OSPF Prefix Suppression

This topic is to discuss the following lesson:

Hi Rene

Could you clarify the differences between this and OSPFv3 Prefix Suppression?

I believe the main difference is that type 8’s and 9’s are suppressed rather than type 1 or 2’s.

Are stub’s advertised as /128’s?

Also how does OSPF detect that a prefix is part of a transit network?

Hi @chrisnewnham17,

In OSPFv3, there is no prefix information in LSA type 1 and 2 anymore, you can find those in LSA type 8 and 9 so yes, OSPFv3 prefix suppression removes it from LSA type 8 and 9.

About the prefix / transit network. Here’s a quick example with three routers connected like this:

R1-R2-R3

All routers are in area 0 and R3 is the DR:

R3#show ipv6 ospf database prefix self-originate  

            OSPFv3 Router with ID (3.3.3.3) (Process ID 1)

                Intra Area Prefix Link States (Area 0)

  LS age: 8
  LS Type: Intra-Area-Prefix-LSA
  Link State ID: 0
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x7834
  Length: 52
  Referenced LSA Type: 2001
  Referenced Link State ID: 0
  Referenced Advertising Router: 3.3.3.3
  Number of Prefixes: 1
  Prefix Address: 2001:DB8:3333:3333::3
  Prefix Length: 128, Options: LA, Metric: 0

  LS age: 1729
  LS Type: Intra-Area-Prefix-LSA
  Link State ID: 3072
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x16FD
  Length: 44
  Referenced LSA Type: 2002
  Referenced Link State ID: 3
  Referenced Advertising Router: 3.3.3.3
  Number of Prefixes: 1
  Prefix Address: 2001:23:23:23::
  Prefix Length: 64, Options: None, Metric: 0

Above you can see the referenced LSA type:

* 2001: Associated with the router
* 2002: Associated with the Network LSA

The 2001:23:23:23:: prefix is configured on an Ethernet network, thus it shows up with LSA type 2002. You can also see the link state ID (3) of the advertising router (R3 our DR).

R3#show ipv6 ospf database network self-originate 

            OSPFv3 Router with ID (3.3.3.3) (Process ID 1)

                Net Link States (Area 0)

  LS age: 426
  Options: (V6-Bit, E-Bit, R-Bit, DC-Bit)
  LS Type: Network Links
  Link State ID: 3 (Interface ID of Designated Router)
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x41A3
  Length: 32
        Attached Router: 3.3.3.3
        Attached Router: 2.2.2.2

About the stub, you mean a stub network? Loopbacks are advertised with a /128, similar to how OSPFv2 advertises them as a /32 by default.

Hi Rene ,
Can explain a bit about how the how ospf detect transit links when performing ospf prefix suppression . Also, you are mentioning that /32 link is considered an invalid link in the lsdb.In such case why aren’ t the Loopback networks considered invalid when prefix suppression is enabled? These are also /32 links.

Regards,
Ruwan

Hello Ruwan

There are two types of LSAs that we have to deal with in this lesson: The Router LSA, that is a type 1 LSA that contains prefix information about the 192.168.30.0/24 network, and the Network LSA< that is a type 2 LSA that contains prefix information about the 192.168.123.0/24 subnet.

If you notice, in each of the LSAs described, the only networks that have a prefix other than /32 are the transit networks, so if you suppress prefixes, the transit networks will be removed.

A subnet mask of /32 on a multi-access network is not possible because then it would not be a multi-access network. So if an OSPF router sees this, it knows that prefix suppression has been applied. It is not invalid in the sense that it cannot exist in the database.

I hope this has been helpful!

Laz

Hi Rene,
Wouldn’t a normal network which isn’t the loopback interface also be suppressed as it is count as another stub network?

Hello Nitay

This is an excellent question. Yes, even though network 3.3.3.3 is considered a stub network in the LSDB it is not removed by the prefix suppression feature. However, the 192.168.34.0 network which is also indicated as a stub network in the LSDB is removed.

The difference between the two is that 192.168.34.0 is also indicated as a point to point network with a neighboring router ID of 4.4.4.4 and a next hop ID of 192.168.34.3. So the router knows that although the network itself is configured as a stub network when viewed as a destination, it is also part of a point to point network. The loopback network 3.3.3.3 which is a stub network does not have a corresponding point to point component, thus, it is not removed.

It is only stub networks that appear in the LSDB that are part of a point to point network that are removed since they are never used as a destination by user traffic.

I hope this has been helpful!

Laz

Thanks you very much,

That was realy helpful but I have another question:

how would R3 advertise his Network LSA to R4?
From the lesson’s statement it says

“The prefix isn’t advertised directly in LSA type 2 but with the IP address of the DR and the subnet mask, the router can calculate what prefix we are using (192.168.123.0/24).”

but there is also a statement which says that the Network LSA will do be advertised so maybe I didn’t understand that right, could you explain me what the quote meant to say?

Hello Nitay

In this statement, Rene is saying that the Network LSA is indeed being advertised, however, the information it is advertising (the 192.168.123.0/24 prefix) is contained within the actual IP address of the DR and its subnet mask. From the IP address and subnet mask, the router calculates the prefix being used. So the Network LSA is being advertised.

I hope this has been helpful!

Laz