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Meddane
VIP
VIP

When moving from OSPFv2 to OSPFv3, there are many changes in the format of the LSAs Type, but the most known changes are: IP prefix informations are no longer carried in Type-1 LSA and Type-2 LSA, new LSAs Type 8 and 9 are added to carry these prefixes.

Let's go indepth the Type-2 LSA in OSPFv2 and OSPFv3.

By definition with OSPFv2 the content of the LSA Type 2 describes the network segment listing the DR address, the attached routers, and the used nework mask. The IP prefix of the transit multi-access network is not carried in the Type-2 LSA, rather it should be computed by other routers that receive this LSA; by ANDing the IP address of the DR with the network mask.

With OSPFv3 the LSA Type 2 does not carry the used network mask and the DR address.

In OSPFv3, there are no information about the subnet mask and the DR address, only the attached routers are carried in the Type-2 LSA.

So how other routers discover and learn the subnet of the transit segment?

The answer is in RFC 5340 OSPF for IPv6

 

4.4.3.3. Network-LSAs

 

  The LS type of a network-LSA is set to the value 0x2002. Network-

  LSAs have area flooding scope. A network-LSA is originated for every

  broadcast or NBMA link with an elected Designated Router that is

  fully adjacent with at least one other router on the link. The

  network-LSA is originated by the link's Designated Router and lists all routers on the link with which it is fully adjacent.

 

  The procedure for originating network-LSAs in IPv6 is the same as the IPv4 procedure documented in Section 12.4.2 of [OSPFV2], with the following exceptions:

     An IPv6 network-LSA's Link State ID is set to the Interface ID of

     the Designated Router on the link.

     IPv6 network-LSAs do not contain a Network Mask. All addressing information formerly contained in the IPv4 network-LSA has now been consigned to intra-Area-Prefix-LSAs originated by the link's Designated Router.

Here comes the Intra-area prefix LSA (Type 9) for help.

OSPFv3 defines a new LSA called Intra-area prefix LSAs (Type 9): A router can originate multiple intra-area prefix LSAs for each router or transit network, each with a unique link-state ID. The link state ID for each intra-area prefix LSA describes its association to either the router LSA or the network LSA. The link-state ID also contains prefixes for stub and transit networks.

This LSA type (Intra Area Prefix) provides information for two different scenarios:

1.     It will provide information about IPv6 address prefixes associated with a transit network by referencing a Network LSA.

2.     It will provide information about IPv6 address prefixes associated with a router by referencing a Router LSA. Type 9 LSAs are only flooded within an area.

When running OSPFv3 in multiaccess segment, the DR generates a unique LSA Type 2 describing the attached routers and unique LSA Type 9 describing the subnet and the subnet mask used in this segment.

 

 

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