OSPF Type-4 LSA

The importance of this article lies in why do the ABR (Area Boarder Routers) need to originate a new type of LSA (Link State Advertisements) side to side to Type-5 LSA.

Type-4 LSA is ASBR (Autonomus System Boarder Routers) summary which contains the summary of the ASBR’s RID (Router ID) where the ABR informs all other areas that they can reach the mentioned ASBR RID through it.

Here is the confusion that may happen, why do we need this Type-4 LSA while we already have Type-3 LSA which is network summary and also is originated by the ABR in order to inform all other areas by the nodes beyond it, so why?

To eliminate this confusion that surely happened to most of you, We will expose the key fields of Type-5 LSA:
Advertising Router: It is the RID of the ASBR who originates this LSA.
Forward Address: It is also known as FA, the default is 0.0.0.0 which means “Follow the path to the RID”

The confusion that happened is due to the RID expression, we know that the RID selection process is like below:

  • 1- highest Loopback interface.
  • 2- highest OSPF enabled interface.

and if the RID is manually configured it will override the selection process.

The OSPF RID is a node-identifer.  It usually uses an IPv4 address format, but it is an ID and does not need to match any interface present on the router.

So that the ABR or any Intera-Area routers can locate the RID but the other areas see Type-5 LSA with unknown Advertising Router ID, that’s why the ABR originates Type-4 LSA to inform all areas about these unknown ASBR RIDs.

lets take an example:

ASBR(Area1) —-(Area1)ABR(Area0)—-(Area0)Router

ASBR
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
end
ASBR#show run | sec ospf
router ospf 1
 router-id 5.5.5.5
 log-adjacency-changes
 redistribute connected subnets
 network 10.0.0.0 0.255.255.255 area 1

This configuration shows that the RID is manually configured to be 5.5.5.5 and ASBR is redistributing 1.1.1.1/32 prefix (connected) to OSPF domain as an external route.

Area 0 router
Router#show ip ospf database external
            OSPF Router with ID (3.3.3.3) (Process ID 1)
                Type-5 AS External Link States
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 803
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 5.5.5.5
  LS Seq Number: 80000003
  Checksum: 0x1F67
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

This output shows us that Area0 is receiving Type-5 LSA about 1.1.1.1/32 and the Advertising Router is 5.5.5.5 while the FA is 0.0.0.0 so that in order to reach this network we should follow the pass to Router 5.5.5.5 which is absolutely unknown to Area0, so that ABR is originating Type-4 LSA to clarify everything about Router 5.5.5.5 as shown below:

Area 0 Router
Router#show ip ospf database asbr-summary
            OSPF Router with ID (3.3.3.3) (Process ID 1)
                Summary ASB Link States (Area 0)
  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 982
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 5.5.5.5 (AS Boundary Router address)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000003
  Checksum: 0x5EBE
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 1

I hope that you found it useful and interesting by eliminating the confusion happened to all of us. For more information check RFC 2328.

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