OSPF Multi-Area Configuration

In single-area OSPF, every router holds a complete copy of the Link State Database (LSDB) for the entire network. As the network grows, this creates problems — every topology change triggers an SPF recalculation on every router, LSDB memory consumption grows, and routing tables become large. Multi-area OSPF solves this by dividing the network into smaller, manageable areas. Each area maintains its own LSDB, runs SPF independently, and only summarised route information crosses area boundaries. For the conceptual overview see OSPF Overview.

The backbone area (Area 0) is the mandatory core — all other areas must connect to it. Routers that sit on the boundary between two areas are called ABRs (Area Border Routers). They maintain one LSDB per connected area and generate summary LSAs that carry route information between areas. This lab builds a three-area topology, configures ABRs, examines LSA types, and implements inter-area route summarization.

Before starting, complete OSPF Single-Area Configuration to understand Router IDs, Hello parameters, neighbor states, and the routing fundamentals that underpin OSPF.

1. Multi-Area OSPF — Key Concepts

Router Roles in Multi-Area OSPF

Role	Abbreviation	Description	LSDB Held
Internal Router	IR	All interfaces in a single area — only knows that area's topology	One area only
Area Border Router	ABR	Interfaces in two or more areas including Area 0 — connects non-backbone areas to the backbone	One LSDB per connected area
Backbone Router	BR	At least one interface in Area 0 — any router connected to the backbone	Area 0 LSDB (+ other areas if ABR)
AS Boundary Router	ASBR	Redistributes routes from outside OSPF (e.g., static routes, BGP) into the OSPF domain	One or more areas + external routes

Why Multi-Area Helps

Problem in Single Area	Multi-Area Solution
Every topology change triggers SPF on all routers	SPF runs per area — changes in Area 1 do not trigger SPF in Area 2
LSDB grows with every router and link added	Each area has a smaller independent LSDB
Routing tables grow with every network	ABRs can summarise many specific routes into one summary route
Instability in one part propagates everywhere	Instability is contained within its area — other areas only see summary changes

2. OSPF LSA Types

OSPF communicates topology and route information using Link State Advertisements (LSAs). Different LSA types carry different information and travel different distances in the network. Understanding LSA types is essential for the CCNA exam and for troubleshooting what routes appear where:

LSA Type	Name	Generated By	Describes	Scope	Route Code
Type 1	Router LSA	Every router	The router's own interfaces and their states	Stays within originating area	O
Type 2	Network LSA	DR on multi-access segments	All routers on a multi-access segment (e.g., Ethernet)	Stays within originating area	O
Type 3	Summary LSA	ABR	Networks from one area advertised into another area	Crosses area boundaries — flooded into adjacent areas	O IA
Type 4	ASBR Summary LSA	ABR	How to reach an ASBR in another area	Crosses area boundaries	—
Type 5	External LSA	ASBR	Routes redistributed from outside OSPF (static, BGP, EIGRP)	Entire OSPF domain (all areas)	O E1 / O E2
Type 7	NSSA External LSA	ASBR inside NSSA	External routes in a Not-So-Stubby Area — converted to Type 5 by ABR	Within NSSA only — converted at ABR	O N1 / O N2

CCNA exam focus: Know Types 1, 2, 3, and 5 thoroughly. Type 1 and 2 = intra-area (stay in the area). Type 3 = inter-area (crosses areas via ABR, shows as O IA in the routing table). Type 5 = external (entire domain, shows as O E2 or O E1).

Route Codes in show ip route

Code	Meaning	LSA Source	AD
O	OSPF intra-area route — same area as the router	Type 1 / Type 2	110
O IA	OSPF inter-area route — from a different area via ABR Type 3 LSA	Type 3	110
O E1	OSPF External Type 1 — metric increases as it crosses routers	Type 5	110
O E2	OSPF External Type 2 — metric stays fixed (default for redistribution)	Type 5	110
*OE2**	Default route redistributed via OSPF (Gateway of Last Resort)	Type 5	110

3. Lab Topology & IP Addressing

Three areas: Area 1 (left), Area 0 (backbone centre), Area 2 (right). R2 is the ABR between Area 1 and Area 0. R3 is the ABR between Area 0 and Area 2.

  ┌───────────────────────────────────────────────────────────────────────────┐
  │  Area 1                  Area 0 (Backbone)               Area 2          │
  │                                                                           │
  │  192.168.1.0/24    10.0.12.0/30   10.0.23.0/30    192.168.3.0/24        │
  │  192.168.2.0/24                                   192.168.4.0/24        │
  │       |                  |               |               |               │
  │    [R1]──Gi0/1────────[R2]──Gi0/1────[R3]──Gi0/1────[R4]               │
  │    RID:1.1.1.1  Gi0/0   RID:2.2.2.2  Gi0/0  RID:3.3.3.3  RID:4.4.4.4 │
  │                                                                           │
  │    R1: Area 1 only      R2: ABR       R3: ABR       R4: Area 2 only     │
  │    (Internal Router)  (Area 1+0)   (Area 0+2)   (Internal Router)      │
  └───────────────────────────────────────────────────────────────────────────┘

Device	Interface	IP Address	OSPF Area	Role
NetsTuts_R1	Gi0/0	192.168.1.1 /24	Area 1	LAN (passive)
NetsTuts_R1	Gi0/1	192.168.2.1 /24	Area 1	LAN (passive)
NetsTuts_R1	Gi0/2	10.0.12.1 /30	Area 1	Link to R2
NetsTuts_R2	Gi0/0	10.0.12.2 /30	Area 1	Link to R1 (ABR — Area 1 side)
NetsTuts_R2	Gi0/1	10.0.23.1 /30	Area 0	Link to R3 (ABR — Area 0 side)
NetsTuts_R3	Gi0/0	10.0.23.2 /30	Area 0	Link to R2 (ABR — Area 0 side)
NetsTuts_R3	Gi0/1	10.0.34.1 /30	Area 2	Link to R4 (ABR — Area 2 side)
NetsTuts_R4	Gi0/0	10.0.34.2 /30	Area 2	Link to R3
NetsTuts_R4	Gi0/1	192.168.3.1 /24	Area 2	LAN (passive)
NetsTuts_R4	Gi0/2	192.168.4.1 /24	Area 2	LAN (passive)

4. Step 1 — Configure R1 (Area 1 Internal Router)

R1 is a pure Area 1 internal router — all interfaces belong to Area 1. LAN interfaces are set passive; only the inter-router link to R2 participates in Hello exchange.

NetsTuts_R1>en
NetsTuts_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

NetsTuts_R1(config)#router ospf 1
NetsTuts_R1(config-router)#router-id 1.1.1.1
NetsTuts_R1(config-router)#auto-cost reference-bandwidth 1000

! ── All R1 interfaces go into Area 1 ─────────────────────
NetsTuts_R1(config-router)#network 192.168.1.0 0.0.0.255 area 1
NetsTuts_R1(config-router)#network 192.168.2.0 0.0.0.255 area 1
NetsTuts_R1(config-router)#network 10.0.12.0 0.0.0.3 area 1

! ── Suppress Hellos on LAN interfaces ────────────────────
NetsTuts_R1(config-router)#passive-interface GigabitEthernet0/0
NetsTuts_R1(config-router)#passive-interface GigabitEthernet0/1
NetsTuts_R1(config-router)#exit
NetsTuts_R1(config)#end
NetsTuts_R1#wr
Building configuration...
[OK]
NetsTuts_R1#

R1 advertises both LANs and its link to R2 — all in Area 1. It has no knowledge of Area 0 or Area 2. R2 (the ABR) will summarise Area 1 routes and inject them into Area 0 as Type 3 LSAs.

5. Step 2 — Configure R2 (ABR — Area 1 and Area 0)

R2 is the ABR — its Gi0/0 interface is in Area 1 and its Gi0/1 interface is in Area 0. The OSPF network command places each interface into its correct area. R2 will maintain two separate LSDBs and generate Type 3 Summary LSAs in both directions.

NetsTuts_R2>en
NetsTuts_R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

NetsTuts_R2(config)#router ospf 1
NetsTuts_R2(config-router)#router-id 2.2.2.2
NetsTuts_R2(config-router)#auto-cost reference-bandwidth 1000

! ── Gi0/0 faces Area 1 (toward R1) ───────────────────────
NetsTuts_R2(config-router)#network 10.0.12.0 0.0.0.3 area 1

! ── Gi0/1 faces Area 0 (toward R3) ───────────────────────
NetsTuts_R2(config-router)#network 10.0.23.0 0.0.0.3 area 0

NetsTuts_R2(config-router)#exit
NetsTuts_R2(config)#end
NetsTuts_R2#wr
Building configuration...
[OK]
NetsTuts_R2#
%OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

R2 places its two interfaces into different areas — this is what makes it an ABR. The adjacency with R1 (1.1.1.1) forms immediately once R1's OSPF is active. R2 will also form adjacency with R3 on the Area 0 side.

What an ABR Does Automatically

Action	Direction	LSA Type Generated
Advertises Area 1 networks into Area 0	Area 1 → Area 0	Type 3 Summary LSA (one per Area 1 prefix)
Advertises Area 0 and other area networks into Area 1	Area 0 → Area 1	Type 3 Summary LSA (one per remote prefix)
Maintains separate LSDB for each connected area	Both	Does not flood Type 1/2 LSAs between areas

6. Step 3 — Configure R3 (ABR — Area 0 and Area 2)

NetsTuts_R3>en
NetsTuts_R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

NetsTuts_R3(config)#router ospf 1
NetsTuts_R3(config-router)#router-id 3.3.3.3
NetsTuts_R3(config-router)#auto-cost reference-bandwidth 1000

! ── Gi0/0 faces Area 0 (toward R2) ───────────────────────
NetsTuts_R3(config-router)#network 10.0.23.0 0.0.0.3 area 0

! ── Gi0/1 faces Area 2 (toward R4) ───────────────────────
NetsTuts_R3(config-router)#network 10.0.34.0 0.0.0.3 area 2

NetsTuts_R3(config-router)#exit
NetsTuts_R3(config)#end
NetsTuts_R3#wr
Building configuration...
[OK]
NetsTuts_R3#
%OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

R3 mirrors R2's role on the other side — ABR between Area 0 and Area 2. Adjacency with R2 (2.2.2.2) forms on the Area 0 link. Once R4 is configured, R3 will also form adjacency on the Area 2 side.

7. Step 4 — Configure R4 (Area 2 Internal Router)

NetsTuts_R4>en
NetsTuts_R4#conf t
Enter configuration commands, one per line.  End with CNTL/Z.

NetsTuts_R4(config)#router ospf 1
NetsTuts_R4(config-router)#router-id 4.4.4.4
NetsTuts_R4(config-router)#auto-cost reference-bandwidth 1000

! ── All R4 interfaces go into Area 2 ─────────────────────
NetsTuts_R4(config-router)#network 192.168.3.0 0.0.0.255 area 2
NetsTuts_R4(config-router)#network 192.168.4.0 0.0.0.255 area 2
NetsTuts_R4(config-router)#network 10.0.34.0 0.0.0.3 area 2

! ── Suppress Hellos on LAN interfaces ────────────────────
NetsTuts_R4(config-router)#passive-interface GigabitEthernet0/1
NetsTuts_R4(config-router)#passive-interface GigabitEthernet0/2
NetsTuts_R4(config-router)#exit
NetsTuts_R4(config)#end
NetsTuts_R4#wr
Building configuration...
[OK]
NetsTuts_R4#
%OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

8. Step 5 — Inter-Area Route Summarization on the ABR

Without summarization, R2 generates one Type 3 Summary LSA for every specific network in Area 1 — so 192.168.1.0/24 and 192.168.2.0/24 each create a separate LSA flooding into Area 0 and Area 2. Summarization replaces these individual LSAs with a single summary covering both: 192.168.0.0/22 encompasses 192.168.0.x through 192.168.3.x. For a broader look at route summarization techniques, see Route Summarization & Aggregation.

For this topology, 192.168.1.0/24 and 192.168.2.0/24 can be summarized as 192.168.0.0/22 (covers .0/24 through .3/24). Summarization is configured on the ABR using area [source-area] range:

! ── On R2: Summarise Area 1 routes advertised into Area 0 ─
NetsTuts_R2(config)#router ospf 1
NetsTuts_R2(config-router)#area 1 range 192.168.0.0 255.255.252.0
NetsTuts_R2(config-router)#end
NetsTuts_R2#wr
Building configuration...
[OK]
NetsTuts_R2#

The command reads: "for routes from Area 1, replace any specific routes falling within 192.168.0.0/22 with a single summary LSA." Instead of two Type 3 LSAs (192.168.1.0/24 and 192.168.2.0/24), Area 0 and Area 2 now receive one Type 3 LSA for 192.168.0.0/22.

Summarize Area 2 on R3 as well

! ── On R3: Summarise Area 2 routes advertised into Area 0 ─
NetsTuts_R3(config)#router ospf 1
NetsTuts_R3(config-router)#area 2 range 192.168.2.0 255.255.254.0
NetsTuts_R3(config-router)#end
NetsTuts_R3#wr
Building configuration...
[OK]
NetsTuts_R3#

192.168.3.0/24 and 192.168.4.0/24 are summarized as 192.168.2.0/23 (covers .2.x and .3.x). Area 0 and Area 1 receive one summary instead of two specific routes.

area range Command Breakdown

Element	Example	Meaning
`area 1`	`area 1 range ...`	The source area whose routes will be summarised
`range [network] [mask]`	`range 192.168.0.0 255.255.252.0`	The summary address and mask — all specific routes that fall within this range are suppressed and replaced by this single entry
`not-advertise` (optional)	`area 1 range 192.168.0.0 255.255.252.0 not-advertise`	Suppresses the summary entirely — useful to hide a set of networks from other areas
`cost [value]` (optional)	`area 1 range 192.168.0.0 255.255.252.0 cost 10`	Manually sets the metric of the summary LSA instead of using the highest component cost

Summarization only works on the ABR. The area range command must be configured on the ABR that connects the source area to Area 0. Configuring it on an internal router has no effect. Also, the summary must accurately cover all the specific routes — a mis-sized summary can cause routing black holes for addresses in the summary range that do not actually exist.

9. Verification

show ip ospf neighbor — All Adjacencies (R2)

NetsTuts_R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:38    10.0.12.1       GigabitEthernet0/0
3.3.3.3           1   FULL/DR         00:00:32    10.0.23.2       GigabitEthernet0/1

R2 (the ABR) has two neighbors — R1 (1.1.1.1) on Area 1 and R3 (3.3.3.3) on Area 0. Both are in FULL state, confirming both adjacencies are complete.

show ip route ospf — R1 sees Inter-Area Routes (O IA)

NetsTuts_R1#show ip route ospf
Codes: O - OSPF, IA - Inter Area

O IA  10.0.23.0/30 [110/2] via 10.0.12.2, GigabitEthernet0/2
O IA  10.0.34.0/30 [110/3] via 10.0.12.2, GigabitEthernet0/2
O IA  192.168.2.0/23 [110/3] via 10.0.12.2, GigabitEthernet0/2

R1 (Area 1 internal router) sees three inter-area routes — all marked O IA. These were generated as Type 3 Summary LSAs by R2 (ABR) from Area 0 information. Notice the Area 2 routes appear as a single summary (192.168.2.0/23) rather than two specific /24 routes — R3's summarization is working.

show ip route ospf — R4 sees Summarized Area 1 (O IA)

NetsTuts_R4#show ip route ospf
Codes: O - OSPF, IA - Inter Area

O IA  10.0.12.0/30 [110/3] via 10.0.34.1, GigabitEthernet0/0
O IA  10.0.23.0/30 [110/2] via 10.0.34.1, GigabitEthernet0/0
O IA  192.168.0.0/22 [110/3] via 10.0.34.1, GigabitEthernet0/0

R4 (Area 2 internal router) receives Area 1's two /24 LANs as a single 192.168.0.0/22 summary — R2's summarization is working. Without summarization, R4 would have separate O IA entries for 192.168.1.0/24 and 192.168.2.0/24.

show ip ospf database — Multiple LSA Types (R2 ABR)

NetsTuts_R2#show ip ospf database

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 1)
Link ID         ADV Router      Age    Seq#       Checksum  Link count
1.1.1.1         1.1.1.1         312    0x80000005 0x00A312  3
2.2.2.2         2.2.2.2         305    0x80000004 0x00B213  1

                Summary Net Link States (Area 1)
Link ID         ADV Router      Age    Seq#       Checksum
10.0.23.0       2.2.2.2         300    0x80000002 0x00C120
192.168.2.0     2.2.2.2         298    0x80000002 0x00D234

                Router Link States (Area 0)
Link ID         ADV Router      Age    Seq#       Checksum  Link count
2.2.2.2         2.2.2.2         305    0x80000003 0x00E312  1
3.3.3.3         3.3.3.3         301    0x80000003 0x00F214  1

                Summary Net Link States (Area 0)
Link ID         ADV Router      Age    Seq#       Checksum
192.168.0.0     2.2.2.2         295    0x80000002 0x00A312
192.168.2.0     3.3.3.3         292    0x80000002 0x00B123

As an ABR, R2 holds two separate LSDBs — one for Area 1 and one for Area 0. The "Summary Net Link States" sections contain the Type 3 LSAs that R2 is generating and flooding between areas. Note the summary addresses (192.168.0.0 in Area 0) confirming summarization is active.

show ip ospf — Confirm ABR Role

NetsTuts_R2#show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 ...
 It is an area border router
 Number of areas in this router is 2. 2 normal 0 stub 0 nssa
 ...
    Area BACKBONE(0)
        Number of interfaces in this area is 1 (0 passive)
    Area 1
        Number of interfaces in this area is 1 (0 passive)

"It is an area border router" — IOS explicitly identifies R2 as an ABR. "Number of areas in this router is 2" confirms R2 participates in both Area 0 and Area 1.

Verification Command Summary

Command	What It Confirms	Run On
`show ip ospf neighbor`	All adjacencies and their state — first check for multi-area	All routers
`show ip route ospf`	O = intra-area, O IA = inter-area. Verify summary routes appear instead of specific routes	Internal routers (R1, R4)
`show ip ospf database`	Full LSDB — ABR shows separate sections for each area. Confirms Type 3 LSAs exist	ABRs (R2, R3)
`show ip ospf`	"It is an area border router" — confirms ABR role and number of areas	ABRs (R2, R3)
`show ip ospf interface brief`	Per-interface area assignment — confirm each interface is in the correct area	All routers
`show ip ospf database summary`	Only Type 3 Summary LSAs — useful to confirm summarization is producing one entry	ABRs (R2, R3)

10. Troubleshooting Multi-Area OSPF

Problem	Symptom	Cause	Fix
Non-backbone area not connected to Area 0	Routes from Area 1 never appear in Area 2 routing tables — O IA routes missing	Area 1 is not connected to Area 0 through an ABR — areas must all touch Area 0	Ensure the ABR has one interface in Area 0 and one in the non-backbone area. Use virtual links as a temporary fix if direct connection is impossible.
Interface placed in wrong area	Adjacency forms but routes appear in wrong area — O routes instead of O IA or vice versa	`network` statement put the interface into the incorrect area	Check `show ip ospf interface brief` on the ABR — verify each interface shows the correct area. Fix the `network` statement.
Summarization not working	Remote routers still see individual /24 routes instead of summary	`area range` command configured on wrong router (not ABR) or the summary range does not cover the specific routes	Verify `area range` is on the ABR only. Check that the summary address and mask mathematically cover all specific routes using subnetting. Use `show ip ospf database summary` to confirm the summary LSA exists.
Routing black hole after summarization	Traffic to some addresses in the summary range is dropped	The summary range covers addresses that do not actually exist — packets match the summary but no specific route exists at the ABR for the actual destination	IOS automatically installs a Null0 discard route for the summary on the ABR to prevent routing loops. Verify the summary range only covers existing networks.
O IA routes missing on internal router	Internal router sees intra-area routes but no inter-area routes from other areas	ABR is not forming adjacency on one side — check both of the ABR's neighbor relationships	Run `show ip ospf neighbor` on the ABR — both adjacencies must be FULL. A missing adjacency on one side means no Type 3 LSAs can be generated for that direction.
Duplicate Router IDs across areas	Unstable adjacencies — log shows "Duplicate Router ID" error	Two routers in the OSPF domain (even in different areas) have the same RID	Router IDs must be unique across the entire OSPF domain — not just within an area. Change the duplicate with `router-id` and run `clear ip ospf process`.

Key Points & Exam Tips

All OSPF areas must connect to Area 0 (the backbone). Traffic between two non-backbone areas must pass through Area 0 via ABRs — there are no direct inter-area shortcuts.
An ABR has interfaces in two or more areas, including Area 0. It maintains a separate LSDB for each connected area and generates Type 3 Summary LSAs to carry route information between areas.
Type 1 and Type 2 LSAs never cross area boundaries — they stay within the originating area. Only Type 3, 4, and 5 LSAs cross boundaries.
Routes from the same area appear as O (intra-area). Routes from a different area appear as O IA (inter-area) — generated by Type 3 LSAs from the ABR.
Routes redistributed from outside OSPF appear as O E2 (default) or O E1 — generated by Type 5 LSAs from an ASBR.
Summarization is configured on the ABR with area [source-area] range [network] [mask]. It reduces LSA flooding, shrinks routing tables, and contains instability within areas. Understanding subnet masks is essential for calculating correct summary ranges.
IOS automatically creates a Null0 discard route on the ABR for each configured summary — this prevents routing loops when the summary range includes non-existent addresses.
show ip ospf on an ABR explicitly states "It is an area border router" — use this to quickly confirm the ABR role is active.
Router IDs must be unique across the entire OSPF domain — not just within an area. Duplicate RIDs cause instability even across area boundaries.
The CCNA exam distinguishes O, O IA, O E1, and O E2 — know which LSA type generates each code and in which direction LSAs flow between areas.

Next Steps: With multi-area OSPF carrying routes across the network, continue to DHCP Server Configuration to provide dynamic addressing to hosts on OSPF-routed subnets. For connecting the OSPF domain to the internet with a default route, revisit Static Route Configuration and the default-information originate command. For MPLS backbones that rely on OSPF as the underlying IGP, see MPLS Fundamentals. For comparing OSPF with another link-state protocol, see OSPF Single-Area Configuration and the OSPF Overview.

TEST WHAT YOU LEARNED

1. Area 2 is directly connected to Area 1 through an ABR but has no connection to Area 0. R4 in Area 2 tries to learn routes from Area 1. What will happen?

A. Routes flow normally — OSPF areas only require connectivity, not specifically through Area 0 B. Routes flow but are marked O IA with a higher administrative distance C. Routes do not propagate correctly — all non-backbone areas must connect to Area 0. Without a path through Area 0, inter-area routing between Area 1 and Area 2 fails D. IOS automatically creates a virtual link between Area 1 and Area 2

Correct answer is C. OSPF's fundamental design rule is that all inter-area traffic must flow through Area 0. If Area 2 connects only to Area 1 (and not to Area 0), OSPF has no valid path for Type 3 LSAs to travel — inter-area routes are not installed. This is by design to prevent routing loops that could occur if areas exchanged routes directly without going through a central backbone. The fix is to connect Area 2 to Area 0, or use a virtual link through Area 1 as a temporary solution.

2. What is the key difference between a Type 1 LSA and a Type 3 LSA?

A. Type 1 (Router LSA) describes a router's own interfaces and stays within its originating area. Type 3 (Summary LSA) is generated by an ABR to carry network prefixes from one area into another — it crosses area boundaries B. Type 1 is generated by internal routers and Type 3 is generated by external routers injecting routes into OSPF C. Type 1 floods the entire OSPF domain and Type 3 stays within the originating area D. They are identical — the number just reflects the order in which LSAs are processed

Correct answer is A. Type 1 (Router LSA) is generated by every router and describes its own links and states. It is flooded only within the area where it originates — it never crosses an area boundary. Type 3 (Summary LSA) is specifically generated by ABRs to carry prefix information between areas. When an ABR receives Type 1 LSAs from Area 1, it does not forward them to Area 0 — instead it creates new Type 3 LSAs summarizing those prefixes and floods them into Area 0.

3. R1 (Area 1 internal router) runs `show ip route ospf` and sees routes marked O IA. What does this tell you about where those routes came from?

A. The routes were learned from another router inside Area 1 B. The routes were redistributed from a non-OSPF protocol into OSPF C. The routes were learned from a static route configured on an ABR D. The routes originated in a different OSPF area and were delivered to Area 1 via a Type 3 Summary LSA generated by the ABR

Correct answer is D. O IA (OSPF Inter-Area) routes are always the result of Type 3 Summary LSAs generated by an ABR. When R2 (ABR) receives Type 1 LSAs from Area 0 or Area 2, it creates Type 3 LSAs and floods them into Area 1. R1 installs these as O IA routes — it knows the destination network but only has the ABR (R2) as its next hop, not the full topology of the remote area.

4. An engineer configures `area 1 range 192.168.0.0 255.255.252.0` on R1, which is an internal router in Area 1. Will summarization work?

A. Yes — any router can perform inter-area summarization with the area range command B. No — the area range command only takes effect on an ABR. On an internal router with all interfaces in one area, the command is accepted but has no effect on LSA generation C. No — the command syntax is wrong, it should use a wildcard mask D. Yes — but only for routes originating from R1 itself

Correct answer is B. IOS accepts the area range command on any router, but it only has functional meaning on an ABR — a router with interfaces in multiple OSPF areas. An ABR is the entity that generates Type 3 Summary LSAs for routes crossing from one area into another. Since R1 has all interfaces in Area 1 only, it never generates inter-area Type 3 LSAs and the command has no effect. Always configure area range on the ABR.

5. After configuring summarization on R2 (ABR), an automatic route to 192.168.0.0/22 via Null0 appears in R2's routing table. Why does IOS create this?

A. It is an error — the Null0 route should be removed manually B. It serves as a default route for traffic that does not match any more specific route in R2's table C. It is a discard route that prevents routing loops — if a packet matches the summary 192.168.0.0/22 but no specific /24 route exists for that destination on R2, the packet is dropped rather than bouncing back toward the ABR and looping D. It is a placeholder that IOS removes once all summarized routes are learned

Correct answer is C. Without the Null0 discard route, consider this scenario: R4 sends a packet for 192.168.0.50 (which is in the summary range but no actual host exists). R4 matches the summary O IA route and sends it to R2. R2 has no specific /24 route for 192.168.0.50 either — so it would send it back toward R4 matching the same summary, creating a routing loop. The Null0 route breaks this loop by dropping the packet at R2 when no specific route exists.

6. Which command on R2 confirms it is functioning as an ABR with interfaces in two areas?

A. show ip ospf neighbor B. show ip route ospf C. show ip ospf interface brief D. show ip ospf — it explicitly states "It is an area border router" and shows "Number of areas in this router is 2"

Correct answer is D. show ip ospf is the definitive command to confirm the ABR role — IOS explicitly states "It is an area border router" in the output. It also shows the number of areas the router participates in. While show ip ospf interface brief shows per-interface area assignments (useful to see which interface is in which area), only show ip ospf explicitly declares the router's role in the OSPF hierarchy.

7. R4 (Area 2 internal router) receives a route marked O E2 in its routing table. What does this indicate?

A. The route was redistributed from outside the OSPF domain (e.g., a static route or BGP route) by an ASBR and carried as a Type 5 External LSA throughout the OSPF domain. The E2 metric stays fixed regardless of how many routers the route crosses B. The route is an inter-area route from Area 2 learned by R4 via the ABR C. The route is a summary generated by the ABR with a cost of 2 D. The route was learned from a second OSPF process running on the same router

Correct answer is A. O E2 means OSPF External Type 2 — a route that originated outside the OSPF domain and was injected by an ASBR via a Type 5 External LSA. Type 5 LSAs flood the entire OSPF domain (all areas). E2 routes keep the same metric (the metric set at the redistribution point) regardless of internal OSPF path cost — the metric does not increase as the route travels through more routers. E1 routes, by contrast, accumulate internal path cost on top of the external metric.

8. Area 1 has routes 192.168.1.0/24 and 192.168.2.0/24. An engineer wants to summarize them on R2 (ABR) into a single route. Which summary address correctly covers both?

A. 192.168.0.0/23 — covers 192.168.0.x and 192.168.1.x only, missing 192.168.2.0/24 B. 192.168.0.0/22 — covers 192.168.0.x through 192.168.3.x, which includes both 192.168.1.0/24 and 192.168.2.0/24 C. 192.168.1.0/23 — covers 192.168.1.x and 192.168.2.x exactly with no wasted space D. 192.168.0.0/24 — a single /24 can summarize multiple /24 routes

Correct answer is B. To summarize correctly you need a supernet that covers both ranges. 192.168.0.0/22 uses a 22-bit mask (255.255.252.0) which covers 192.168.0.0 through 192.168.3.255 — this includes both 192.168.1.0/24 and 192.168.2.0/24. Option C (192.168.1.0/23) would actually also work mathematically (covers .1.x and .2.x), but 192.168.0.0/22 is the more common answer because it aligns on a /22 boundary. Option A misses 192.168.2.0/24. Option D is incorrect — a /24 cannot summarize other /24s.

9. How many Link State Databases does an ABR with interfaces in Area 0 and Area 1 maintain?

A. One — OSPF merges all area information into a single global LSDB on the ABR B. One for the backbone only — non-backbone area LSDB is discarded after Type 3 LSAs are generated C. Three — one for each area plus one global summary database D. Two — one LSDB for Area 0 and one for Area 1, each kept completely separate. The ABR runs SPF independently for each area

Correct answer is D. An ABR maintains a completely independent LSDB for every area it participates in. With interfaces in Area 0 and Area 1, R2 holds two LSDBs — the Area 0 topology database and the Area 1 topology database. Type 1 and Type 2 LSAs from Area 1 never enter the Area 0 LSDB and vice versa. The ABR runs the SPF algorithm separately for each area. This separation is what contains topology changes within their area and reduces SPF computation on remote routers.

10. R2 is the ABR between Area 1 and Area 0. R3 is the ABR between Area 0 and Area 2. R3 loses its adjacency with R2 (the Area 0 link goes down). What happens to O IA routes on R4 in Area 2?

A. O IA routes from Area 1 are retained — OSPF holds them in the table for 60 seconds before removing them B. O IA routes from Area 1 are removed from R4's routing table — without an adjacency to Area 0, R3 cannot receive Type 3 LSAs from R2 and therefore cannot generate them for Area 2 C. O IA routes remain — they were already in the LSDB and OSPF does not remove installed routes until a full SPF recalculation D. O IA routes convert to O routes since R4 now has a direct path through Area 2

Correct answer is B. When R3 loses its Area 0 adjacency with R2, R3 can no longer receive Type 3 LSAs from R2 (which carry Area 1 route information). R3's Area 0 LSDB ages out these LSAs (LSA max age is 3600 seconds, but the neighbor going down triggers immediate LSA withdrawal). Without valid Type 3 LSAs for Area 1 networks, R3 stops generating them into Area 2, and R4's O IA routes for Area 1 networks are removed from the routing table during the next SPF run.

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