Lab Objective
Configure a three-router multi-area OSPF topology with a dedicated backbone area and a leaf area containing several subnets, then configure route summarization at the ABR so only a single aggregated route is advertised into the backbone.
Lab Purpose
This lab combines two concepts discussed earlier in this series — multi-area OSPF design and the area range summarization command — into a single practical exercise, reinforcing why careful, contiguous address planning within an area is what makes clean summarization possible at all.
Lab Topology
R1 (Area 1, leaf) ---- R2 (ABR) ---- R3 (Area 0, backbone)
R1 has four LANs, all within a contiguous
/22 block, in Area 1:
192.168.4.0/24 (Loopback1)
192.168.5.0/24 (Loopback2)
192.168.6.0/24 (Loopback3)
192.168.7.0/24 (Loopback4)
R2-R3 link and R2-R1 link both in their
respective areas as notedTask 1: Configure Basic Addressing
Configure R1's four loopbacks and the two inter-router links.
Task 2: Configure Multi-Area OSPF
Enable OSPF on all three routers, placing R1 and R2's shared link plus R1's loopbacks in Area 1, and R2 and R3's shared link in Area 0.
Task 3: Verify Individual Routes Reach R3 Before Summarization
Confirm R3 initially learns all four /24 subnets as separate Type 3 summary LSAs.
Task 4: Configure Summarization on the ABR
On R2, configure area range summarization for Area 1 covering all four subnets as a single /22 block.
Task 5: Verify R3 Now Receives Only the Summary
Confirm R3's routing table shows a single summarized /22 route instead of four individual entries.
Solution and Verification
R1(config)# interface loopback1
R1(config-if)# ip address 192.168.4.1 255.255.255.0
R1(config-if)# exit
R1(config)# interface loopback2
R1(config-if)# ip address 192.168.5.1 255.255.255.0
R1(config-if)# exit
R1(config)# interface loopback3
R1(config-if)# ip address 192.168.6.1 255.255.255.0
R1(config-if)# exit
R1(config)# interface loopback4
R1(config-if)# ip address 192.168.7.1 255.255.255.0
R1(config-if)# exit
R1(config)# interface serial0/0/0
R1(config-if)# ip address 10.1.1.1 255.255.255.252
R1(config-if)# no shutdownR2(config)# interface serial0/0/0
R2(config-if)# ip address 10.1.1.2 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown
R2(config-if)# exit
R2(config)# interface serial0/0/1
R2(config-if)# ip address 10.2.2.1 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdownR3(config)# interface serial0/0/1
R3(config-if)# ip address 10.2.2.2 255.255.255.252
R3(config-if)# no shutdownR1(config)# router ospf 1
R1(config-router)# network 192.168.4.0 0.0.0.255 area 1
R1(config-router)# network 192.168.5.0 0.0.0.255 area 1
R1(config-router)# network 192.168.6.0 0.0.0.255 area 1
R1(config-router)# network 192.168.7.0 0.0.0.255 area 1
R1(config-router)# network 10.1.1.0 0.0.0.3 area 1R2(config)# router ospf 1
R2(config-router)# network 10.1.1.0 0.0.0.3 area 1
R2(config-router)# network 10.2.2.0 0.0.0.3 area 0R3(config)# router ospf 1
R3(config-router)# network 10.2.2.0 0.0.0.3 area 0R3# show ip route ospf
O IA 192.168.4.0/24 [110/66] via 10.2.2.1
O IA 192.168.5.0/24 [110/66] via 10.2.2.1
O IA 192.168.6.0/24 [110/66] via 10.2.2.1
O IA 192.168.7.0/24 [110/66] via 10.2.2.1
-- Four separate inter-area routes, before
-- summarization is configuredR2(config)# router ospf 1
R2(config-router)# area 1 range 192.168.4.0 255.255.252.0
-- Covers exactly 192.168.4.0 through
-- 192.168.7.255, the smallest contiguous
-- block containing all four subnetsR3# show ip route ospf
O IA 192.168.4.0/22 [110/66] via 10.2.2.1
-- A single summarized route now replaces
-- all four individual entriesKey Takeaway
This summarization was only possible because all four of R1's LANs were deliberately assigned as a contiguous /22 block within the same area — the careful, hierarchical address planning discussed earlier in this series regarding IPv4 addressing and VLSM is the actual prerequisite that makes clean summarization achievable, not merely the area range command itself.