Inter-Area Summarization (ABRs)
Command: area [id] range [network] [mask]
Configuration:
router ospf 1
area 10 range 172.16.0.0 255.255.0.0
What it does:
- Summarizes all 172.16.x.x networks in Area 10
- ABR advertises single /16 into Area 0
- Blocks individual /24 LSAs
External Summarization (ASBRs)
Command: summary-address [network] [mask]
Configuration:
router ospf 1
summary-address 192.168.0.0 255.255.0.0
What it does:
- Summarizes all 192.168.x.x external routes
- ASBR advertises single /16 Type 5 LSA
Example Configuration
Topology
Area 0 --- R1 (ABR) --- Area 10 (172.16.1.0/24, 172.16.2.0/24, etc.)
R1 Configuration
router ospf 1
area 10 range 172.16.0.0 255.255.0.0
Result
Before:
O IA 172.16.1.0/24
O IA 172.16.2.0/24
O IA 172.16.3.0/24
After:
O IA 172.16.0.0/16 ← One route instead of many
Verification
R1# show ip ospf | include range
Area 10
It is an area border router
Area has range for advertisement: 172.16.0.0/16
Summary
✅ Inter-area — area X range on ABR
✅ External — summary-address on ASBR
✅ Benefits — Smaller tables, faster SPF
✅ Plan IP addressing for clean summarization
Next: OSPF Redistribution (Article 29)
Internal Links:
- ← SPF Algorithm (Article 27)
- ← OSPF Cost (Article 6)
- ← Multi-Area OSPF (Article 12)
- → OSPF Redistribution (Article 29)
