OSPFv2 vs OSPFv3

This topic is to discuss the following lesson:

Hi Rene,

Links, not networks” what do mean at this point, can you please explain in detail ? and can you give me an example about multiple instance ID in the same link ?

Hello Hussein

This is essentially a change in semantics. In OSPFv2 we speak about networks. The destination network, the networks that are advertised. A network is expressed as, for example, This is a destination or advertise network as far as OSPFv2 is concerned.

IN OSPFv3, the term that is used is link. This means that 2001:AB::0/64 found in the routing table for example is called a link in the context of OSPFv3.

The command to implement OSPFv3 on an interface is the following:

**ipv6 ospf** process-id area area-id [instance instance-id]

You can input this command several times with a different instance-id in each case and have the same interface (and subsequently its link) participate in multiple instances of OSPFv3.

I hope this has been helpful!


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Thanks Laz

That was very helpful indeed.

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Hi all,
The flooding scope 0x2 says in the lesson that “is used for LSAs that are flooded throughout a single area”. LSA Type 3 and 4 are flooded throughout all ospf areas like the LSA Type5. Shouldn’t they also have the flooding scope 0x4?
Thanks in advance.

Hello Marios

Actually, if you take a closer look at Type 3 and Type 4 LSAs, you will find that they are not flooded throughout the whole AS scope. Looking at the related lesson about OSPFv2 LSAs, you can see that both are generated by an ABR that receives a Type 1 LSA. They are flooded to the rest of the network, but not in the OSPF area from which the Type 1 LSA is received.

The only LSA that truly exists throughout the whole AS in all OSPF areas is a Type 5 LSA, and it is the only one that begins with 0x4.

I hope this has been helpful!


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Regarding the statement:

By separating the SPF tree and prefixes, OSPFv3 is more efficient. When the link-local address on an interface changes, the router only has to flood an updated link LSA and intra-area-prefix LSA. Since there are no changes to the topology, we don’t have to flood type 1 and 2 LSA(s). Other routers won’t have to run SPF in this case.

Is it also correct to say that is more efficient because prefix information is not being repeated, as Type-1 and Type-2 LSAs can contain the same network information. e.g. if router 1 and router 2 are connected network. Both their Type-1 LSAs contain the same prefixes for the same link.



Hello Samir

Yes, what you say makes sense because you are not duplicating information that has already been sent.


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Hi dear,

I have a question? How can loops occur in OSPF? and how to differentiate L2 loops and L3 loops?

Hello Zahid

Concerning loops in OSPF, take a look at this post:

Concerning L2 and L3 loops, a layer 2 loop also known as a switching loop, or a bridging loop, is one where there is more than one layer 2 paths between two endpoints. A layer 2 loop will take place when

  • there are multiple connections between two network switches on the same VLAN
  • two ports on the same switch on the same VLAN are directly connected
  • three or more switches are connected in a physical loop using ports on the same VLAN

Unlike Layer 3 loops, which employ a time to live (TTL) function, switching loop packets will circulate the network until they are dropped, e.g. due to resource exhaustion.

Layer 2 loops are dealt with using features such as STP, Etherchannel, or the creation of VLANs within the topology

A layer 3 loop also known as a routing loop takes place when routing is configured in such a way to send an IP packet continuously around the same path. This differs from a switching loop in that the loop is created due to routing decisions. This means that a looped IP packet will be routed from one subnet to another (or from one VLAN to another) resulting in a continuously looped packet.

This is primarily due to misconfiguration or a routing algorithm error. Unlike Layer 2 loops, IP packets have a TTL value that is decremented every hop, and when it reaches zero, it is dropped. Layer 3 loops are mitigated against using TTL as well as using correct routing configurations.

I hope this has been helpful!


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