OSPF Area Types & Where Filtering Works

OSPF area types control which LSAs may exist in an area – to reduce routing-table size, LSA flooding and to improve stability and performance.

Area type overview

Area type	Allowed LSAs	External routes	Default route	ASBR in area	Cisco command
Normal area	1, 2, 3, 4, 5	✔️	❌	✔️	(default)
Stub area	1, 2, 3	❌ (type 5 blocked)	✔️	❌	`area X stub`
Totally stub (Cisco)	1, 2	❌	✔️	❌	`area X stub no-summary`
NSSA	1, 2, 3, 7	✔️ (type 7)	✔️	✔️	`area X nssa`
Totally NSSA (Cisco)	1, 2, 7	✔️ (type 7)	✔️	✔️	`area X nssa no-summary`

Stub area

A stub area does not allow external routes: it blocks Type 5 LSAs and receives one default route (0.0.0.0) from the ABR. Allowed: types 1, 2, 3. Not allowed: type 5.

✅ Smaller routing table, less flooding, faster convergence, simple design
❌ No external routes, only one way out, unsuitable if the area has an ASBR

router ospf 1
 area 1 stub

All routers in the area MUST match.

NSSA (Not-So-Stubby Area)

A stub area with an exception: it allows an ASBR inside; external routes are injected as Type 7 LSAs, and the ABR translates Type 7 → Type 5 toward the backbone.

✅ Less flooding, allows local redistribution, more flexible than stub
❌ More complex, still limited, requires correct type 7→5 translation

router ospf 1
 area 2 nssa

Totally stubby & totally NSSA (Cisco proprietary)

Totally stubby: blocks types 3, 4 and 5 – only the default route: area 1 stub no-summary
Totally NSSA: blocks type 3, allows type 7: area 2 nssa no-summary

⚠️ Cisco-only – not standard OSPF.

When to use what?

Situation	Area type
Small site without external routes	Stub
Site with local internet/MPLS	NSSA
Very small site	Totally stub
Strict routing control	Totally NSSA

Exam one-liners ⚠️

Stub: no Type 5, default route only
NSSA: Type 7 inside, Type 5 outside
Totally stub: only default route
NSSA ≠ Stub because NSSA allows an ASBR

Typical faults

Area type not matching on all routers
ASBR placed in a stub (wrong)
Expecting Type 5 in stub/NSSA
Forgetting no-summary on the ABR

OSPF filtering & summarization cheat sheet

1. ABR – prefix-list (area filtering) — filters inter-area routes (Type 3):

ip prefix-list BLOCK seq 5 deny 10.0.0.0/8
ip prefix-list BLOCK seq 10 permit 0.0.0.0/0 le 32
router ospf 1
 area 1 filter-list prefix BLOCK in

2. Router – distribute-list — filters only the routing table (RIB), NOT the LSDB:

ip access-list standard BLOCK
 deny 10.0.0.0 0.255.255.255
 permit any
router ospf 1
 distribute-list BLOCK in

3. ASBR – route-map (redistribution filtering) — filters external routes (Type 5):

ip access-list standard REDIST
 permit 192.168.0.0 0.0.255.255
route-map FILTER permit 10
 match ip address REDIST
router ospf 1
 redistribute static subnets route-map FILTER

4. ABR – summarization (inter-area):

router ospf 1
 area 1 range 10.0.0.0 255.0.0.0

5. ASBR – summarization (external):

router ospf 1
 summary-address 192.168.0.0 255.255.0.0

What you MUST remember (exam!)

ABR = area filter-list / area range
ASBR = route-map / summary-address
Distribute-list = local filtering only (the trap!)
Missing subnets = routes don’t show up
OSPF does not filter the LSDB directly → only indirect methods

5-second rule: ABR → areas · ASBR → external · route-map → redistribution · prefix-list → ABR filtering · distribute-list → RIB only.

Tagged ccnp, ospf