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 both versions cannot form 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)

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