16, FastEthernet0/1
O E2     10.0.30.0 [110/20] via 172.17.0.2, 00:00:16, FastEthernet0/1

Configuration:

Let’s apply the summarization at the ASBR level (R1 in our case):

R1(config)#router ospf 10
R1(config-router)#summary-address 10.0.0.0 255.255.0.0


Verification:

R2# show ip route

      10.0.0.0/16 is subnetted, 1 subnets
O E2     10.0.0.0 [110/20] via 172.17.0.2, 00:01:22, FastEthernet0/1



https://www.ciscopress.com/articles/article.asp?p=2294214&seqNum=3

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