Understanding OSPF LSA Type 4

Understanding OSPF LSA Type 4 is essential for anyone preparing for the CCIE lab, as it plays a critical role in OSPF routing. This LSA type, generated by Area Border Routers (ABRs), informs other areas how to reach an Autonomous System Boundary Router (ASBR).

Before diving into LSA Type 4, it's important to familiarize yourself with the various OSPF LSA types. Engaging in Cisco enterprise training can provide you with the knowledge needed to navigate these concepts effectively.

OSPF LSA Types

OSPF (Open Shortest Path First) uses Link State Advertisements (LSAs) to share routing information and build a comprehensive view of the network topology. There are several types of LSAs, each serving a specific purpose:

Type 1 - Router LSA

A router LSA is generated by each router in an OSPF area and describes the state of the router's links. It includes.

● Router ID

● Link of the router

● Cost of each Link

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Type 2 - Network LSA

A network LSA is generated by the designated router (DR) on a multi-access network and describes the state of the network. It includes 

● List of routers connected to the network.

● Cost of each link

Type 3 - Summary LSA

A summary LSA is generated by an Area Border Router (ABR) and describes the state of routes outside the area. It includes 

● IP address of the destination network

● Subnet mask

● Cost of the path to reach the destination.

Type 4 - ASBR Summary LSA

An ASBR summary LSA is generated by an ABR and describes the location of an ASBR (Autonomous System Boundary Router) that connects to an external network. It includes. 

● IP address of the ASBR

● Cost of the path

Type 5 - External LSA

An external LSA is generated by an ASBR and describes the state of external networks outside the OSPF domain. It includes 

● IP address of the external network

● Subnet mask

● Cost of the path

Type 6 - Multicast LSA

A multicast LSA is used to distribute information about multicast groups within an OSPF domain. It includes the multicast group address and a list of routers that have joined the group.

Type 7 - NSSA External LSA

An NSSA external LSA is generated by an ASBR in an NSSA (Not-So-Stubby Area) and describes the state of external networks that are injected into the area. It includes. 

● IP address of the external network

● Subnet mask

● Cost of the path

Each type of LSA serves a specific purpose in OSPF and helps to ensure efficient and accurate routing within the network. Understanding the different types of LSAs and their functions is crucial for network administrators who are managing OSPF networks.

You will get many books and notes on all other types of OSPF, and they are relatively easier to understand, In this blog article I will be discussing only type-4 LSA. Consider a small OSPF domain for a better understanding of this LSA. The diagram is also given below for your reference. 

—–RIP Domain —–R1—-AREA0——R2——AREA1——–R3

Here R1 is an ASBR since it is redistributing RIP into OSPF, and R2 is an ABR because both area 0 is attached to it. R3 is an internal router in area 1. In order to understand LSA Type-5 we have to understand how a router performs lookup to reach an external prefix.

When R1 performs redistribution from rip into OSPF, R1 generates an LSA Type-5 which contains external prefixes with its attributes. In this LSA, R1 attaches in this LSA, its router-I’d in the advertising router field and forwarding address (this may be 0.0.0.0 or any IP address depends on some parameters of the external link).

Now when a router in the same area in which ASBR is present like R2 performs a few steps of recursive look-up.

First look is for external prefix i.e.

Show ip ospf database external <> output of this command will contain the advertising router ip address and forward address along with other details.

If the forwarder address field is a non-zero value, then further look up will perform on this address otherwise look up will perform on the advertising router field IP address which is R1’s router I’d.

R2 will then search for this address in its ospf database

show ip ospf database router

Since this router is in the same area so it will perform SPF calculation in order to reach R1 and ultimately to external prefix.

When an ABR i.e R2 pass on the LSA Type-5 to area 1 (LSA Tpye-5 are supposed to get flooded through out the ospf domain) it also generates an LSA Type-4 for area 1 in order to inform all the routers in area 1 that the external prefixes are advertised by the router address (ASBR) contained in the LSA Type-4. Since R2 the ABR forwarding this information to the internal routers in area 1 so they can contact R2 in order to reach to ASBR and ultimately the external prefixes.

Any internal router performs the first look up on external prefix

show ip ospf database external <> (assuming forwarder address field is 0.0.0.0) then look up will perform on advertising router ( R1’s router-id ) appeared in this output.

Show ip ospf database router

This will not give any output because this router does not belong to this area 1. Therefore internal router will now have to perform look up on asbr

Show ip ospf database asbr-summary <>

This contains the advertising router as R2’s router-id the ABR so further look ups will perform on this ABR in order to reach it.

Show ip ospf database router

now internal router will do SPF calculation to reach ABR to reach ABBR and ultimately to external prefixes.

If you are interested in learning more such concepts check out more "IT infrastructure courses" that cover various concepts.