Cisco CCNP Enterprise (ENCOR 350-401) · lesson 2 of 12 · 30 min
Why Summarize OSPF Routes?
Route summarization (also called aggregation) is one of the most powerful tools in a network engineer’s toolkit. Instead of advertising 20 individual /24 subnets from Area 1 into Area 0, an ABR can advertise a single /20 summary that covers all of them. This reduces the number of LSAs in the backbone, shrinks routing tables on every router, and — critically — hides topology instability. If one /24 subnet flaps, routers in other areas never see the change because the summary stays stable. Imagine you’re giving someone directions to a neighborhood. Instead of listing every house address, you say “take Exit 42 for the Riverside District.” That’s summarization — one piece of information covers many destinations. If a single house gets renovated, the exit sign doesn’t change.
Inter-Area Summarization on ABRs
Inter-area summarization is configured on the ABR using the
area range
command. The ABR suppresses the individual Type 3 LSAs and generates a single summary Type 3 LSA instead.
The
area range
command is configured on the ABR in the direction of the area being summarized — not on routers inside the area. The summary is advertised into Area 0 (or the other area the ABR connects to).
External Route Summarization on ASBRs
When an ASBR redistributes external routes into OSPF, it can summarize them using the
summary-address
command. This reduces the number of Type 5 LSAs flooded throughout the domain.
OSPF Route Filtering Techniques
OSPF offers several ways to filter routes. Unlike distance-vector protocols, you cannot filter LSAs themselves (that would break the LSDB consistency guarantee), but you can filter which routes are installed into the routing table or which prefixes are advertised as Type 3 LSAs. Key distinction distribute-list filters routes from entering the routing table — the LSA still exists in the LSDB. area filter-list prevents the ABR from generating Type 3 LSAs — the prefix is truly hidden from the other area. Use area filter-list when you want to prevent a remote area from even knowing a prefix exists.
OSPF Default Route Injection
To inject a default route into OSPF, use
default-information originate
under the OSPF process. This generates a Type 5 LSA for 0.0.0.0/0.
E1 (metric-type 1) adds the internal OSPF cost to the external metric as the route propagates — the cost grows as it travels further from the ASBR. E2 (default) keeps the metric constant regardless of internal path cost. E1 is more accurate; E2 is simpler.
Worked Scenario: Summarization and Filtering
Scenario
Area 1 has subnets 10.1.0.0/24 through 10.1.15.0/24 (16 subnets). Area 2 should not see the 10.1.8.0/24 management subnet. The ASBR redistributes BGP prefixes in the 192.168.0.0/16 range.
Step 1 — Summarize Area 1 on the ABR:
area 1 range 10.1.0.0 255.255.240.0
— this covers all 16 /24s with a single /20 summary.
Step 2 — Filter the management subnet from Area 2: Create a prefix-list that permits 10.1.0.0/20 but denies 10.1.8.0/24. Apply it with
area 0 filter-list prefix MGMT-FILTER out
on the ABR facing Area 2.
Step 3 — Summarize external BGP routes on the ASBR:
summary-address 192.168.0.0 255.255.0.0
— reduces dozens of BGP prefixes to one Type 5 LSA.
Result: Area 0 sees one /20 for Area 1 and one /16 for external BGP. Area 2 sees the /20 but not the management /24. The LSDB is clean and stable.
Flashcard Drill
area range ABR command to summarize inter-area routes — suppresses individual Type 3 LSAs and generates one summary. summary-address ASBR command to summarize external routes — reduces Type 5 LSAs flooded domain-wide. area filter-list Filters Type 3 LSAs at the ABR — prevents a prefix from being advertised into or out of an area. distribute-list Filters routes from entering the routing table — LSA still exists in LSDB. default-information originate always Injects a default route into OSPF even if no default exists in the routing table. E1 vs E2 E1 adds internal OSPF cost to external metric (grows with distance). E2 keeps metric constant. E2 is the default. not-advertise Option on area range or summary-address that suppresses the summary entirely — prefix hidden from other areas. Type 3 LSA filtering Only possible at the ABR using area filter-list or area range not-advertise — cannot filter LSAs inside an area.
Knowledge Check — 12 Questions
OSPF Summarization & Filtering
Which command summarizes Area 1 routes on an ABR?
- summary-address
- area 1 range
- ip summary-address ospf
- redistribute summary
Answer: area 1 range
area range is the ABR command for inter-area summarization. summary-address is used on ASBRs for external routes.
What type of LSA does area range suppress and replace?
- Type 1
- Type 2
- Type 3
- Type 5
Answer: Type 3
area range suppresses individual Type 3 Summary LSAs and generates a single summary Type 3 LSA.
Which filtering method prevents a prefix from appearing in the LSDB of another area?
- distribute-list in
- area filter-list
- ip prefix-list deny
- passive-interface
Answer: area filter-list
area filter-list prevents the ABR from generating Type 3 LSAs for specific prefixes — the prefix never enters the other area’s LSDB.
A distribute-list applied to OSPF removes the LSA from the LSDB.
- True
- False
Answer: False
distribute-list only filters routes from being installed in the routing table. The LSA still exists in the LSDB.
What is the difference between E1 and E2 OSPF external metric types?
- E1 is for internal routes, E2 for external
- E1 adds internal cost to external metric; E2 keeps metric constant
- E1 uses bandwidth; E2 uses hop count
- E1 is for Type 5; E2 is for Type 7
Answer: E1 adds internal cost to external metric; E2 keeps metric constant
E1 accumulates internal OSPF cost as the route propagates — more accurate. E2 keeps the metric set at the ASBR constant — simpler but less precise.
To always inject a default route into OSPF regardless of routing table state, use: default-information originate _____.
Answer: always
The always keyword forces OSPF to advertise a default route even when no default route exists in the local routing table.
Which command summarizes external routes on an ASBR?
- area range
- summary-address
- aggregate-address
- ip summary-address eigrp
Answer: summary-address
summary-address under the OSPF process on an ASBR summarizes redistributed external routes into fewer Type 5 LSAs.
What does area 1 range 10.1.0.0 255.255.0.0 not-advertise do?
- Summarizes Area 1 routes into a /16
- Suppresses the summary — Area 1 prefixes are hidden from other areas
- Filters only the 10.1.0.0/16 prefix
- Converts Type 3 to Type 5
Answer: Suppresses the summary — Area 1 prefixes are hidden from other areas
not-advertise suppresses the summary LSA entirely — no Type 3 LSA is generated for this range, hiding all covered prefixes from other areas.
On which router is area range configured?
- Internal router
- ASBR
- ABR
- DR
Answer: ABR
area range is configured on the ABR — the router that sits between the area being summarized and the backbone.
Summarization hides individual subnet flaps from other areas, improving stability.
- True
- False
Answer: True
When a component subnet flaps, the summary LSA remains stable as long as at least one component route exists — other areas never see the instability.
Which OSPF filtering technique is applied per-neighbor to stop all LSA flooding?
- area filter-list
- distribute-list
- database-filter all out
- passive-interface
Answer: database-filter all out
database-filter all out prevents all LSAs from being sent to a specific neighbor — useful for hub-and-spoke where spokes don’t need full LSDB.
A summary covers 10.1.0.0/24 through 10.1.15.0/24. What is the correct summary prefix?
- 10.1.0.0/20
- 10.1.0.0/22
- 10.1.0.0/16
- 10.1.0.0/21
Answer: 10.1.0.0/20
16 /24 subnets require 4 bits of host space (2^4=16). A /24 minus 4 bits = /20. The summary is 10.1.0.0/20.
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