Internal router, backbone router, ABR, ASBR

1.10 Troubleshoot OSPF (v2/v3)

1.10.c Network types, area types, and router types

📘CCNP Enterprise – ENARSI (300-410)

OSPF Router Types

OSPF routers are classified based on where they are in the OSPF topology and how they connect different areas. Understanding these types is crucial for troubleshooting OSPF because each router type has specific roles and behaviors.

There are four main OSPF router types:

Internal Router
Backbone Router
Area Border Router (ABR)
Autonomous System Boundary Router (ASBR)

Let’s go through them one by one.

1. Internal Router

Definition: A router where all of its interfaces belong to the same OSPF area.
Key Feature: It does not connect multiple areas; it is entirely within one area.
Routing Behavior:
- Only maintains the Link-State Database (LSDB) for its own area.
- Shares OSPF LSAs (Link-State Advertisements) within the same area.
Example in IT Environment:
- Imagine a router inside a data center area where all servers and subnets are within Area 1. This router only knows about other devices in Area 1.

Exam Tip:

Internal routers are usually the simplest OSPF routers.
Commands like show ip ospf neighbor will show neighbors only in the same area.

2. Backbone Router

Definition: A router with at least one interface in Area 0 (the OSPF backbone).
Key Feature: Any router that connects to Area 0 is automatically a backbone router.
Routing Behavior:
- Handles traffic between different OSPF areas.
- Can be an internal router too if all its interfaces are in Area 0.
Example in IT Environment:
- A router connecting the main data center (Area 0) to other branches. Traffic from other areas must pass through the backbone router to reach the backbone area.

Exam Tip:

Backbone is mandatory for OSPF inter-area routing.
All other areas must connect to Area 0 either directly or through a virtual link.

3. Area Border Router (ABR)

Definition: A router that connects two or more OSPF areas, typically one being the backbone (Area 0) and another non-backbone area.
Key Feature: ABRs maintain LSDBs for multiple areas.
Routing Behavior:
- Summarizes routes between areas (inter-area routing).
- Generates Type 3 LSAs (Summary LSAs) to advertise networks from one area to another.
Example in IT Environment:
- A branch office router connecting Area 1 (branch) to Area 0 (core network). It tells the backbone about the branch subnets and learns the backbone routes for branch use.

Exam Tip:

ABRs are critical for scalability.
They can have different LSDBs for each area, unlike internal routers.
Config commands like show ip ospf database will show multiple area LSAs.

4. Autonomous System Boundary Router (ASBR)

Definition: A router that connects OSPF to other routing protocols (like EIGRP, BGP) or another OSPF autonomous system.
Key Feature: Converts external routing information into OSPF routes (Type 5 LSAs).
Routing Behavior:
- Injects external routes into OSPF.
- Can redistribute routes from another protocol into OSPF.
Example in IT Environment:
- A router connecting the OSPF network to the Internet via BGP. It advertises external networks to internal OSPF routers.

Exam Tip:

ASBR is always responsible for external routes.
Use commands like show ip ospf border-routers to identify ASBRs.
On an OSPF topology diagram, ASBRs are where external connectivity enters.

Quick Comparison Table

Router Type	Area Connection	LSDB Scope	Key LSAs	Main Role
Internal Router	Single area only	Single area only	Type 1, Type 2	Routes inside one area
Backbone Router	At least Area 0	Area 0	Type 1, Type 2	Backbone traffic handling
ABR	Multiple areas (Area 0 + others)	LSDB per area	Type 1, 2, 3	Connects areas, summarizes routes
ASBR	Any area	Depends on placement	Type 5	Connects OSPF to external networks

Exam Tips and Tricks

Always remember:
- Internal → one area only.
- Backbone → at least in Area 0.
- ABR → connects multiple areas.
- ASBR → injects external routes.
Use show ip ospf and show ip ospf database to identify router types.
Know LSA types for each router type.
ABRs and ASBRs are the routers that cause inter-area or external routes to appear in OSPF routing tables.