> ## Content Index > Fetch the complete content index at: https://www.pinglabz.com/llms.txt > Use this file to discover other available public pages before exploring further. # Configuring Multi-Area OSPF on Cisco Routers - URL: https://www.pinglabz.com/configure-multi-area-ospf-cisco/ - Published: 2026-03-24T03:16:22.000Z - Updated: 2026-07-04T22:56:54.000Z - Description: Multi-area OSPF across Area 0, 10, and 20 with two ABRs. Full R1 to R5 config plus show commands to confirm inter-area routes and ABR behavior. - Author: Jaime - Tags: OSPF, #Import 2026-08-01 19:54 ## Multi-Area OSPF Rules 1. **Area 0 is the backbone** \- All inter-area traffic flows through it 2. **All ABRs must connect to Area 0** \- At least one interface in Area 0 3. **Non-backbone areas connect to Area 0** \- Not directly to each other 4. **Area 0 must be contiguous** \- Cannot be split ## Lab Topology ``` Area 0 (Backbone) [R1]----[R2]----[R3] | | Area 10 Area 20 | | [R4] [R5] ``` **Design:** New to OSPF? Start with the [full OSPF guide](https://www.pinglabz.com/ospf/), then come back here for the deep dive. - **R1, R2, R3:** Area 0 (backbone routers) - **R1:** ABR between Area 0 and Area 10 - **R3:** ABR between Area 0 and Area 20 - **R4:** Internal router in Area 10 - **R5:** Internal router in Area 20 **Networks:** - Area 0: 10.0.x.x - Area 10: 172.16.x.x - Area 20: 192.168.x.x ## Configuration ### R1 (ABR: Area 0 + Area 10) ``` interface loopback 0 ip address 1.1.1.1 255.255.255.255 interface gi0/0 description Link to R2 - Area 0 ip address 10.0.12.1 255.255.255.252 interface gi0/1 description Link to R4 - Area 10 ip address 172.16.14.1 255.255.255.252 router ospf 1 router-id 1.1.1.1 network 1.1.1.1 0.0.0.0 area 0 network 10.0.12.0 0.0.0.3 area 0 network 172.16.14.0 0.0.0.3 area 10 ``` **Key:** Same router, different areas on different interfaces. ### R2 (Backbone Router: Area 0 only) ``` interface loopback 0 ip address 2.2.2.2 255.255.255.255 interface gi0/0 description Link to R1 - Area 0 ip address 10.0.12.2 255.255.255.252 interface gi0/1 description Link to R3 - Area 0 ip address 10.0.23.2 255.255.255.252 router ospf 1 router-id 2.2.2.2 network 2.2.2.2 0.0.0.0 area 0 network 10.0.12.0 0.0.0.3 area 0 network 10.0.23.0 0.0.0.3 area 0 ``` ### R3 (ABR: Area 0 + Area 20) ``` interface loopback 0 ip address 3.3.3.3 255.255.255.255 interface gi0/1 description Link to R2 - Area 0 ip address 10.0.23.3 255.255.255.252 interface gi0/2 description Link to R5 - Area 20 ip address 192.168.35.3 255.255.255.252 router ospf 1 router-id 3.3.3.3 network 3.3.3.3 0.0.0.0 area 0 network 10.0.23.0 0.0.0.3 area 0 network 192.168.35.0 0.0.0.3 area 20 ``` ### R4 (Internal Router: Area 10) ``` interface loopback 0 ip address 4.4.4.4 255.255.255.255 interface loopback 1 ip address 172.16.100.1 255.255.255.0 description User Network interface gi0/1 description Link to R1 - Area 10 ip address 172.16.14.4 255.255.255.252 router ospf 1 router-id 4.4.4.4 network 4.4.4.4 0.0.0.0 area 10 network 172.16.0.0 0.0.255.255 area 10 ``` ### R5 (Internal Router: Area 20) ``` interface loopback 0 ip address 5.5.5.5 255.255.255.255 interface loopback 1 ip address 192.168.200.1 255.255.255.0 description User Network interface gi0/2 description Link to R3 - Area 20 ip address 192.168.35.5 255.255.255.252 router ospf 1 router-id 5.5.5.5 network 5.5.5.5 0.0.0.0 area 20 network 192.168.0.0 0.0.255.255 area 20 ``` ## Verification ### Check OSPF Process and Areas **R1 (ABR):** ``` R1# show ip ospf Routing Process "ospf 1" with ID 1.1.1.1 Supports opaque LSA This router is an ABR Area BACKBONE(0) Number of interfaces in this area is 2 SPF algorithm last executed 00:05:23.456 ago Area 10 Number of interfaces in this area is 1 SPF algorithm last executed 00:02:15.123 ago ``` **Key:** "This router is an ABR" + two areas listed ### Check OSPF Neighbors **R1:** ``` R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/ - 00:00:35 10.0.12.2 Gi0/0 4.4.4.4 1 FULL/ - 00:00:38 172.16.14.4 Gi0/1 ``` **R2 (backbone only):** ``` R2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/ - 00:00:36 10.0.12.1 Gi0/0 3.3.3.3 1 FULL/ - 00:00:34 10.0.23.3 Gi0/1 ``` ### Check Routing Table **R4 (Area 10):** ``` R4# show ip route ospf 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/2] via 172.16.14.1, Gi0/1 2.0.0.0/32 is subnetted, 1 subnets O IA 2.2.2.2 [110/3] via 172.16.14.1, Gi0/1 3.0.0.0/32 is subnetted, 1 subnets O IA 3.3.3.3 [110/4] via 172.16.14.1, Gi0/1 5.0.0.0/32 is subnetted, 1 subnets O IA 5.5.5.5 [110/5] via 172.16.14.1, Gi0/1 10.0.0.0/8 is variably subnetted O IA 10.0.12.0/30 [110/2] via 172.16.14.1, Gi0/1 O IA 10.0.23.0/30 [110/3] via 172.16.14.1, Gi0/1 O IA 192.168.35.0/30 [110/4] via 172.16.14.1, Gi0/1 O IA 192.168.200.0/24 [110/5] via 172.16.14.1, Gi0/1 ``` **Key indicators:** - **O IA** \= Inter-Area routes (from other areas) - Routes to Area 0 and Area 20 visible ### Test Connectivity **R4 to R5 (Area 10 → Area 20):** ``` R4# ping 5.5.5.5 source 4.4.4.4 !!!!! Success rate is 100 percent (5/5) R4# ping 192.168.200.1 source 172.16.100.1 !!!!! Success rate is 100 percent (5/5) ``` ✅ **Full inter-area connectivity!** ### Check OSPF Database **R1 (ABR):** ``` R1# show ip ospf database OSPF Router with ID (1.1.1.1) Router Link States (Area 0) Link ID ADV Router 1.1.1.1 1.1.1.1 2.2.2.2 2.2.2.2 3.3.3.3 3.3.3.3 Summary Net Link States (Area 0) Link ID ADV Router 172.16.14.0 1.1.1.1 ← R1 advertising Area 10 into Area 0 172.16.100.0 1.1.1.1 192.168.35.0 3.3.3.3 ← R3 advertising Area 20 into Area 0 192.168.200.0 3.3.3.3 Router Link States (Area 10) Link ID ADV Router 1.1.1.1 1.1.1.1 4.4.4.4 4.4.4.4 Summary Net Link States (Area 10) Link ID ADV Router 10.0.12.0 1.1.1.1 ← R1 advertising Area 0 into Area 10 10.0.23.0 1.1.1.1 192.168.35.0 1.1.1.1 ← R1 advertising Area 20 into Area 10 192.168.200.0 1.1.1.1 ``` **Key:** ABRs generate Summary (Type 3) LSAs between areas. ## Inter-Area Routing Flow **Path from R4 (Area 10) to R5 (Area 20):** 1. R4 sends traffic to R1 (ABR) 2. R1 forwards to Area 0 (R2) 3. R2 forwards to R3 (ABR) 4. R3 forwards to R5 (Area 20) **All inter-area traffic flows through Area 0.** ## Common Multi-Area Mistakes ### Mistake 1: Non-Backbone Areas Connected Directly **Bad design:** ``` R4 (Area 10) ---- R5 (Area 20) ← Direct link ``` **Problem:** OSPF won't route between them. Must go through Area 0. **Fix:** Make the direct link part of Area 0, or route via Area 0. ### Mistake 2: ABR Not in Area 0 **Bad config:** ``` R1: Gi0/0 in Area 10 Gi0/1 in Area 20 (No interface in Area 0!) ``` **Problem:** R1 can't be an ABR. All ABRs need at least one Area 0 interface. **Fix:** Ensure ABR has an interface in Area 0. ### Mistake 3: Discontiguous Area 0 **Bad design:** ``` R1 (Area 0) ---- R2 (Area 10) ---- R3 (Area 0) ``` Area 0 is split by Area 10. **Problem:** ABRs can't reach each other through Area 0. **Fix:** Use virtual link (temporary) or redesign topology. ## Route Summarization at ABRs **ABRs can summarize routes between areas:** **R1 (summarize Area 10 networks):** ``` router ospf 1 area 10 range 172.16.0.0 255.255.0.0 ``` **Result:** R1 advertises a single `172.16.0.0/16` into Area 0 instead of individual subnets. **Learn more:** [OSPF Route Summarization](https://www.pinglabz.com/ospf-route-summarization-configuration/) ## Best Practices ### 1\. Design Area 0 with Redundancy Area 0 is critical. Use redundant links and routers. ### 2\. Keep Internal Routers Simple Internal routers (like R4, R5) only see their area + summaries. This is good - keep them simple. ### 3\. Place ABRs Strategically ABRs are high-traffic routers (inter-area forwarding). Use powerful hardware. ### 4\. Document Area Assignments Maintain a diagram showing which routers/links are in which areas. ### 5\. Use Summarization Reduce routing table size by summarizing at ABR boundaries. ## Summary Now you know: ✅ **Why multi-area OSPF** \- Scalability, SPF isolation ✅ **Multi-area rules** \- Area 0 backbone, ABRs connect to Area 0 ✅ **How to configure** \- Different areas on different interfaces ✅ **How to verify** \- Check ABR status, inter-area routes (O IA) ✅ **Common mistakes** \- Direct area connections, missing Area 0 ✅ **Best practices** \- Redundant Area 0, strategic ABR placement **Next Step:** Multi-area OSPF needs security. Learn [**How to Configure OSPF Authentication**](https://www.pinglabz.com/ospf-authentication-cisco-configuration/) next. **Internal Links:** - ← [OSPF Areas Explained](https://www.pinglabz.com/ospf-areas-explained/) - ← [Interface-Based OSPF](https://www.pinglabz.com/interface-based-ospf-configuration/) - → [OSPF Authentication](https://www.pinglabz.com/ospf-authentication-cisco-configuration/) - → [OSPF Stub Areas](https://www.pinglabz.com/ospf-stub-area-configuration/) - → [OSPF Route Summarization](https://www.pinglabz.com/ospf-route-summarization-configuration/)