OSPFv3 Fundamentals

OSPFv3 is the evolution of OSPF designed to support IPv6.
For the CCNP ENCOR exam, you need to understand how OSPFv3 differs from OSPFv2.

OSPF exists in two main versions:

• OSPFv2 → Designed for IPv4

• OSPFv3 → Designed for IPv6 (and can also carry IPv4)

Figure 1 - OSPFv2 vs OSPFv3 fundamentals

Even though the core logic of OSPF remains the same (SPF algorithm, areas, DR/BDR election, LSAs), OSPFv3 introduces important structural changes.

OSPFv3 is not backward compatible with OSPFv2, and the two versions cannot form an adjacency with each other.

OSPFv3 Packet Transport and Multicast Communication

OSPFv3 still uses IP protocol 89, but it runs over IPv6 instead of IPv4.
It uses the IPv6 multicast addresses:

• FF02::5 (AllSPFRouters)

• FF02::6 (AllDRouters)

These two addresses are used for neighbor communication.

To form an OSPFv3 adjacency, a valid Router ID is mandatory.

Router ID Requirement

You must understand that:

• The Router ID is still 32-bit

• It is written in IPv4 format

• It must be unique in the OSPF domain

Figure 2 - Adjacency requirement not respected

If a router does not have a Router ID configured and no IPv4 address is available, the Router ID becomes 0.0.0.0, and adjacency will not form.
Once both routers have valid Router IDs, adjacency can be established.

Figure 3 - Adjacency requirement is respected

In addition to the Router ID, neighbors must also match:

• Area ID

• Hello and Dead timers

• Network type

• MTU

Link-Local Based Adjacency

Another key concept you must remember:

OSPFv3 forms adjacency using IPv6 link-local addresses only.
Global IPv6 addresses are not used for neighbor relationships.

Figure 4 - OSPFv3 link-local adjacency

For the exam, think:

OSPFv3 adjacency = link-local based communication.

In OSPFv2, you are used to advertising networks using the network statement under the routing process.

Figure 5 - OSPFv2 network statement configuration

In OSPFv3, this changes.
The network statement no longer exists.

Instead, you:

• Create the OSPFv3 process

• Configure the Router ID

• Enable OSPF directly on the interface

This is called interface-based activation.

Figure 6 - OSPFv3 interface-based activation

Interface-based activation provides more granular control and eliminates wildcard mask errors.
You must be comfortable recognizing this difference in configuration questions.

A major architectural change in OSPFv3 is how LSAs are structured.

Separation of Topology and Prefixes

In OSPFv2:

• The Router LSA (Type 1) contains both topology and prefixes.

In OSPFv3:

• The Router LSA (Type 1) contains topology only

• Prefixes are advertised separately

OSPFv3 builds the shortest path tree based on links, not networks.

Renamed LSAs

Type 3 and Type 4 LSAs are renamed in OSPFv3 but keep the same logical purpose.

• Type 3 → Inter-Area Prefix LSA

• Type 4 → Inter-Area Router LSA

New LSAs in OSPFv3

Two additional LSAs were introduced:

• Type 8 — Link LSA

• Type 9 — Intra-Area Prefix LSA

You do not need the deep internal details for ENCOR.

SPF Behavior in OSPFv3

What you must remember is:
OSPFv3 separates topology from prefix advertisement.

SPF calculation runs only on:

• Type 1 (Router LSA)

• Type 2 (Network LSA)

This means prefix changes do not trigger a full SPF recalculation.
That is the core architectural improvement.

OSPFv3 supports multi-area design just like OSPFv2.

You still have:

• Area 0 as the backbone

• ABRs connecting multiple areas

• Type 3 LSAs for inter-area routes

• Type 5 LSAs for external routes

Figure 7 - OSPFv3 Multi Area Topology

The ABR behavior remains identical to OSPFv2.
From a design perspective, nothing changes.

The difference is internal behavior (prefix handling), not area logic.
In the next lesson, you will configure and verify OSPFv3 step by step and see these concepts in action.