9.2 OSPF LSA Types & Multi-Area Design

9.2 OSPF LSA Types & Multi-Area Design

In a single-area OSPF network, every router maintains a detailed copy of all link states, requiring significant memory and processing power as the topology expands. To scale enterprise routing domains, OSPF partitions the network into multiple areas. Link-State Advertisements (LSAs) are categorized into specific functional types, each with defined boundaries, flooding scopes, and aggregation behaviors.

+-----------------------------------------------------------------------------------+
|                         OSPF LSA TYPES MASTER OVERVIEW                            |
+-----------------------------------------------------------------------------------+
|  LSA Type 1: Router LSA           - Generated by ALL routers; intra-area scope.   |
|  LSA Type 2: Network LSA          - Generated by DR on multi-access; intra-area.  |
|  LSA Type 3: Summary LSA          - Generated by ABRs; advertises inter-area nets.|
|  LSA Type 4: ASBR Summary LSA     - Generated by ABRs; advertises ASBR host route.|
|  LSA Type 5: AS External LSA      - Generated by ASBRs; domain-wide external nets.|
|  LSA Type 7: NSSA External LSA    - Generated by ASBRs in NSSA; NSSA-only scope.  |
+-----------------------------------------------------------------------------------+

1. OSPF LSA Types & Flooding Scopes

Every LSA serves a specific purpose in building the topological map or advertising reachable subnets across area boundaries.

+-----------------------------------------------------------------------------------+
|                       LSA GENERATION & BOUNDARY PROPAGATION                       |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|    [ STANDARD AREA 10 ]               [ BACKBONE AREA 0 ]        [ EXTERNAL AS ]  |
|  +--------------------+             +--------------------+      +---------------+ |
|  | Internal Router 1  |             |  Backbone Router   |      |  BGP / EIGRP  | |
|  | - Generates Type 1 |             | - Generates Type 1 |      |    Domain     | |
|  +--------------------+             +--------------------+      +---------------+ |
|           |                                  |                          |         |
|      (Type 1 & 2)                       (Type 1 & 2)                    |         |
|           v                                  v                          v         |
|    +------------------+             +------------------+      +-----------------+ |
|    |   ABR (Area 10)  |<============|   ABR (Area 20)  |<=====|  ASBR (Area 20) | |
|    | - Floods Type 3  |  (Type 3)   | - Floods Type 4  |      | - Injects Type 5| |
|    +------------------+             +------------------+      +-----------------+ |
+-----------------------------------------------------------------------------------+

Comprehensive LSA Catalog

1. Type 1: Router LSA:

   • Originator: Generated by every OSPF router for each area in which it has active interfaces.
   • Flooding Scope: Flooded exclusively within the originating area. Never crosses an ABR.
   • Link-State ID: The Router ID (RID) of the originating router.
   • Contents: Lists all active interfaces in that area, link types (Point-to-Point, Transit, Stub, Virtual Link), neighbor IDs, and assigned link costs.

2. Type 2: Network LSA:

   • Originator: Generated by the Designated Router (DR) on multi-access broadcast and NBMA links.
   • Flooding Scope: Flooded exclusively within the originating area. Never crosses an ABR.
   • Link-State ID: The IP address of the DR's interface on that multi-access link.
   • Contents: Lists the network subnet mask and the Router IDs of all routers currently attached to and fully adjacent with the DR on that segment.

3. Type 3: Summary LSA (Net-Summary):

   • Originator: Generated by Area Border Routers (ABRs).
   • Flooding Scope: Flooded across Area 0 and into other non-backbone areas. Regenerated by downstream ABRs.
   • Link-State ID: The destination network prefix / IP address (e.g., 10.10.0.0).
   • Contents: Advertises inter-area network prefixes learned from one area into another area, along with the ABR's metric cost to reach that prefix. (Note: Does not contain internal topology details—operates like distance-vector between areas).

4. Type 4: ASBR Summary LSA:

   • Originator: Generated by ABRs.
   • Flooding Scope: Flooded across Area 0 and into all standard non-backbone areas.
   • Link-State ID: The Router ID of the ASBR.
   • Contents: Informs routers in other areas how to reach the ASBR. Contains the host route metric from the ABR to the ASBR.

5. Type 5: AS External LSA:

   • Originator: Generated by Autonomous System Boundary Routers (ASBRs).
   • Flooding Scope: Flooded throughout the entire OSPF domain (all standard areas and Area 0). Blocked from entering all Stub and NSSA areas.
   • Link-State ID: The external destination network prefix.
   • Metric Types:
     • External Type 1 (E1): Metric equals the external redistribution cost plus the cumulative internal OSPF path cost to reach the ASBR. Used when multiple ASBR paths exist.
     • External Type 2 (E2 - Default): Metric is fixed at the external redistribution cost (internal OSPF cost to the ASBR is not added). Used when a single exit path exists.

6. Type 7: NSSA External LSA:

   • Originator: Generated by ASBRs located within a Not-So-Stubby Area (NSSA).
   • Flooding Scope: Flooded only within the originating NSSA. Never enters Area 0 directly.
   • Link-State ID: The external destination network prefix.
   • Contents: Transports external redistributed routes within an NSSA. Converted to Type 5 LSAs at the NSSA ABR before propagation into Area 0.

2. OSPF Special Area Types

In large networks, leaf routers (such as branch office or edge distribution switches) do not need visibility into thousands of external routes or inter-area prefixes. Cisco OSPF supports four specialized area types to reduce LSDB size and memory consumption.

+-----------------------------------------------------------------------------------+
|                         OSPF SPECIAL AREA CLASSIFICATIONS                         |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|  1. STANDARD AREA:                                                                |
|     - Allows all LSAs: Type 1, 2, 3, 4, 5. Full LSDB database.                    |
|                                                                                   |
|  2. STUB AREA:                                                                    |
|     - Blocks Type 4 and Type 5 LSAs (No external routes).                         |
|     - ABR injects a Type 3 Summary Default Route (0.0.0.0/0).                     |
|     - Command: 'area <id> stub' (Must be configured on ALL routers in area).      |
|                                                                                   |
|  3. TOTALLY STUBBY AREA (Cisco Proprietary):                                      |
|     - Blocks Type 3, Type 4, and Type 5 LSAs.                                     |
|     - Allows only Type 1 and 2 LSAs (intra-area) + Single Type 3 Default Route.   |
|     - Command: 'area <id> stub no-summary' on ABR; 'area <id> stub' on internals. |
|                                                                                   |
|  4. NOT-SO-STUBBY AREA (NSSA - RFC 3101):                                         |
|     - Blocks Type 4 and Type 5 LSAs from external areas.                          |
|     - Permits local ASBR to redistribute external routes as Type 7 LSAs.          |
|     - ABR translates Type 7 to Type 5 for Area 0.                                 |
|     - Command: 'area <id> nssa' (Must be configured on ALL routers in area).      |
|                                                                                   |
|  5. TOTALLY NSSA (Cisco Proprietary):                                             |
|     - Blocks Type 3, 4, and 5 LSAs; allows Type 7 external routes.                |
|     - ABR automatically injects Type 3 Default Route (0.0.0.0/0).                 |
|     - Command: 'area <id> nssa no-summary' on ABR; 'area <id> nssa' on internals. |
+-----------------------------------------------------------------------------------+

Detailed Area Comparison Matrix

3. LSA Type 7-to-5 Translation & Forwarding Address (FA)

Because Type 7 LSAs cannot leave an NSSA, an NSSA Area Border Router must translate eligible Type 7 LSAs into standard Type 5 External LSAs before propagating them across Area 0.

+-----------------------------------------------------------------------------------+
|                         LSA 7-TO-5 TRANSLATION WORKFLOW                           |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|   [ ASBR in NSSA ]                                                                |
|   - Redistributes static/BGP route into NSSA Area 30.                             |
|   - Generates Type 7 LSA with P-bit (Propagate) = 1.                              |
|                               |                                                   |
|                               v (Type 7 LSA)                                      |
|   +-----------------------------------------------+                               |
|   |             NSSA ABR 1 (RID: 1.1.1.1)         |                               |
|   +-----------------------------------------------+                               |
|   |             NSSA ABR 2 (RID: 2.2.2.2)         | <--- Elected 7-to-5 Translator|
|   +-----------------------------------------------+      (Highest Router ID)      |
|                               |                                                   |
|                               v (Type 5 LSA)                                      |
|   [ BACKBONE AREA 0 & STANDARD AREAS ]                                            |
|   - Receives standard Type 5 LSA. Routers forward to Forwarding Address (FA).     |
+-----------------------------------------------------------------------------------+

Key Mechanics of 7-to-5 Translation

1. Translator Election: If multiple ABRs connect the NSSA to Area 0, only one ABR performs the translation to prevent duplicate Type 5 LSAs. The ABR with the highest Router ID is elected as the translator.
2. The P-Bit (Propagate Bit): In the Type 7 LSA header, the originating ASBR sets the P-bit (Propagate bit) to 1. When the elected NSSA ABR receives the LSA, it verifies that P=1, translates the LSA to Type 5, and clears the P-bit. (If an NSSA ABR generates a default Type 7 LSA, it sets P=0, ensuring it is never translated to Type 5).
3. Forwarding Address (FA): The Type 7 LSA contains a Forwarding Address (FA) field. During translation, the ABR preserves the non-zero FA in the Type 5 LSA header. Routers in Area 0 and other standard areas compute their SPF path directly toward the FA rather than routing toward the translator ABR, avoiding suboptimal triangular routing.

4. OSPF Route Summarization & Loop Prevention

Because OSPF routers inside an area must maintain identical link-state databases, route summarization cannot be performed within an area. Summarization is strictly confined to area boundaries on ABRs or redistribution boundaries on ASBRs.

+-----------------------------------------------------------------------------------+
|                         OSPF ROUTE SUMMARIZATION BOUNDARIES                       |
+-----------------------------------------------------------------------------------+
|                                                                                   |
|  1. INTER-AREA SUMMARIZATION (ON ABR):                                            |
|     - Summarizes Type 1 and Type 2 LSAs from an area into a single Type 3 LSA.    |
|     - Command: 'area <area-id> range <ip-prefix> <subnet-mask>'                   |
|                                                                                   |
|  2. EXTERNAL ROUTE SUMMARIZATION (ON ASBR):                                       |
|     - Summarizes redistributed external routes into a single Type 5 or 7 LSA.     |
|     - Command: 'summary-address <ip-prefix> <subnet-mask>'                        |
|                                                                                   |
|  3. AUTOMATIC DISCARD ROUTE (NULL0):                                              |
|     - ABRs and ASBRs automatically install a route to 'Null0' for the summary.   |
|     - Prevents routing loops if packets arrive for unallocated subnets.           |
+-----------------------------------------------------------------------------------+

1. Inter-Area Summarization on ABR (area range)

• Configured under the OSPF process on an ABR to consolidate contiguous subnets from a specific area before advertising them into other areas.
• Metric Calculation: By default, the metric of the summarized Type 3 LSA equals the lowest (or highest depending on platform, default is lowest in modern IOS-XE) metric among the contributing component subnets, or can be explicitly assigned using the cost <value> parameter.
• Prefix Suppression: Appending the not-advertise keyword suppresses the summary and all constituent subnets, effectively filtering those prefixes from entering other areas.

2. External Summarization on ASBR (summary-address)

• Configured on an ASBR to aggregate external redistributed routes (from BGP, EIGRP, or static routes) before injecting them as a single Type 5 or Type 7 LSA.

3. Loop Prevention via Discard Route to Null0

When a router summarizes routes, it creates an aggregate prefix covering subnets that may not physically exist in the network. If an external host sends traffic destined for an unassigned IP within that summary range, the packet would normally match a default route and be routed back into the network, creating a continuous forwarding loop until TTL expires.

To prevent this, OSPF automatically installs a Discard Route in the local routing table pointing to interface Null0 with an administrative distance of 110 (e.g., 10.1.0.0/16 is a summary, Null0). Packets matching the summary that do not have a more specific route entry are silently dropped at the ABR/ASBR.

5. Cisco IOS-XE Configuration & Verification

1. Configuring Special Areas and Summarization

! --- Configure ABR for Area 10 as Totally Stubby with Inter-Area Summarization ---
ABR-1(config)# router ospf 1
ABR-1(config-router)# router-id 1.1.1.1
! Define Area 10 as Totally Stubby (blocks Type 3, 4, 5 and injects Type 3 Default)
ABR-1(config-router)# area 10 stub no-summary
! Summarize Area 10 subnets (10.10.0.0/24 to 10.10.3.0/24) into a /22
ABR-1(config-router)# area 10 range 10.10.0.0 255.255.252.0

! --- Configure Internal Router in Area 10 (Standard Stub Router) ---
Router-10A(config)# router ospf 1
Router-10A(config-router)# router-id 10.10.1.1
Router-10A(config-router)# area 10 stub

! --- Configure ABR for Area 20 as Totally NSSA with External Summarization ---
ABR-2(config)# router ospf 1
ABR-2(config-router)# router-id 2.2.2.2
! Define Area 20 as Totally NSSA (blocks Type 3, 4, 5; allows Type 7; injects Type 3 Default)
ABR-2(config-router)# area 20 nssa no-summary

! --- Configure ASBR in Area 20 Redistributing Static Routes with Summarization ---
ASBR-20(config)# router ospf 1
ASBR-20(config-router)# router-id 20.20.1.1
ASBR-20(config-router)# area 20 nssa
! Summarize external redistributed static routes into a single /16
ASBR-20(config-router)# summary-address 172.16.0.0 255.255.0.0
ASBR-20(config-router)# redistribute static subnets metric 20 metric-type 1

2. Verification Commands & Output

! Verify OSPF Database LSA types on ABR
ABR-1# show ip ospf database

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)
Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         412         0x80000003 0x00A124 2
2.2.2.2         2.2.2.2         450         0x80000002 0x007812 2

                Summary Net Link States (Area 0)
Link ID         ADV Router      Age         Seq#       Checksum
10.10.0.0       1.1.1.1         182         0x80000001 0x00B291

                Summary Net Link States (Area 10)
Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         1.1.1.1         520         0x80000001 0x00F83A

! Verify Discard Route installed for summary prefix
ABR-1# show ip route ospf
O    10.10.0.0/22 [110/10] is a summary, 00:08:14, Null0
O*IA 0.0.0.0/0 [110/11] via 10.0.0.1, 00:15:32, GigabitEthernet0/0/0